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In evolutionary biology, abiogenesis, or informally the origin of life (OoL),[a] is the natural process by which life has arisen from non-living matter, such as simple organic compounds. While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but an evolutionary process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. Although the occurrence of abiogenesis is uncontroversial among scientists, its possible mechanisms are poorly understood. There are several principles and hypotheses for how abiogenesis could have occurred.
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from biology, chemistry, and geophysics, with more recent approaches attempting a synthesis of all three: more specifically, astrobiology, biochemistry, biophysics, geochemistry, molecular biology, oceanography and paleontology. Life functions through the specialized chemistry of carbon and water and builds largely upon four key families of chemicals: lipids (cell membranes), carbohydrates (sugars, cellulose), amino acids (protein metabolism), and nucleic acids (DNA and RNA). Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world, although other self-replicating molecules may have preceded RNA.
The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.
The alternative panspermia hypothesis speculates that microscopic life arose outside Earth by unknown mechanisms, and spread to the early Earth on space dust and meteoroids. It is known that complex organic molecules occur in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth.
Earth remains the only place in the universe known to harbour life, and fossil evidence from the Earth informs most studies of abiogenesis. The age of the Earth is 4.54 Gy (billion year); the earliest undisputed evidence of life on Earth dates from at least 3.5 Gya (Gy ago), and possibly as early as the Eoarchean Era (3.6–4.0 Gya). In 2017 scientists found possible evidence of early life on land in 3.48 Gyo (Gy old) geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia. However, a number of discoveries suggest that life may have appeared on Earth even earlier. As of 2017[update], microfossils (fossilised microorganisms) within hydrothermal-vent precipitates dated 3.77 to 4.28 Gya in rocks in Quebec may harbour the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 Gya during the Hadean Eon.
The NASA strategy on abiogenesis states that it is necessary to identify interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems. Emphasis must continue to map the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection likely was a critical step in the emergence of prebiotic chemical evolution.
Thermodynamics, self-organization, and information: Physics
Thermodynamics principles: Energy and entropy
In antiquity it was commonly thought, for instance by Empedocles and Aristotle, that the life of the individuals of some species, and more generally, life itself, could start with high temperature, i.e. implicitly by thermal cycling.
Similarly, it was realized early on that life requires a loss of entropy, or disorder, when molecules organize themselves into living matter. This Second Law of thermodynamics needs to be considered when self-organization of matter to higher complexity happens. Because living organisms are machines, the Second Law applies to life as well.
Thus, contrary to a naive view of the Second Law, the emergence of life and increased complexity does not contradict this law: First, a living organism creates order in some places (e.g. its living body or dwelling) on the expense of an increase of entropy elsewhere (e.g. heat and waste production). Second, the Second Law of thermodynamics actually predicts an increase in complexity and in correlations between a system and its surrounding, when undergoing interaction - with memory and genetic adaptation being examples of such correlations between a living organism and its environment.
Obtaining free energy
Bernal said on the Miller–Urey experiment that
it is not enough to explain the formation of such molecules, what is necessary, is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy.
Multiple sources of energy were available for chemical reactions on the early Earth. For example, heat (such as from geothermal processes) is a standard energy source for chemistry. Other examples include sunlight and electrical discharges (lightning), among others. In fact, lightning is a plausible energy source for the origin of life, given that just in the tropics lightning strikes about 100 million times a year.
Unfavourable reactions can also be driven by highly favourable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for carbon fixation (the conversion of carbon from its inorganic form to an organic one).[b] Carbon fixation via iron-sulfur chemistry is highly favourable, and occurs at neutral pH and 100C. Iron-sulfur surfaces, which are abundant near hydrothermal vents, are also capable of producing small amounts of amino acids and other biological metabolites.
The discipline of synergetics studies self-organization in physical systems. In his book Synergetics Hermann Haken has pointed out that different physical systems can be treated in a similar way. He gives as examples of self-organization several types of lasers, instabilities in fluid dynamics, including convection, and chemical and biochemical oscillations. In his preface he mentions the origin of life, but only in general terms:
The spontaneous formation of well organized structures out of germs or even out of chaos is one of the most fascinating phenomena and most challenging problems scientists are confronted with. Such phenomena are an experience of our daily life when we observe the growth of plants and animals. Thinking of much larger time scales, scientists are led into the problems of evolution, and, ultimately, of the origin of living matter. When we try to explain or understand in some sense these extremely complex biological phenomena it is a natural question, whether processes of self-organization may be found in much simpler systems of the unanimated world.
In recent years it has become more and more evident that there exists numerous examples in physical and chemical systems where well organized spatial, temporal, or spatio-temporal structures arise out of chaotic states. Furthermore, as in living organisms, the functioning of these systems can be maintained only by a flux of energy (and matter) through them. In contrast to man-made machines, which are devised to exhibit special structures and functionings, these structures develop spontaneously—they are selforganizing. ...
This theory postulates that the hallmark of the origin and evolution of life is the microscopic dissipative structuring under UVC light of organic pigments and their proliferation over the entire Earth surface. Present day life augments the entropy production of Earth in its solar environment by dissipating ultraviolet and visible photons into heat through organic pigments in water. This heat then catalyzes a host of secondary dissipative processes such as the water cycle, ocean and wind currents, hurricanes, etc.
Self-organization by dissipative structures
The 19th-century physicist Ludwig Boltzmann first recognized that the struggle for existence of living organisms was neither over raw material nor energy, but instead had to do with entropy production derived from the conversion of the solar spectrum into heat by these systems. Boltzmann thus realized that living systems, like all irreversible processes, were dependent on the dissipation of a generalized chemical potential for their existence. In his book "What is Life", the 20th-century physicist Erwin Schrödinger emphasized the importance of Boltzmann's deep insight into the irreversible thermodynamic nature of living systems, suggesting that this was the physics and chemistry behind the origin and evolution of life.
However, irreversible processes, and much less living systems, could not be conveniently analyzed under this perspective until Lars Onsager, and later Ilya Prigogine, developed an elegant mathematical formalism for treating the "self-organization" of material under a generalized chemical potential. This formalism became known as Classical Irreversible Thermodynamics and Prigogine was awarded the Nobel Prize in Chemistry in 1977 "for his contributions to non-equilibrium thermodynamics, particularly the theory of dissipative structures". The analysis by Prigogine showed that if a system were left to evolve under an imposed external potential, material could spontaneously organize (lower its entropy) forming what he called "dissipative structures" which would increase the dissipation of the externally imposed potential (augment the global entropy production). Non-equilibrium thermodynamics has since been successfully applied to the analysis of living systems, from the biochemical production of ATP to optimizing bacterial metabolic pathways to complete ecosystems.
Current life, the result of abiogenesis: biology
Definition of life
As many as 123 definitions of life have been compiled.
The definition of life is somewhat disagreed upon; different biology textbooks define life differently. James Gould notes:
Most dictionaries define life as the property that distinguishes the living from the dead, and define dead as being deprived of life. These singularly circular and unsatisfactory definitions give us no clue to what we have in common with protozoans and plants.
Neil Campbell and Jane Reece wrote:
The phenomenon we call life defies a simple, one-sentence definition.
This difference can also be found in books on the origin of life. John Casti gives a single-sentence definition:
By more or general consensus nowadays, an entity is considered to be "alive" if it has the capacity to carry out three basic functional activities: metabolism, self-repair, and replication. 
In contrast, Dirk Schulze-Makuch and Louis Irwin devote the entire first chapter of their book to discussing a definition of life.
Since living organisms probably first arose in an atmosphere lacking oxygen, anaerobic fermentation is the simplest and most primitive type of biological mechanism for obtaining energy from nutrient molecules.
Fermentation involves glycolysis, which, rather inefficiently, transduces the chemical energy of sugar into the chemical energy of ATP.
As Fermentation had around 1970 been elucidated, whereas the mechanism of oxidative phosphorylation had not and some controversies still existed, fermentation may have looked too complex for investigators of the origin of life at that time. Peter Mitchell's Chemiosmosis is now however generally accepted as correct.
Even Peter Mitchell himself assumed that fermentation preceded chemiosmosis. Chemiosmosis is however ubiquitous in life. A model for the origin of life has been presented in terms of chemiosmosis. 
Both respiration by mitochondria and photosynthesis in chloroplasts make use of chemiosmosis to generate most of their ATP.
Today the energy source of all life can be linked to photosynthesis, and one speaks of primary production by sunlight. The oxygen used for oxidizing reducing compounds by organisms at hydrothermal vents at the bottom of the ocean is the result of photosynthesis at the Oceans' surface.
The mechanism of ATP synthesis is complex and involves a closed membrane in which the ATP synthase is embedded. The ATP is synthesized by the F1 subunit of ATP synthase by the binding change mechanism discovered by Paul Boyer. The energy required to release formed strongly-bound ATP has its origin in protons that move across the membrane. These protons have been set across the membrane during respiration or photosynthesis.
The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. It is widely accepted that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to exist. This conclusion is drawn from many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical groups and information transfers required for life. The structure of the ribosome has been called the "smoking gun," as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed.
The concept of the RNA world was first proposed in 1962 by Alexander Rich, and the term was coined by Walter Gilbert in 1986. In March 2020, astronomer Tomonori Totani presented a statistical approach for explaining how an initial active RNA molecule might have been produced randomly in the universe sometime since the Big Bang.
Phylogeny and LUCA
The most commonly accepted location of the root of the tree of life is between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota of what is referred to as the "traditional tree of life" based on several molecular studies starting with Carl Woese.
A very small minority of studies have concluded differently, namely that the root is in the domain Bacteria, either in the phylum Firmicutes or that the phylum Chloroflexi is basal to a clade with Archaea+Eukaryotes and the rest of Bacteria as proposed by Thomas Cavalier-Smith. More recently, Peter Ward has proposed an alternative view which is rooted in abiotic RNA synthesis which becomes enclosed within a capsule and then creates RNA ribozyme replicates. It is proposed that this then bifurcates between Dominion Ribosa (RNA life), and after the loss of ribozymes RNA viruses as Domain Viorea, and Dominion Terroa[clarification needed], which after creating a large cell within a lipid wall, creating DNA the 20 based amino acids and the triplet code, is established as the last universal common ancestor or LUCA, of earlier phylogenic trees.
In 2016, a set of 355 genes likely present in the Last Universal Common Ancestor (LUCA) of all organisms living on Earth was identified. A total of 6.1 million prokaryotic protein coding genes from various phylogenic trees were sequenced, identifying 355 protein clusters from amongst 286,514 protein clusters that were probably common to LUCA. The results
. . . depict LUCA as anaerobic, CO2-fixing, H2-dependent with a Wood–Ljungdahl pathway, N2-fixing and thermophilic. LUCA's biochemistry was replete with FeS clusters and radical reaction mechanisms. Its cofactors reveal dependence upon transition metals, flavins, S-adenosyl methionine, coenzyme A, ferredoxin, molybdopterin, corrins and selenium. Its genetic code required nucleoside modifications and S-adenosylmethionine-dependent methylations."
The results depict methanogenic clostridia as a basal clade in the 355 phylogenies examined, and suggest that LUCA inhabited an anaerobic hydrothermal vent setting in a geochemically active environment rich in H2, CO2 and iron.
A study at the University of Düsseldorf created phylogenic trees based upon 6 million genes from bacteria and archaea, and identified 355 protein families that were probably present in the LUCA. They were based upon an anaerobic metabolism fixing carbon dioxide and nitrogen. It suggests that the LUCA evolved in an environment rich in hydrogen, carbon dioxide and iron.
Key issues in abiogenesis
Protein vs. nucleic acid as the precursor to protein synthesis
Possible precursors for the evolution of protein synthesis include a mechanism to synthesize short peptide cofactors or form a mechanism for the duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.
Eugene Koonin said,
Despite considerable experimental and theoretical effort, no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system. The MWO ["many worlds in one"] version of the cosmological model of eternal inflation could suggest a way out of this conundrum because, in an infinite multiverse with a finite number of distinct macroscopic histories (each repeated an infinite number of times), emergence of even highly complex systems by chance is not just possible but inevitable.
Emergence of the genetic code
See: Genetic code.
Error in translation catastrophe
Hoffmann has shown that an early error-prone translation machinery can be stable against an error catastrophe of the type that had been envisaged as problematical for the origin of life, and was known as "Orgel's paradox".
Homochirality refers to a geometric uniformity of some materials composed of chiral units. Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands. Living organisms use molecules that have the same chirality ("handedness"): with almost no exceptions, amino acids are left-handed while nucleotides and sugars are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enantiomers (called a racemic mixture). Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation, statistical fluctuations during racemic synthesis, and spontaneous symmetry breaking.
Once established, chirality would be selected for. A small bias (enantiomeric excess) in the population can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction. In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.
Clark has suggested that homochirality may have started in outer space, as the studies of the amino acids on the Murchison meteorite showed that L-alanine is more than twice as frequent as its D form, and L-glutamic acid was more than three times prevalent than its D counterpart. Various chiral crystal surfaces can also act as sites for possible concentration and assembly of chiral monomer units into macromolecules. Compounds found on meteorites suggest that the chirality of life derives from abiogenic synthesis, since amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias, the same as found in living organisms.
Early universe and Earth: astronomy and geology
Early universe with first stars
Soon after the Big Bang, which occurred roughly 14 Gya, the only chemical elements present in the universe were hydrogen, helium, and lithium, the three lightest atoms in the periodic table. These elements gradually came together to form stars. These early stars were massive and short-lived, producing heavier elements through stellar nucleosynthesis. Carbon, currently the fourth most abundant chemical element in the universe (after hydrogen, helium and oxygen), was formed mainly in white dwarf stars, particularly those bigger than two solar masses.
As these stars reached the end of their lifecycles, they ejected these heavier elements, among them carbon and oxygen, throughout the universe. These heavier elements allowed for the formation of new objects, including rocky planets and other bodies.
Emergence of the Solar System
According to the nebular hypothesis, the formation and evolution of the Solar System began 4.6 Gya with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.
Emergence of Earth
The Earth, formed 4.5 Gya, was at first inhospitable to any living organisms. Based on numerous observations and studies of the geological time-scale, the Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane, ammonia and Water vapor—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. According to later models, suggested by studying ancient minerals, the atmosphere in the late Hadean period consisted largely of water vapor, nitrogen and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, and rapidly lost it during the Hadean period, along with the bulk of the original inert gases. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, giving them a pH of about 5.5. The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry.
Emergence of the ocean
Oceans may have appeared first in the Hadean Eon, as soon as 200 My after the Earth formed, in a hot, 100 C, reducing environment, and the pH of about 5.8 rose rapidly towards neutral. This scenario has found support from the dating of 4.404 Gyo zircon crystals from metamorphosed quartzite of Mount Narryer in the Western Australia Jack Hills of the Pilbara, which provide evidence that oceans and continental crust existed within 150 Ma of Earth's formation. Despite the likely increased volcanism and existence of many smaller tectonic "platelets," it has been suggested that between 4.4 and 4.3 Gyo, the Earth was a water world, with little if any continental crust, an extremely turbulent atmosphere and a hydrosphere subject to intense ultraviolet (UV) light, from a T Tauri stage Sun, cosmic radiation and continued bolide impacts.
Late heavy bombardment
The Hadean environment would have been highly hazardous to modern life. Frequent collisions with large objects, up to 500 km in diameter, would have been sufficient to sterilize the planet and vaporize the oceans within a few months of impact, with hot steam mixed with rock vapor becoming high altitude clouds that would completely cover the planet. After a few months, the height of these clouds would have begun to decrease but the cloud base would still have been elevated for about the next thousand years. After that, it would have begun to rain at low altitude. For another two thousand years, rains would slowly have drawn down the height of the clouds, returning the oceans to their original depth only 3,000 y after the impact event.
Traditionally it was thought that during the period between 4.28 and 3.8 Gya, changes in the orbits of the giant planets may have caused a heavy bombardment by asteroids and comets that pockmarked the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have repeatedly sterilized the planet, had life appeared before that time. Geologically, the Hadean Earth would have been far more active than at any other time in its history. Studies of meteorites suggests that radioactive isotopes such as aluminium-26 with a half-life of 7.17 ky, and potassium-40 with a half-life of 1.25 Gy, isotopes mainly produced in supernovae, were much more common. Internal heating as a result of gravitational sorting between the core and the mantle would have caused a great deal of mantle convection, with the probable result of many more smaller and more active tectonic plates than now exist.
The time periods between such devastating environmental events give time windows for the possible origin of life in the early environments. If the deep marine hydrothermal setting was the site for the origin of life, then abiogenesis could have happened as early as 4.0-4.2 Gya. If the site was at the surface of the Earth, abiogenesis could only have occurred between 3.7 and 4.0 Gya.
Estimates of the production of organics from these sources suggest that the Late Heavy Bombardment before 3.5 Ga within the early atmosphere made available quantities of organics comparable to those produced by terrestrial sources.
It has been estimated that the Late Heavy Bombardment may also have effectively sterilized the Earth's surface to a depth of tens of meters. If life evolved deeper than this, it would have also been shielded from the early high levels of ultraviolet radiation from the T Tauri stage of the Sun's evolution. Simulations of geothermically heated oceanic crust yield far more organics than those found in the Miller–Urey experiments. In the deep hydrothermal vents, Everett Shock has found "there is an enormous thermodynamic drive to form organic compounds, as seawater and hydrothermal fluids, which are far from equilibrium, mix and move towards a more stable state." Shock has found that the available energy is maximized at around 100–150 C, precisely the temperatures at which the hyperthermophilic bacteria and thermoacidophilic archaea have been found, at the base of the phylogenetic tree of life closest to the Last Universal Common Ancestor (LUCA).
Earliest evidence of life: palaeontology
The earliest life on Earth existed more than 3.5 Gya (billion years ago), during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence so far found consists of microfossils in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation rocks at least 3.77 and possibly 4.28 Gya. This finding suggested life developed very soon after oceans formed. The structure of the microbes was noted to be similar to bacteria found near hydrothermal vents in the modern era, and provided support for the hypothesis that abiogenesis began near hydrothermal vents.
Biogenic graphite has been found in 3.7 Gya metasedimentary rocks from southwestern Greenland and microbial mat fossils found in 3.48 Gyo sandstone from Western Australia. Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya have shown biogenic carbon isotopes. In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 Gya. At Strelley Pool, in the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing sandstone in a fossilized beach, that showed rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen. Further research on zircons from Western Australia in 2015 suggested that life likely existed on Earth at least 4.1 Gya.
In 2019 Raphael Baumgartner at the University of New South Wales in Australia and his colleagues looked at rocks in the Pilbara region of Western Australia. This area contains some of the oldest preserved rocks on Earth. Of the three most important sites, the Dresser Formation is the oldest, with rocks that are 3.48 billion years old. The Dresser Formation appears to contain layered structures called stromatolites. These stromatolites lie within undeformed hydrothermal-sedimentary strata and show textural features that are indicative of biogenic origins. In 2017, Tara Djokic and her team showed that parts of the Dresser formation preserve hot spring on lands, but other regions seem to have been shallow seas.
Conceptual history until the 1960s: biology
The panspermia hypothesis does not attempt to explain how life first originated but merely shifts the origin to another planet or a comet. The advantage of an extraterrestrial origin of primitive life is that life is not required to have formed on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence for the panspermia hypothesis is scant, but it finds some support in studies of Martian meteorites found in Antarctica and in studies of extremophile microbes' survival in outer space tests.
In August 2020, scientists reported that bacteria from Earth, particularly Deinococcus radiodurans, which is highly resistant to environmental hazards, were found to survive for three years in outer space, based on studies conducted on the International Space Station.
Origin of life posited directly after the Big Bang and have spread over the entire Universe
An extreme speculation is that the biochemistry of life could have begun as early as 17 My (million years) after the Big Bang, during a habitable epoch, and that life may exist throughout the universe.
Panspermia by life brought from Mars to Earth
Carl Zimmer has speculated that the chemical conditions, including the presence of boron, molybdenum and oxygen needed for the initial production of RNA, may have been better on early Mars than on early Earth. If so, life-suitable molecules originating on Mars may have later migrated to Earth via meteor ejections.
General acceptance of spontaneous generation until the 19th century
Traditional religion attributed the origin of life to supernatural deities who created the natural world. Spontaneous generation, the first naturalistic theory of life arising from non-life, goes back to Aristotle and ancient Greek philosophy, and continued to have support in Western scholarship until the 19th century. Classical notions of spontaneous generation held that certain "lower" or "vermin" animals are generated by decaying organic substances. According to Aristotle, it was readily observable that aphids arise from dew on plants, flies from putrid matter, mice from dirty hay, crocodiles from rotting sunken logs, and so on. A related theory was heterogenesis: that some forms of life could arise from different forms (e.g. bees from flowers). The modern scientist John Bernal said that the basic idea of such theories was that life was continuously created as a result of chance events.
In the 17th century, people began to question such assumptions. In 1646, Thomas Browne published his Pseudodoxia Epidemica (subtitled Enquiries into Very many Received Tenets, and commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His contemporary, Alexander Ross, erroneously refuted him, stating:
To question this [spontaneous generation], is to question Reason, Sense, and Experience: If he doubts of this, let him go to Ægypt, and there he will find the fields swarming with mice begot of the mud of Nylus, to the great calamity of the Inhabitants.
In 1665, Robert Hooke published the first drawings of a microorganism. Hooke was followed in 1676 by Antonie van Leeuwenhoek, who drew and described microorganisms that are now thought to have been protozoa and bacteria. Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for sexual reproduction, and asexual reproduction through cell division had not yet been observed. Van Leeuwenhoek took issue with the ideas common at the time that fleas and lice could spontaneously result from putrefaction, and that frogs could likewise arise from slime. Using a broad range of experiments ranging from sealed and open meat incubation and the close study of insect reproduction he became, by the 1680s, convinced that spontaneous generation was incorrect.
The first experimental evidence against spontaneous generation came in 1668 when Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative hypothesis was biogenesis: that every living thing came from a pre-existing living thing (omne vivum ex ovo, Latin for "every living thing from an egg"). In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could be killed by boiling. In 1861, Louis Pasteur performed a series of experiments that demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but could only appear by invasion from without.
Spontaneous generation considered disproven in the 19th century
By the middle of the 19th century, biogenesis had accumulated so much evidence in support that the alternative theory of spontaneous generation had been effectively disproven. Pasteur remarked, about a finding of his in 1864 which he considered definitive,
gave a mechanism by which life diversified from a few simple organisms to a variety of to complex forms. Today, scientists agree that all current life descends from earlier life, which has become progressively more complex and diverse through Charles Darwin's mechanism of evolution by natural selection. Darwin wrote to Hooker in 1863 stating that,
It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter.
In On the Origin of Species, he had referred to life having been "created", by which he "really meant 'appeared' by some wholly unknown process", but had soon regretted using the Old Testament term "creation".
Etymology of biogenesis and abiogenesis
The term biogenesis is usually credited to either Henry Bastian or to Thomas Huxley. Bastian used the term around 1869 in an unpublished exchange with John Tyndall to mean "life-origination or commencement". In 1870, Huxley, as new president of the British Association for the Advancement of Science, delivered an address entitled Biogenesis and Abiogenesis. In it he introduced the term biogenesis (with an opposite meaning to Bastian's) as well as abiogenesis:
- And thus the hypothesis that living matter always arises by the agency of pre-existing living matter, took definite shape; and had, henceforward, a right to be considered and a claim to be refuted, in each particular case, before the production of living matter in any other way could be admitted by careful reasoners. It will be necessary for me to refer to this hypothesis so frequently, that, to save circumlocution, I shall call it the hypothesis of Biogenesis; and I shall term the contrary doctrine—that living matter may be produced by not living matter—the hypothesis of Abiogenesis.
Subsequently, in the preface to Bastian's 1871 book, The Modes of Origin of Lowest Organisms, Bastian referred to the possible confusion with Huxley's usage and explicitly renounced his own meaning:
- A word of explanation seems necessary with regard to the introduction of the new term Archebiosis. I had originally, in unpublished writings, adopted the word Biogenesis to express the same meaning—viz., life-origination or commencement. But in the meantime, the word Biogenesis has been made use of, quite independently, by a distinguished biologist [Huxley], who wished to make it bear a totally different meaning. He also introduced the word Abiogenesis. I have been informed, however, on the best authority, that neither of these words can—with any regard to the language from which they are derived—be supposed to bear the meanings which have of late been publicly assigned to them. Wishing to avoid all needless confusion, I therefore renounced the use of the word Biogenesis, and being, for the reason just given, unable to adopt the other term, I was compelled to introduce a new word, in order to designate the process by which living matter is supposed to come into being, independently of pre-existing living matter.
Since the end of the nineteenth century, 'evolutive abiogenesis' means increasing complexity and evolution of matter from inert to living states.
Oparin: Primordial soup hypothesis
There is no single generally accepted model for the origin of life. Scientists have proposed several plausible hypotheses which share some common elements. While differing in details, these hypotheses are based on the framework laid out by Alexander Oparin (in 1924) and John Haldane (in 1925), that the first molecules constituting the earliest cells
. . . were synthesized under natural conditions by a slow process of molecular evolution, and these molecules then organized into the first molecular system with properties with biological order".
Oparin and Haldane suggested that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43−), with molecular oxygen (O2) and ozone (O3) either rare or absent. According to later models, the atmosphere in the late Hadean period consisted largely of nitrogen (N2) and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen (H2), and sulfur compounds; while it did lack molecular oxygen and ozone, it was not as chemically reducing as Oparin and Haldane supposed.
No new notable research or hypothesis on the subject appeared until 1924, when Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for life. In his book The Origin of Life, he proposed (echoing Darwin) that the "spontaneous generation of life" that had been attacked by Pasteur did, in fact, occur once, but was now impossible because the conditions found on the early Earth had changed, and preexisting organisms would immediately consume any spontaneously generated organism. Oparin argued that a "primeval soup" of organic molecules could be created in an oxygenless atmosphere through the action of sunlight. These would combine in ever more complex ways until they formed coacervate droplets. These droplets would "grow" by fusion with other droplets, and "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which factors that promote "cell integrity" survive, and those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.
About this time, Haldane suggested that the Earth's prebiotic oceans (quite different from their modern counterparts) would have formed a "hot dilute soup" in which organic compounds could have formed. Bernal called this idea biopoiesis or biopoesis, the process of living matter evolving from self-replicating but non-living molecules, and proposed that biopoiesis passes through a number of intermediate stages.
- The early Earth had a chemically reducing atmosphere.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
- These compounds accumulated in a "soup" that may have concentrated at various locations (shorelines, oceanic vents etc.).
- By further transformation, more complex organic polymers—and ultimately life—developed in the soup.
John Bernal showed that based upon this and subsequent work there is no difficulty in principle in forming most of the molecules we recognize as the necessary molecules for life from their inorganic precursors. The underlying hypothesis held by Oparin, Haldane, Bernal, Miller and Urey, for instance, was that multiple conditions on the primeval Earth favoured chemical reactions that synthesized the same set of complex organic compounds from such simple precursors. Bernal coined the term biopoiesis in 1949 to refer to the origin of life. In 1967, he suggested that it occurred in three "stages":
- the origin of biological monomers
- the origin of biological polymers
- the evolution from molecules to cells
Bernal suggested that evolution commenced between stages 1 and 2. Bernal regarded the third stage, in which biological reactions were incorporated behind a cell's boundary, as the most difficult. Modern work on the way that cell membranes self-assemble, and the work on micropores in various substrates, may be a key step towards understanding the development of independent free-living cells.
One of the most important pieces of experimental support for the "soup" theory came in 1952. Stanley Miller and Harold Urey performed an experiment that demonstrated how organic molecules could have spontaneously formed from inorganic precursors under conditions like those posited by the Oparin-Haldane hypothesis. The now-famous Miller–Urey experiment used a highly reducing mixture of gases—methane, ammonia, and hydrogen, as well as water vapor—to form simple organic monomers such as amino acids. The mixture of gases was cycled through an apparatus that delivered electrical sparks to the mixture. After one week, it was found that about 10% to 15% of the carbon in the system was then in the form of a racemic mixture of organic compounds, including amino acids, which are the building blocks of proteins. This provided direct experimental support for the second point of the "soup" theory, and it is around the remaining two points of the theory that much of the debate now centers.
A 2011 reanalysis of the saved vials containing the original extracts that resulted from the Miller and Urey experiments, using current and more advanced analytical equipment and technology, has uncovered more biochemicals than originally discovered in the 1950s. One of the more important findings was 23 amino acids, far more than the five originally found.
In November 2020, a team of international scientists reported studies which suggest that the primeval atmosphere of the Earth was much different than the conditions used in the Miller-Urey studies.
Primordial origin of biological molecules: Chemistry
The chemical processes on the pre-biotic early Earth are called chemical evolution. The elements, except for hydrogen and helium, ultimately derive from stellar nucleosynthesis. In 2016, astronomers reported that the very basic chemical ingredients of life—the carbon-hydrogen molecule (CH, or methylidyne radical), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+)—are largely the result of ultraviolet light from stars, rather than other forms of radiation from supernovae and young stars, as thought earlier. Complex molecules, including organic molecules, form naturally both in space and on planets. There are two possible sources of organic molecules on the early Earth:
- Terrestrial origins – organic molecule synthesis driven by impact shocks or by other energy sources (such as UV light, redox coupling, or electrical discharges; e.g., Miller's experiments)
- Extraterrestrial origins – formation of organic molecules in interstellar dust clouds, which rain down on planets. (See pseudo-panspermia)
Observed extraterrestrial organic molecules
An organic compound is any member of a large class of gaseous, liquid, or solid chemicals whose molecules contain carbon. Carbon is the fourth most abundant element in the Universe by mass after hydrogen, helium, and oxygen. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets. Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation. Based on computer model studies, the complex organic molecules necessary for life may have formed on dust grains in the protoplanetary disk surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets.
NASA announced in 2009 that scientists had identified another fundamental chemical building block of life in a comet for the first time, glycine, an amino acid, which was detected in material ejected from comet Wild 2 in 2004 and grabbed by NASA's Stardust probe. Glycine has been detected in meteorites before. Carl Pilcher, who leads the NASA Astrobiology Institute commented that
The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare.
Comets are encrusted with outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by ionizing radiation. It is possible that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth. Amino acids which were formed extraterrestrially may also have arrived on Earth via comets. It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year.
PAH world hypothesis
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the observable universe, and are considered a likely constituent of the primordial sea. In 2010, PAHs, have been detected in nebulae.
Polycyclic aromatic hydrocarbons (PAH) are known to be abundant in the universe, including in the interstellar medium, in comets, and in meteorites, and are some of the most complex molecules so far found in space.
Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of PAHs in a nebula. In 2010, another team also detected PAHs, along with fullerenes, in nebulae. The use of PAHs has also been proposed as a precursor to the RNA world in the PAH world hypothesis. The Spitzer Space Telescope has detected a star, HH 46-IR, which is forming by a process similar to that by which the Sun formed. In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. In 2012, NASA scientists reported that PAHs, subjected to interstellar medium conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively." Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."
NASA maintains a database for tracking PAHs in the universe. More than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.
Observations suggest that the majority of organic compounds introduced on Earth by interstellar dust particles are considered principal agents in the formation of complex molecules, thanks to their peculiar surface-catalytic activities. Studies reported in 2008, based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite, suggested that the RNA component uracil and related molecules, including xanthine, were formed extraterrestrially. In 2011, a report based on NASA studies of meteorites found on Earth was published suggesting DNA components (adenine, guanine and related organic molecules) were made in outer space. Scientists also found that the cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created naturally, and rapidly, by stars. Sun Kwok of The University of Hong Kong suggested that these compounds may have been related to the development of life on Earth said that "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
The sugar glycolaldehyde
Glycolaldehyde, the first example of an interstellar sugar molecule, was detected in the star-forming region near the centre of our galaxy. It was discovered in 2000 by Jes Jørgensen and Jan Hollis. In 2012, Jørgensen's team reported the detection of glycolaldehyde in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422 400 light years from Earth. Glycolaldehyde is needed to form RNA, which is similar in function to DNA. These findings suggest that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation. Because sugars are associated with both metabolism and the genetic code, two of the most basic aspects of life, it is thought the discovery of extraterrestrial sugar increases the likelihood that life may exist elsewhere in our galaxy.
A problem in most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphates. Polyphosphates are formed by polymerization of ordinary monophosphate ions PO43-. Several mechanisms of organic molecule synthesis have been investigated. Polyphosphates cause polymerization of amino acids into peptides. They are also logical precursors in the synthesis of such key biochemical compounds as adenosine triphosphate (ATP). A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate. There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early Earth. Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets. (Also see Extraterrestrial organic molecules).
The accumulation and concentration of organic molecules on a planetary surface is also considered an essential early step for the origin of life. Identifying and understanding the mechanisms that led to the production of prebiotic molecules in various environments is critical for establishing the inventory of ingredients from which life originated on Earth, assuming that the abiotic production of molecules ultimately influenced the selection of molecules from which life emerged.
In 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based origin of life on Earth, and possibly, as well, the notion of panspermia.
Chemical synthesis in the laboratory
As early as the 1860s, experiments have demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts.
In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal, Sidney Fox in the 1950s and 1960s studied the spontaneous formation of peptide structures (small chains of amino acids) under conditions that might plausibly have existed early in Earth's history. In one of his experiments, he allowed amino acids to dry out as if puddled in a warm, dry spot in prebiotic conditions: In an experiment to set suitable conditions for life to form, Fox collected volcanic material from a cinder cone in Hawaii. He discovered that the temperature was over 100 C just 4 inches (100 mm) beneath the surface of the cinder cone, and suggested that this might have been the environment in which life was created—molecules could have formed and then been washed through the loose volcanic ash into the sea. He placed lumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials, and baked the lava over the amino acids for a few hours in a glass oven. A brown, sticky substance formed over the surface, and when the lava was drenched in sterilized water, a thick, brown liquid leached out. He found that, as they dried, the amino acids formed long, often cross-linked, thread-like, submicroscopic polypeptide molecules.
In particular, experiments by Butlerov (the formose reaction) showed that tetroses, pentoses, and hexoses are produced when formaldehyde is heated under basic conditions with divalent metal ions like calcium. The reaction was scrutinized and subsequently proposed to be autocatalytic by Breslow in 1959.
Similar experiments (see below) demonstrate that nucleobases like guanine and adenine could be synthesized from simple carbon and nitrogen sources like hydrogen cyanide and ammonia.
Formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Formamide is ubiquitous in the Universe, produced by the reaction of water and hydrogen cyanide (HCN). It has several advantages as a biotic precursor, including the ability to easily become concentrated through the evaporation of water. Although HCN is poisonous, it only affects aerobic organisms (eukaryotes and aerobic bacteria), which did not yet exist. It can play roles in other chemical processes as well, such as the synthesis of the amino acid glycine.
In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like PAHs, the most carbon-rich chemical found in the Universe, may have been formed in red giant stars or in interstellar dust and gas clouds. A group of Czech scientists reported that all four RNA-bases may be synthesized from formamide in the course of high-energy density events like extraterrestrial impacts.
Use of reactions for ammonium cyanide
Effects with temperatures around the freezing point of water
Other pathways for synthesizing bases from inorganic materials were also reported. Orgel and colleagues have shown that freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide. Research by Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Research by the Miller group notes the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide were left in a freezer from 1972 to 1997. Other work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source). The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often. Mechanistic exploration using quantum chemical methods provide a more detailed understanding of some of the chemical processes involved in chemical evolution, and a partial answer to the fundamental question of molecular biogenesis.
Use of less-reducing gas in Miller–Urey experiment
At the time of the Miller–Urey experiment, scientific consensus was that the early Earth had a reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen (e.g., CH4 and NH3 as opposed to CO2 and nitrogen dioxide (NO2)). However, current scientific consensus describes the primitive atmosphere as either weakly reducing or neutral (see also Oxygen Catastrophe). Such an atmosphere would diminish both the amount and variety of amino acids that could be produced, although studies that include iron and carbonate minerals (thought present in early oceans) in the experimental conditions have again produced a diverse array of amino acids. Other scientific research has focused on two other potential reducing environments: outer space and deep-sea thermal vents.
Synthesis based on hydrogen cyanide
A research project completed in 2015 by John Sutherland and others found that a network of reactions beginning with hydrogen cyanide and hydrogen sulfide, in streams of water irradiated by UV light, could produce the chemical components of proteins and lipids, as well as those of RNA, while not producing a wide range of other compounds. The researchers used the term "cyanosulfidic" to describe this network of reactions.
Issues during laboratory synthesis
The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments. The Miller–Urey experiment, for example, produces many substances that would react with the amino acids or terminate their coupling into peptide chains. Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible set. Most models of the origin of life suggest organisms developed from environmentally available organic compounds. The fundamental roles that peptides and proteins play in today's biology makes it almost indisputable that peptides were key players in the origin of life. Insofar as it is appropriate to extrapolate back from extant biology to the prebiotic world, one must acknowledge the critical importance that interconnected molecular networks, likely with peptides as key components, would have played in life's origin.
Autocatalysts are substances that catalyze the production of themselves and therefore are "molecular replicators." The simplest self-replicating chemical systems are autocatalytic, and typically contain three components: a product molecule and two precursor molecules. The product molecule joins together the precursor molecules, which in turn produce more product molecules from more precursor molecules. The product molecule catalyzes the reaction by providing a complementary template that binds to the precursors, thus bringing them together. Such systems have been demonstrated both in biological macromolecules and in small organic molecules. Systems that do not proceed by template mechanisms, such as the self-reproduction of micelles and vesicles, have also been observed.
It has been proposed that life initially arose as autocatalytic chemical networks. British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor's Tale. In his book, Dawkins cites experiments performed by Julius Rebek and his colleagues in which they combined amino adenosine and pentafluorophenyl esters with the autocatalyst amino adenosine triacid ester (AATE). One product was a variant of AATE, which catalyzed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of natural selection.
Encapsulation without a membrane
Membraneless polyester droplets
Researchers Tony Jia and Kuhan Chandru have proposed that membraneless polyesters droplets could have been significant in the Origins of Life. Given the "messy" nature of prebiotic chemistry, the spontaneous generation of these combinatorial droplets may have played a role in early cellularization before the innovation of lipid vesicles. Protein function within and RNA function in the presence of certain polyester droplets was shown to be preserved within the droplets. Additionally, the droplets have scaffolding ability, by allowing lipids to assemble around them that may have prevented leakage of genetic materials.
The amino acids had combined to form proteinoids, and the proteinoids had combined to form small globules that Fox called "microspheres". His proteinoids were not cells, although they formed clumps and chains reminiscent of cyanobacteria, but they contained no functional nucleic acids or any encoded information. Based upon such experiments, Colin Pittendrigh stated in 1967 that "laboratories will be creating a living cell within ten years," a remark that reflected the typical contemporary naivety about the complexity of cell structures.
The lipid world theory postulates that the first self-replicating object was lipid-like. It is known that phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favourably. Studies on vesicles from potentially prebiotic amphiphiles have so far been limited to systems containing one or two types of amphiphiles. This in contrast to the output of simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures of compounds. Within the hypothesis of a lipid bilayer membrane composed of a mixture of various distinct amphiphilic compounds there is the opportunity of a huge number of theoretically possible combinations in the arrangements of these amphiphiles in the membrane. Among all these potential combinations, a specific local arrangement of the membrane would have favoured the constitution of a hypercycle, actually a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles which would have allowed Darwinian natural selection.
A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life. A central question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation driving the evolution of life. Although a functional protocell has not yet been achieved in a laboratory setting, there are scientists who think the goal is well within reach.
Self-assembled vesicles are essential components of primitive cells. The second law of thermodynamics requires that the universe move in a direction in which entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate life processes from non-living matter. Researchers Irene Chen and Szostak amongst others, suggest that simple physicochemical properties of elementary protocells can give rise to essential cellular behaviours, including primitive forms of differential reproduction competition and energy storage. Such cooperative interactions between the membrane and its encapsulated contents could greatly simplify the transition from simple replicating molecules to true cells. Furthermore, competition for membrane molecules would favour stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today. Such micro-encapsulation would allow for metabolism within the membrane, the exchange of small molecules but the prevention of passage of large substances across it. The main advantages of encapsulation include the increased solubility of the contained cargo within the capsule and the storage of energy in the form of an electrochemical gradient.
Another approach to the notion of a protocell concerns the term "chemoton" (short for 'chemical automaton') which refers to an abstract model for the fundamental unit of life introduced by Hungarian theoretical biologist Tibor Gánti. It is the oldest known computational abstract of a protocell. Gánti conceived the basic idea in 1952 and formulated the concept in 1971 in his book The Principles of Life (originally written in Hungarian, and translated to English only in 2003). He surmised the chemoton as the original ancestor of all organisms, or the last universal common ancestor.
The basic assumption of the chemoton model is that life should fundamentally and essentially have three properties: metabolism, self-replication, and a bilipid membrane. The metabolic and replication functions together form an autocatalytic subsystem necessary for the basic functions of life, and a membrane encloses this subsystem to separate it from the surrounding environment. Therefore, any system having such properties may be regarded as alive, and it will be subjected to natural selection and contain a self-sustaining cellular information. Some consider this model a significant contribution to origin of life as it provides a philosophy of evolutionary units.
Nonetheless, a 2012 study led by Mulkidjanian of the University of Osnabrück, suggests that inland pools of condensed and cooled geothermal vapor have the ideal characteristics for the origin of life. Scientists confirmed in 2002 that by adding a montmorillonite clay to a solution of fatty acid micelles (lipid spheres), the clay sped up the rate of vesicles formation 100-fold. Furthermore, recent studies have found that the repeated actions of dehydration and rehydration trapped biomolecules like RNA inside the lipid protocells found within hot springs and providing the necessary preconditions for evolution by natural selection.
Lipid vesicles formation in fresh water
Bruce Damer and David Deamer have come to the conclusion that cell membranes cannot be formed in salty seawater, and must therefore have originated in freshwater. Before the continents formed, the only dry land on Earth would be volcanic islands, where rainwater would form ponds where lipids could form the first stages towards cell membranes. These predecessors of true cells are assumed to have behaved more like a superorganism rather than individual structures, where the porous membranes would house molecules which would leak out and enter other protocells. Only when true cells had evolved would they gradually adapt to saltier environments and enter the ocean.
Vesicles consisting of mixtures of RNA-like biochemicals
Another protocell model is the Jeewanu. First synthesized in 1963 from simple minerals and basic organics while exposed to sunlight, it is still reported to have some metabolic capabilities, the presence of semipermeable membrane, amino acids, phospholipids, carbohydrates and RNA-like molecules. However, the nature and properties of the Jeewanu remains to be clarified.
Electrostatic interactions induced by short, positively charged, hydrophobic peptides containing 7 amino acids in length or fewer, can attach RNA to a vesicle membrane, the basic cell membrane.
William Martin and Michael Russell have suggested
. . . . that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments,..."
Pertinent geological environments
Darwin's little pond
An early concept, that life originated from non-living matter in slow stages, appeared in Herbert Spencer's 1864–1867 book Principles of Biology. In 1879 William Turner Thiselton-Dyer referred to this in a paper "On spontaneous generation and evolution". On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker, and set out his own speculation, suggesting that the original spark of life may have begun in a
warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes.
He went on to explain that
at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.
It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.
— Darwin, 1 February 1871
More recent studies, in 2017, support the notion that life may have begun right after the Earth was formed as RNA molecules emerging from "warm little ponds".
Volcanic hot springs and hydrothermal vents, shallow or deep
Martin Brazier has shown that early micro-fossils came from a hot world of gases such as methane, ammonia, carbon dioxide and hydrogen sulphide, which are toxic to much current life. Another analysis of the conventional threefold tree of life shows thermophilic and hyperthermophilic bacteria and archaea are closest to the root, suggesting that life may have evolved in a hot environment.
Deep sea hydrothermal vents
that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments,...
These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see Iron–sulfur world theory. These are highly exothermic reactions.[c]
Russell demonstrated that alkaline vents created an abiogenic proton motive force (PMF) chemiosmotic gradient, in which conditions are ideal for an abiogenic hatchery for life. Their microscopic compartments "provide a natural means of concentrating organic molecules," composed of iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser. This movement of ions across the membrane depends on a combination of two factors:
- Diffusion force caused by concentration gradient—all particles including ions tend to diffuse from higher concentration to lower.
- Electrostatic force caused by electrical potential gradient—cations like protons H+ tend to diffuse down the electrical potential, anions in the opposite direction.
These two gradients taken together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. The proton motive force can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential).
Szostak suggested that geothermal activity provides greater opportunities for the origination of life in open lakes where there is a buildup of minerals. In 2010, based on spectral analysis of sea and hot mineral water, Ignat Ignatov and Oleg Mosin demonstrated that life may have predominantly originated in hot mineral water. The hot mineral water that contains hydrocarbonate and calcium ions has the most optimal range. This case is similar to the origin of life in hydrothermal vents, but with hydrocarbonate and calcium ions in hot water. The research with spectral analyses was performed in Rupite, Bulgaria with hot mineral water with calcium and hydrocarbonate ions, Anoxybacillus rupiences sp., bacteria, archaea and cyanobacteria  Mineral water with pH of 9–11 is possible to have the reactions in seawater. According to Melvin Calvin, certain reactions of condensation-dehydration of amino acids and nucleotides in individual blocks of peptides and nucleic acids can take place in the primary hydrosphere with pH 9–11 at a later evolutionary stage. Some of these compounds like hydrocyanic acid (HCN) have been proven in the experiments of Miller. This is the environment in which the stromatolites have been created. David Ward of Montana State University described the formation of stromatolites in hot mineral water at the Yellowstone National Park. Stromatolites survive in hot mineral water and in proximity to areas with volcanic activity. Processes have evolved in the sea near geysers of hot mineral water. In 2011, Tadashi Sugawara from the University of Tokyo created a protocell in hot water.
Experimental research and computer modelling suggest that the surfaces of mineral particles inside hydrothermal vents have catalytic properties similar to those of enzymes and are able to create simple organic molecules, such as methanol (CH3OH) and formic, acetic and pyruvic acid out of the dissolved CO2 in the water.
The research reported above by Martin in 2016 supports the thesis that life arose at hydrothermal vents, that spontaneous chemistry in the Earth's crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life's origin and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism. Martin suggests, based upon this evidence that LUCA "may have depended heavily on the geothermal energy of the vent to survive".
Fluctuating hydrothermal pools on volcanic islands or proto-continents
Mulkidjanian and co-authors think that the marine environments did not provide the ionic balance and composition universally found in cells, as well as of ions required by essential proteins and ribozymes found in virtually all living organisms, especially with respect to K+/Na+ ratio, Mn2+, Zn2+ and phosphate concentrations. The only known environments that mimic the needed conditions on Earth are found in terrestrial hydrothermal pools fed by steam vents. Additionally, mineral deposits in these environments under an anoxic atmosphere would have suitable pH (as opposed to current pools in an oxygenated atmosphere), contain precipitates of sulfide minerals that block harmful UV radiation, have wetting/drying cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of polymers of nucleic acids, polyesters and depsipeptides, both by chemical reactions in the hydrothermal environment, as well as by exposure to UV light during transport from vents to adjacent pools. Their hypothesized pre-biotic environments are similar to the deep-oceanic vent environments most commonly hypothesized, but add additional components that help explain peculiarities found in reconstructions of the Last Universal Common Ancestor (LUCA) of all living organisms.
Colín-García et al. (2016) discuss the advantages and disadvantages of hydrothermal vents as primitive environments. They mention the exergonic reactions in such systems could have been a source of free energy that promoted chemical reactions, additional to their high mineralogical diversity which implies the induction of important chemical gradients, thus favoring the interaction between electron donors and acceptors. Colín-García et al. (2016) also summarize a set of experiments proposed to test the role of hydrothermal vents in prebiotic synthesis.
Volcanic ash in the ocean
Geoffrey W. Hoffmann has argued that a complex nucleation event as the origin of life involving both polypeptides and nucleic acid is compatible with the time and space available in the primary oceans of Earth Hoffmann suggests that volcanic ash may provide the many random shapes needed in the postulated complex nucleation event. This aspect of the theory can be tested experimentally.
Gold's deep-hot biosphere
In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. It is claimed that the discovery of microbial life below the surface of another body in our Solar System would lend significant credence to this theory. Gold also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold's theory is that the flow of such food is due to out-gassing of primordial methane from the Earth's mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks.
Radioactive beach hypothesis
Zachary Adam claims that tidal processes that occurred during a time when the Moon was much closer may have concentrated grains of uranium and other radioactive elements at the high-water mark on primordial beaches, where they may have been responsible for generating life's building blocks. According to computer models, a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the Oklo uranium ore seam in Gabon. Such radioactive beach sand might have provided sufficient energy to generate organic molecules, such as amino acids and sugars from acetonitrile in water. Radioactive monazite material also has released soluble phosphate into the regions between sand-grains, making it biologically "accessible." Thus amino acids, sugars, and soluble phosphates might have been produced simultaneously, according to Adam. Radioactive actinides, left behind in some concentration by the reaction, might have formed part of organometallic complexes. These complexes could have been important early catalysts to living processes.
John Parnell has suggested that such a process could provide part of the "crucible of life" in the early stages of any early wet rocky planet, so long as the planet is large enough to have generated a system of plate tectonics which brings radioactive minerals to the surface. As the early Earth is thought to have had many smaller plates, it might have provided a suitable environment for such processes.
Origin of metabolism: physiology
Different forms of life with variable origin processes may have appeared quasi-simultaneously in the early history of Earth. The other forms may be extinct (having left distinctive fossils through their different biochemistry—e.g., hypothetical types of biochemistry). It has been proposed that:
The first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids. If biosynthesis recapitulates biopoiesis, then the synthesis of amino acids preceded the synthesis of the purine and pyrimidine bases. Furthermore, the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides.
Metabolism-like reactions could have occurred naturally in early oceans, before the first organisms evolved. Metabolism may predate the origin of life, which may have evolved from the chemical conditions in the earliest oceans. Reconstructions in laboratories show that some of these reactions can produce RNA, and some others resemble two essential reaction cascades of metabolism: glycolysis and the pentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids and lipids.
Montmorillonite, an abundant clay, is a catalyst for the polymerization of RNA and for the formation of membranes from lipids. A model for the origin of life using clay was forwarded by Alexander Cairns-Smith in 1985 and explored as a plausible mechanism by several scientists. The clay hypothesis postulates that complex organic molecules arose gradually on pre-existing, non-organic replication surfaces of silicate crystals in solution.
At the Rensselaer Polytechnic Institute, James Ferris' studies have also confirmed that montmorillonite clay minerals catalyze the formation of RNA in aqueous solution, by joining nucleotides to form longer chains.
In 2007, Bart Kahr from the University of Washington and colleagues reported their experiments that tested the idea that crystals can act as a source of transferable information, using crystals of potassium hydrogen phthalate. "Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. They then examined the distribution of imperfections in the new crystals and found that the imperfections in the mother crystals were reproduced in the daughters, but the daughter crystals also had many additional imperfections. For gene-like behavior to be observed, the quantity of inheritance of these imperfections should have exceeded that of the mutations in the successive generations, but it did not. Thus Kahr concluded that the crystals "were not faithful enough to store and transfer information from one generation to the next."
In the 1980s, Günter Wächtershäuser, encouraged and supported by Karl Popper, postulated his iron–sulfur world, a theory of the evolution of pre-biotic chemical pathways as the starting point in the evolution of life. It systematically traces today's biochemistry to primordial reactions which provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.
In contrast to the classical Miller experiments, which depend on external sources of energy (simulated lightning, ultraviolet irradiation), "Wächtershäuser systems" come with a built-in source of energy: sulfides of iron (iron pyrite) and other minerals. The energy released from redox reactions of these metal sulfides is available for the synthesis of organic molecules, and such systems may have evolved into autocatalytic sets constituting self-replicating, metabolically active entities predating the life forms known today. Experiments with such sulfides in an aqueous environment at 100 °C produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) although under the same conditions, dipeptides were quickly broken down.
Several models reject the self-replication of a "naked-gene", postulating instead the emergence of a primitive metabolism providing a safe environment for the later emergence of RNA replication. The centrality of the Krebs cycle (citric acid cycle) to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, suggests that it was one of the first parts of the metabolism to evolve. Concordantly, geochemists Szostak and Adamala demonstrated non enzymatic RNA replication in primitive protocells is only possibly in presence of weak cation chelator like citric acid, providing further evidence for central role of citric acid in primordial metabolism. doi:10.1126/science.1241888
Russell has proposed that "the purpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first," rather than a "genetics-first," scenario). Physicist Jeremy England has proposed that life was inevitable from general thermodynamic considerations:
... when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. This could mean that under certain conditions, matter inexorably acquires the key physical attribute associated with life.
One of the earliest incarnations of this idea was put forward in 1924 with Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. Variants in the 1980s and 1990s include Wächtershäuser's iron–sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman's notion of collectively autocatalytic sets, discussed later that decade.
Orgel summarized his analysis by stating,
There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral."
It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the "open" acetyl-CoA pathway (another one of the five recognized ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organization on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase, harbors mixed nickel-iron-sulfur clusters in its reaction centers and catalyzes the formation of acetyl-CoA (similar to acetyl-thiol) in a single step. There are increasing concerns, however, that prebiotic thiolated and thioester compounds are thermodynamically and kinetically unfavorable to accumulate in presumed prebiotic conditions (i.e. hydrothermal vents). It has also been proposed that cysteine and homocysteine may have reacted with nitriles resulting from the Stecker reaction, readily forming catalytic thiol-reach poplypeptides.
The zinc world (Zn-world) hypothesis of Mulkidjanian  is an extension of Wächtershäuser's pyrite hypothesis. Wächtershäuser based his theory of the initial chemical processes leading to informational molecules (RNA, peptides) on a regular mesh of electric charges at the surface of pyrite that may have facilitated the primeval polymerization by attracting reactants and arranging them appropriately relative to each other. The Zn-world theory specifies and differentiates further. Hydrothermal fluids rich in H2S interacting with cold primordial ocean (or Darwin's "warm little pond") water leads to the precipitation of metal sulfide particles. Oceanic vent systems and other hydrothermal systems have a zonal structure reflected in ancient volcanogenic massive sulfide deposits (VMS) of hydrothermal origin. They reach many kilometers in diameter and date back to the Archean Eon. Most abundant are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). ZnS and MnS have a unique ability to store radiation energy, e.g. from UV light. During the relevant time window of the origins of replicating molecules, the primordial atmospheric pressure was high enough (>100 bar, about 100 atmospheres) to precipitate near the Earth's surface, and UV irradiation was 10 to 100 times more intense than now; hence the unique photosynthetic properties mediated by ZnS provided just the right energy conditions to energize the synthesis of informational and metabolic molecules and the selection of photostable nucleobases.
The Zn-world theory has been further filled out with experimental and theoretical evidence for the ionic constitution of the interior of the first proto-cells before archaea, bacteria and proto-eukaryotes evolved. Archibald Macallum noted the resemblance of body fluids such as blood and lymph to seawater; however, the inorganic composition of all cells differ from that of modern seawater, which led Mulkidjanian and colleagues to reconstruct the "hatcheries" of the first cells combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. The authors conclude that ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K+, Zn2+, Mn2+, and [PO
. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in what we today call marine settings but is compatible with emissions of vapor-dominated zones of what we today call inland geothermal systems. Under the oxygen depleted, CO2-dominated primordial atmosphere, the chemistry of water condensates and exhalations near geothermal fields would resemble the internal milieu of modern cells. Therefore, the precellular stages of evolution may have taken place in shallow "Darwin ponds" lined with porous silicate minerals mixed with metal sulfides and enriched in K+, Zn2+, and phosphorus compounds.
Other abiogenesis scenarios
We define a scenario as a set of related concepts pertinent to the origin of life that is or has been investigated. The concepts related to the Iron-Sulfur world can be considered as a scenario. We consider some other scenarios that may partially overlap with scenarios discussed above or with each other.
Chemical pathways described by computer
In September 2020, chemists described, for the first time, possible chemical pathways from nonliving prebiotic chemicals to complex biochemicals that could give rise to living organisms, based on a new computer program named ALLCHEMY.
In the early 1970s, Manfred Eigen and Peter Schuster examined the transient stages between the molecular chaos and a self-replicating hypercycle in a prebiotic soup. In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery of ribozymes capable of catalyzing their own chemical reactions. The hypercycle theory requires the existence of complex biochemicals, such as nucleotides, which do not form under the conditions proposed by the Miller–Urey experiment.
Organic pigments in dissipative structures
In his "Thermodynamic Dissipation Theory of the Origin and Evolution of Life", Karo Michaelian has taken the insight of Boltzmann and the work of Prigogine to its ultimate consequences regarding the origin of life. This theory postulates that the hallmark of the origin and evolution of life is the microscopic dissipative structuring of organic pigments and their proliferation over the entire Earth surface. Present day life augments the entropy production of Earth in its solar environment by dissipating ultraviolet and visible photons into heat through organic pigments in water. This heat then catalyzes a host of secondary dissipative processes such as the water cycle, ocean and wind currents, hurricanes, etc. Michaelian argues that if the thermodynamic function of life today is to produce entropy through photon dissipation in organic pigments, then this probably was its function at its very beginnings. It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 230–290 nm wavelength (UV-C) region, which is a part of the Sun's spectrum that could have penetrated the prebiotic atmosphere. In fact, not only RNA and DNA, but many fundamental molecules of life (those common to all three domains of life) are also pigments that absorb in the UV-C, and many of these also have a chemical affinity to RNA and DNA. Nucleic acids may thus have acted as acceptor molecules to the UV-C photon excited antenna pigment donor molecules by providing an ultrafast channel for dissipation. Michaelian has shown using the formalism of non-linear irreversible thermodynamics that there would have existed during the Archean a thermodynamic imperative to the abiogenic UV-C photochemical synthesis and proliferation of these pigments over the entire Earth surface if they acted as catalysts to augment the dissipation of the solar photons. By the end of the Archean, with life-induced ozone dissipating UV-C light in the Earth's upper atmosphere, it would have become ever more improbable for a completely new life to emerge that did not rely on the complex metabolic pathways already existing since now the free energy in the photons arriving at Earth's surface would have been insufficient for direct breaking and remaking of covalent bonds. It has been suggested, however, that such changes in the surface flux of ultraviolet radiation due to geophysical events affecting the atmosphere could have been what promoted the development of complexity in life based on existing metabolic pathways, for example during the Cambrian explosion
Some of the most difficult problems concerning the origin of life, such as enzyme-less replication of RNA and DNA, homochirality of the fundamental molecules, and the origin of information encoding in RNA and DNA, also find an explanation within the same dissipative thermodynamic framework by considering the probable existence of a relation between primordial replication and UV-C photon dissipation. Michaelian suggests that it is erroneous to expect to describe the emergence, proliferation, or even evolution, of life without overwhelming reference to entropy production through the dissipation of a generalized thermodynamic potential, in particular, the prevailing solar photon flux.
A new origin-of-life theory based on self-replicating beta-sheet structures has been put forward by Maury in 2009. The theory suggest that self-replicating and self-assembling catalytic amyloids were the first informational polymers in a primitive pre-RNA world. The main arguments for the amyloid hypothesis is based on the structural stability, autocatalytic and catalytic properties, and evolvability of beta-sheet based informational systems. Such systems are also error correcting and chiroselective.
Fluctuating salinity: dilute and dry-down
Theories of abiogenesis seldom address the caveat raised by Harold Blum: if the key informational elements of life – proto-nucleic acid chains – spontaneously form duplex structures, then there is no way to dissociate them.
Somewhere in this cycle work must be done, which means that free energy must be expended. If the parts assemble themselves on a template spontaneously, work has to be done to take the replica off; or, if the replica comes off the template of its own accord, work must be done to put the parts on in the first place.
The Oparin–Haldane conjecture addresses the formation, but not the dissociation, of nucleic acid polymers and duplexes. However, nucleic acids are unusual because, in the absence of counterions (low salt) to neutralize the high charges on opposing phosphate groups, the nucleic acid duplex dissociates into single chains. Early tides, driven by a close moon, could have generated rapid cycles of dilution (high tide, low salt) and concentration (dry-down at low tide, high salt) that exclusively promoted the replication of nucleic acids through a process dubbed tidal chain reaction (TCR). This theory has been criticized on the grounds that early tides may not have been so rapid, although regression from current values requires an Earth–Moon juxtaposition at around two Ga, for which there is no evidence, and early tides may have been approximately every seven hours. Another critique is that only 2–3% of the Earth's crust may have been exposed above the sea until late in terrestrial evolution.
The TCR (tidal chain reaction) theory has mechanistic advantages over thermal association/dissociation at deep-sea vents because TCR requires that chain assembly (template-driven polymerization) takes place during the dry-down phase, when precursors are most concentrated, whereas thermal cycling needs polymerization to take place during the cold phase, when the rate of chain assembly is lowest and precursors are likely to be more dilute.
A first protein that condenses substrates during thermal cycling: thermosynthesis
Emergence of chemiosmotic machinery Today's bioenergetic process of fermentation is carried out by either the aforementioned citric acid cycle or the Acetyl-CoA pathway, both of which have been connected to the primordial Iron–sulfur world.
In a different approach, the thermosynthesis hypothesis considers the bioenergetic process of chemiosmosis, which plays an essential role in cellular respiration and photosynthesis, more basal than fermentation: the ATP synthase enzyme, which sustains chemiosmosis, is proposed as the currently extant enzyme most closely related to the first metabolic process.
First life needed an energy source to bring about the condensation reaction that yielded the peptide bonds of proteins and the phosphodiester bonds of RNA. In a generalization and thermal variation of the binding change mechanism of today's ATP synthase, the "first protein" would have bound substrates (peptides, phosphate, nucleosides, RNA 'monomers') and condensed them to a reaction product that remained bound until it was released after a temperature change by a thermal unfolding. The primordial first protein would therefore have strongly resembled the beta subunits of the ATP synthase alpha/beta subunits of today's F1 moiety in the FoF1 ATP synthase. Note however that today's enzymes function during isothermal conditions, whereas the hypothetical first protein worked on and during thermal cycling.
The energy source under the thermosynthesis hypothesis was thermal cycling, the result of suspension of protocells in a convection current, as is plausible in a volcanic hot spring; the convection accounts for the self-organization and dissipative structure required in any origin of life model. The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis.
By phosphorylating cell membrane lipids, this first protein gave a selective advantage to the lipid protocell that contained the protein. This protein also synthesized a library of many proteins, of which only a minute fraction had thermosynthesis capabilities. As proposed by Dyson, it propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Functioning daughters consisted of different amino acid sequences.
Whereas the iron–sulfur world identifies a circular pathway as the most simple, the thermosynthesis hypothesis does not even invoke a pathway: ATP synthase's binding change mechanism resembles a physical adsorption process that yields free energy, rather than a regular enzyme's mechanism, which decreases the free energy.
The described first protein may be simple in the sense that is requires only a short sequence of conserved amino acid residues, a sequent sufficient for the appropriate catalytic cleft. In contrast, it has been claimed that the emergence of cyclic systems of protein catalysts such as required by fermentation is implausible because of the length of many required sequences.
Pre-RNA world: The ribose issue and its bypass
It is possible that a different type of nucleic acid, such as peptide nucleic acid, threose nucleic acid or glycol nucleic acid, was the first to emerge as a self-reproducing molecule, only later replaced by RNA. Larralde et al., say that
and they conclude that their
results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability.
The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.
Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesized by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by using nitrogenous or oxygenous chemistries. Sutherland has demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater. This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallized out from a mixture of the other pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry. In 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerize into RNA. This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates.
While features of self-organization and self-replication are often considered the hallmark of living systems, there are many instances of abiotic molecules exhibiting such characteristics under proper conditions. Stan Palasek suggested based on a theoretical model that self-assembly of ribonucleic acid (RNA) molecules can occur spontaneously due to physical factors in hydrothermal vents. Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.
Recent evidence for a "virus first" hypothesis, which may support theories of the RNA world, has been suggested. One of the difficulties for the study of the origins of viruses is their high rate of mutation; this is particularly the case in RNA retroviruses like HIV. A 2015 study compared protein fold structures across different branches of the tree of life, where researchers can reconstruct the evolutionary histories of the folds and of the organisms whose genomes code for those folds. They argue that protein folds are better markers of ancient events as their three-dimensional structures can be maintained even as the sequences that code for those begin to change. Thus, the viral protein repertoire retain traces of ancient evolutionary history that can be recovered using advanced bioinformatics approaches. Those researchers think that "the prolonged pressure of genome and particle size reduction eventually reduced virocells into modern viruses (identified by the complete loss of cellular makeup), meanwhile other coexisting cellular lineages diversified into modern cells." The data suggest that viruses originated from ancient cells that co-existed with the ancestors of modern cells. These ancient cells likely contained segmented RNA genomes.
A computational model (2015) has shown that virus capsids may have originated in the RNA world and that they served as a means of horizontal transfer between replicator communities since these communities could not survive if the number of gene parasites increased, with certain genes being responsible for the formation of these structures and those that favored the survival of self-replicating communities. The displacement of these ancestral genes between cellular organisms could favor the appearance of new viruses during evolution. Viruses retain a replication module inherited from the prebiotic stage since it is absent in cells. So this is evidence that viruses could originate from the RNA world and could also emerge several times in evolution through genetic escape in cells.
A number of hypotheses of formation of RNA have been put forward. As of 1994[update], there were difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil. Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Early cell membranes could have formed spontaneously from proteinoids, which are protein-like molecules produced when amino acid solutions are heated while in the correct concentration of aqueous solution. These are seen to form micro-spheres which are observed to behave similarly to membrane-enclosed compartments. Other possible means of producing more complicated organic molecules include chemical reactions that take place on clay substrates or on the surface of the mineral pyrite.
Factors supporting an important role for RNA in early life include its ability to act both to store information and to catalyze chemical reactions (as a ribozyme); its many important roles as an intermediate in the expression of and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the RNA molecule under the conditions that approximated the early Earth.
Relatively short RNA molecules have been synthesized, capable of replication. Such replicase RNA, which functions as both code and catalyst provides its own template upon which copying can occur. Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. If these conditions were present, Darwinian natural selection would favour the proliferation of such autocatalytic sets, to which further functionalities could be added. Such autocatalytic systems of RNA capable of self-sustained replication have been identified. The RNA replication systems, which include two ribozymes that catalyze each other's synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the conditions that existed in the experiment. In evolutionary competition experiments, this led to the emergence of new systems which replicated more efficiently. This was the first demonstration of evolutionary adaptation occurring in a molecular genetic system.
Depending on the definition, life started when RNA chains began to self-replicate, initiating the three mechanisms of Darwinian selection: heritability, variation of type, and differential reproductive output. The fitness of an RNA replicator (its per capita rate of increase) would likely be a function of its intrinsic adaptive capacities, determined by its nucleotide sequence, and the availability of resources. The three primary adaptive capacities may have been: (1) replication with moderate fidelity, giving rise to both heritability while allowing variation of type, (2) resistance to decay, and (3) acquisition of process resources. These capacities would have functioned by means of the folded configurations of the RNA replicators resulting from their nucleotide sequences.
In the latter half of 2020, evidence, based on a plausibly prebiotic simple compound named diamidophosphate (DAP), supporting the notion of a RNA-DNA mixture coevolution, has been presented. The mixture of RNA-DNA sequences, called chimeras, have weak affinity and form weaker duplex structures. This property is advantageous in an abiotic scenario and these chimeras have been shown to replicate RNA and DNA – overcoming the "template-product" inhibition problem, where a pure RNA or pure DNA strand is unable to replicate non-enzymatically because it binds too strongly to its partners. This behavior of chimeric RNA-DNA sequences could lead to an abiotic cross-catalytic amplification of RNA and DNA—a key step toward the simultaneous emergence of RNA and DNA.
Experiments on the origin of life
Both Eigen and Sol Spiegelman demonstrated that evolution, including replication, variation, and natural selection, can occur in populations of molecules as well as in organisms. Following on from chemical evolution came the initiation of biological evolution, which led to the first cells. No one has yet synthesized a "protocell" using simple components with the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to focus on chemosynthesis. However, some researchers work in this field, notably Steen Rasmussen and Szostak.
Others have argued that a "top-down approach" is more feasible, starting with simple forms of current life. Spiegelman took advantage of natural selection to synthesize the Spiegelman Monster, which had a genome with just 218 nucleotide bases, having deconstructively evolved from a 4500-base bacterial RNA. Eigen built on Spiegelman's work and produced a similar system further degraded to just 48 or 54 nucleotides—the minimum required for the binding of the replication enzyme. Craig Venter and others at J. Craig Venter Institute engineered existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life are reached.
In October 2018, researchers at McMaster University announced the development of a new technology, called a Planet Simulator, to help study the origin of life on planet Earth and beyond. It consists of a sophisticated climate chamber to study how the building blocks of life were assembled and how these prebiotic molecules transitioned into self-replicating RNA molecules.
- Anthropic principle – Philosophical premise that all scientific observations presuppose a universe compatible with the emergence of sentient organisms that make those observations
- Artificial cell
- Artificial life – A field of study wherein researchers examine systems related to natural life, its processes, and its evolution, through the use of simulations, wet-lab experimentation, and robotics
- Bathybius haeckelii
- Entropy and life
- Formamide-based prebiotic chemistry
- GADV-protein world hypothesis
- Hemolithin – Protein claimed to be of extraterrestrial origin
- Hypothetical types of biochemistry – Possible alternative biochemicals used by life forms
- Mediocrity principle – philosophical concept
- Nexus for Exoplanet System Science – Dedicated to the search for life on exoplanets
- Noogenesis – Emergence and evolution of intelligence
- Planetary habitability – Extent to which a planet is suitable for life as we know it
- Protocell – Lipid globule proposed as a precursor of living cells
- Rare Earth hypothesis – Hypothesis that complex extraterrestrial life is improbable and extremely rare
- Shadow biosphere – A hypothetical microbial biosphere of Earth that would use radically different biochemical and molecular processes from that of currently known life
- Tholin – Class of molecules formed by ultraviolet irradiation of organic compounds
- Also occasionally called biopoiesis (Bernal, 1960, p. 30)
- The reactions are:
- FeS + H2S → FeS2 + 2H+ + 2e−
- FeS + H2S + CO2 → FeS2 + HCOOH
- The reactions are:
Reaction 1: Fayalite + water → magnetite + aqueous silica + hydrogen
- 3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2
- 3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4
- 2Mg2SiO4 + 3H2O → Mg3Si2O5(OH)4 + Mg(OH)2
- 2 Ca2SiO4 + 4 H2O → 3 CaO · 2 SiO2 · 3 H2O + Ca(OH)2
- Dodd, Matthew S.; Papineau, Dominic; Grenne, Tor; Slack, John F.; Rittner, Martin; Pirajno, Franco; O'Neil, Jonathan; Little, Crispin T.S. (1 March 2017). "Evidence for early life in Earth's oldest hydrothermal vent precipitates". Nature. 543 (7643): 60–64. Bibcode:2017Natur.543...60D. doi:10.1038/nature21377. PMID 28252057. Archived from the original on 8 September 2017. Retrieved 2 March 2017.
- Zimmer, Carl (1 March 2017). "Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest". The New York Times. Archived from the original on 2 March 2017. Retrieved 2 March 2017.
- Oparin, Aleksandr Ivanovich (1938). The Origin of Life. Phoenix Edition Series. Translated by Morgulis, Sergius (2 ed.). Mineola, New York: Courier Corporation (published 2003). ISBN 978-0486495224. Retrieved 16 June 2018.
- Peretó, Juli (2005). "Controversies on the origin of life" (PDF). International Microbiology. 8 (1): 23–31. PMID 15906258. Archived from the original (PDF) on 24 August 2015. Retrieved 1 June 2015.
Ever since the historical contributions by Aleksandr I. Oparin, in the 1920s, the intellectual challenge of the origin of life enigma has unfolded based on the assumption that life originated on Earth through physicochemical processes that can be supposed, comprehended, and simulated; that is, there were neither miracles nor spontaneous generations.
- Compare: Scharf, Caleb; et al. (18 December 2015). "A Strategy for Origins of Life Research". Astrobiology. 15 (12): 1031–1042. Bibcode:2015AsBio..15.1031S. doi:10.1089/ast.2015.1113. PMC 4683543. PMID 26684503.
What do we mean by the origins of life (OoL)? [...] Since the early 20th century the phrase OoL has been used to refer to the events that occurred during the transition from non-living to living systems on Earth, i.e., the origin of terrestrial biology (Oparin, 1924; Haldane, 1929). The term has largely replaced earlier concepts such as abiogenesis (Kamminga, 1980; Fry, 2000).
- Oparin 1953, p. vi
- Warmflash, David; Warmflash, Benjamin (November 2005). "Did Life Come from Another World?". Scientific American. 293 (5): 64–71. Bibcode:2005SciAm.293e..64W. doi:10.1038/scientificamerican1105-64. PMID 16318028.
According to the conventional hypothesis, the earliest living cells emerged as a result of chemical evolution on our planet billions of years ago in a process called abiogenesis.
- Yarus 2010, p. 47
- Witzany, Guenther (2016). "Crucial steps to life: From chemical reactions to code using agents" (PDF). BioSystems. 140: 49–57. doi:10.1016/j.biosystems.2015.12.007. PMID 26723230.
- Howell, Elizabeth (8 December 2014). "How Did Life Become Complex, And Could It Happen Beyond Earth?". Astrobiology Magazine. Retrieved 14 February 2018.
- Tirard, Stephane (20 April 2015). Abiogenesis – Definition. Encyclopedia of Astrobiology. p. 1. doi:10.1007/978-3-642-27833-4_2-4. ISBN 978-3-642-27833-4.
Thomas Huxley (1825–1895) used the term abiogenesis in an important text published in 1870. He strictly made the difference between spontaneous generation, which he did not accept, and the possibility of the evolution of matter from inert to living, without any influence of life. [...] Since the end of the nineteenth century, evolutive abiogenesis means increasing complexity and evolution of matter from inert to living state in the abiotic context of evolution of primitive Earth.
- Levinson, Gene (2020). Rethinking evolution: the revolution that's hiding in plain sight. World Scientific. ISBN 978-1786347268.
- Voet & Voet 2004, p. 29
- Dyson 1999
- Davies, Paul (1998). The Fifth Miracle, Search for the origin and meaning of life. Penguin. Bibcode:1998fmso.book.....D.[page needed]
- Ward, Peter; Kirschvink, Joe (2015). A New History of Life: the radical discoveries about the origins and evolution of life on earth. Bloomsbury Press. pp. 39–40. ISBN 978-1608199105.
- *Copley, Shelley D.; Smith, Eric; Morowitz, Harold J. (December 2007). "The origin of the RNA world: Co-evolution of genes and metabolism" (PDF). Bioorganic Chemistry. 35 (6): 430–443. doi:10.1016/j.bioorg.2007.08.001. PMID 17897696. Archived (PDF) from the original on 5 September 2013. Retrieved 8 June 2015.
The proposal that life on Earth arose from an RNA world is widely accepted.
- Orgel, Leslie E. (April 2003). "Some consequences of the RNA world hypothesis". Origins of Life and Evolution of the Biosphere. 33 (2): 211–218. Bibcode:2003OLEB...33..211O. doi:10.1023/A:1024616317965. PMID 12967268. S2CID 32779859.
It now seems very likely that our familiar DNA/RNA/protein world was preceded by an RNA world...
- Robertson & Joyce 2012: "There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life."
- Neveu, Kim & Benner 2013: "[The RNA world's existence] has broad support within the community today."
- Orgel, Leslie E. (April 2003). "Some consequences of the RNA world hypothesis". Origins of Life and Evolution of the Biosphere. 33 (2): 211–218. Bibcode:2003OLEB...33..211O. doi:10.1023/A:1024616317965. PMID 12967268. S2CID 32779859.
- Robertson, Michael P.; Joyce, Gerald F. (May 2012). "The origins of the RNA world". Cold Spring Harbor Perspectives in Biology. 4 (5): a003608. doi:10.1101/cshperspect.a003608. PMC 3331698. PMID 20739415.
- Cech, Thomas R. (July 2012). "The RNA Worlds in Context". Cold Spring Harbor Perspectives in Biology. 4 (7): a006742. doi:10.1101/cshperspect.a006742. PMC 3385955. PMID 21441585.
- Keller, Markus A.; Turchyn, Alexandra V.; Ralser, Markus (25 March 2014). "Non‐enzymatic glycolysis and pentose phosphate pathway‐like reactions in a plausible Archean ocean". Molecular Systems Biology. 10 (725): 725. doi:10.1002/msb.20145228. PMC 4023395. PMID 24771084.
- Rampelotto, Pabulo Henrique (26 April 2010). Panspermia: A Promising Field of Research (PDF). Astrobiology Science Conference 2010. Houston, TX: Lunar and Planetary Institute. p. 5224. Bibcode:2010LPICo1538.5224R. Archived (PDF) from the original on 27 March 2016. Retrieved 3 December 2014. Conference held at League City, TX
- Berera, Arjun (6 November 2017). "Space dust collisions as a planetary escape mechanism". Astrobiology. 17 (12): 1274–1282. arXiv:1711.01895. Bibcode:2017AsBio..17.1274B. doi:10.1089/ast.2017.1662. PMID 29148823. S2CID 126012488.
- Chan, Queenie H.S. (10 January 2018). "Organic matter in extraterrestrial water-bearing salt crystals". Science Advances. 4 (1, eaao3521): eaao3521. Bibcode:2018SciA....4O3521C. doi:10.1126/sciadv.aao3521. PMC 5770164. PMID 29349297.
- Ehrenfreund, Pascale; Cami, Jan (December 2010). "Cosmic carbon chemistry: from the interstellar medium to the early Earth". Cold Spring Harbor Perspectives in Biology. 2 (12): a002097. doi:10.1101/cshperspect.a002097. PMC 2982172. PMID 20554702.
- Perkins, Sid (8 April 2015). "Organic molecules found circling nearby star". Science (News). Washington, DC: American Association for the Advancement of Science. Retrieved 2 June 2015.
- King, Anthony (14 April 2015). "Chemicals formed on meteorites may have started life on Earth". Chemistry World (News). London: Royal Society of Chemistry. Archived from the original on 17 April 2015. Retrieved 17 April 2015.
- Saladino, Raffaele; Carota, Eleonora; Botta, Giorgia; et al. (13 April 2015). "Meteorite-catalyzed syntheses of nucleosides and of other prebiotic compounds from formamide under proton irradiation". Proc. Natl. Acad. Sci. U.S.A. 112 (21): E2746–E2755. Bibcode:2015PNAS..112E2746S. doi:10.1073/pnas.1422225112. PMC 4450408. PMID 25870268.
- Graham, Robert W. (February 1990). "Extraterrestrial Life in the Universe" (PDF) (NASA Technical Memorandum 102363). Lewis Research Center, Cleveland, Ohio: NASA. Archived (PDF) from the original on 3 September 2014. Retrieved 2 June 2015.
- Altermann 2009, p. xvii
- "Age of the Earth". United States Geological Survey. 9 July 2007. Archived from the original on 23 December 2005. Retrieved 10 January 2006.
- Dalrymple 2001, pp. 205–221
- Manhesa, Gérard; Allègre, Claude J.; Dupréa, Bernard; Hamelin, Bruno (May 1980). "Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics". Earth and Planetary Science Letters. 47 (3): 370–382. Bibcode:1980E&PSL..47..370M. doi:10.1016/0012-821X(80)90024-2.
- Schopf, J. William; Kudryavtsev, Anatoliy B.; Czaja, Andrew D.; Tripathi, Abhishek B. (5 October 2007). "Evidence of Archean life: Stromatolites and microfossils". Precambrian Research. 158 (3–4): 141–155. Bibcode:2007PreR..158..141S. doi:10.1016/j.precamres.2007.04.009.
- Schopf, J. William (29 June 2006). "Fossil evidence of Archaean life". Philosophical Transactions of the Royal Society B. 361 (1470): 869–885. doi:10.1098/rstb.2006.1834. PMC 1578735. PMID 16754604.
- Raven & Johnson 2002, p. 68
- Staff (9 May 2017). "Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks". Phys.org. Archived from the original on 10 May 2017. Retrieved 13 May 2017.
- Djokic, Tara; Van Kranendonk, Martin J.; Campbell, Kathleen A.; Walter, Malcolm R.; Ward, Colin R. (9 May 2017). "Earliest signs of life on land preserved in ca. 3.5 Gao hot spring deposits". Nature Communications. 8: 15263. Bibcode:2017NatCo...815263D. doi:10.1038/ncomms15263. PMC 5436104. PMID 28486437.
- Schopf, J. William; Kitajima, Kouki; Spicuzza, Michael J.; Kudryavtsev, Anatolly B.; Valley, John W. (2017). "SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions". PNAS. 115 (1): 53–58. Bibcode:2018PNAS..115...53S. doi:10.1073/pnas.1718063115. PMC 5776830. PMID 29255053.
- Tyrell, Kelly April (18 December 2017). "Oldest fossils ever found show life on Earth began before 3.5 billion years ago". University of Wisconsin-Madison. Retrieved 18 December 2017.
- Ghosh, Pallab (1 March 2017). "Earliest evidence of life on Earth found". BBC News. Archived from the original on 2 March 2017. Retrieved 2 March 2017.
- Dunham, Will (1 March 2017). "Canadian bacteria-like fossils called oldest evidence of life". Reuters. Archived from the original on 2 March 2017. Retrieved 1 March 2017.
- "Researchers uncover 'direct evidence' of life on Earth 4 billion years ago". Deutsche Welle. Retrieved 5 March 2017.
- "NASA Astrobiology Strategy" (PDF). NASA. 2015. Archived from the original (PDF) on 22 December 2016. Retrieved 24 September 2017.
- Guthrie, W. K. C. (1957). In the beginning: Some Greek views on the origins of life and the early state of man. Methuen, London.
- Simon, Michael A. (1971). The Matter of Life (1 ed.). New Haven and London: Yale University Press.
- Ladyman, J.; Lambert, J.; Weisner, K.B. What is a Complex System? Eur. J. Philos. Sci. 2013, 3, 33–67.
- Esposito, M., Lindenberg, K., & Van den Broeck, C. (2010). Entropy production as correlation between system and reservoir. New Journal of Physics, 12(1), 013013.
- Bernal 1967, p. 143
- Follmann, Hartmut; Brownson, Carol (November 2009). "Darwin's warm little pond revisited: from molecules to the origin of life". Naturwissenschaften. 96 (11): 1265–1292. Bibcode:2009NW.....96.1265F. doi:10.1007/s00114-009-0602-1. PMID 19760276. S2CID 23259886.
- Gora, Evan M.; Burchfield, Jeffrey C.; Muller‐Landau, Helene C.; Bitzer, Phillip M.; Yanoviak, Stephen P. (2020). "Pantropical geography of lightning-caused disturbance and its implications for tropical forests". Global Change Biology. n/a (n/a): 5017–5026. Bibcode:2020GCBio..26.5017G. doi:10.1111/gcb.15227. ISSN 1365-2486. PMID 32564481.
- Kalson, Natan-Haim; Furman, David; Zeiri, Yehuda (2017). "Cavitation-Induced Synthesis of Biogenic Molecules on Primordial Earth". ACS Central Science. 3 (9): 1041–1049. doi:10.1021/acscentsci.7b00325. PMC 5620973. PMID 28979946.
- Haken, Hermann (1978). Synergetics. An Introduction. Berlin: Springer.
- Haken, Hermann (1978). Synergetics. An Introduction. Springer.
- Michaelian, K (2011). "Thermodynamic dissipation theory for the origin of life". Earth System Dynamics. 2 (1): 37–51. arXiv:0907.0042. Bibcode:2011ESD.....2...37M. doi:10.5194/esd-2-37-2011. S2CID 14574109.
- Michaelian, K.; Simeonov, A. (19 August 2015). "Fundamental molecules of life are pigments which arose and co-evolved as a response to the thermodynamic imperative of dissipating the prevailing solar spectrum". Biogeosciences. 12 (16): 4913–4937. Bibcode:2015BGeo...12.4913M. doi:10.5194/bg-12-4913-2015. ISSN 1726-4170.
- Michaelian, Karo (2017). "Microscopic dissipative structuring and proliferation at the origin of life". Heliyon. 3 (10): e00424. doi:10.1016/j.heliyon.2017.e00424. PMC 5647473. PMID 29062973.
- Michaelian, K (2012). "HESS Opinions 'Biological catalysis of the hydrological cycle: Life's thermodynamic function'". Hydrology and Earth System Sciences. 16 (8): 2629–2645. arXiv:0907.0040. Bibcode:2012HESS...16.2629M. doi:10.5194/hess-16-2629-2012.
- Boltzmann, L. (1886) The Second Law of Thermodynamics, in: Ludwig Boltzmann: Theoretical physics and Selected writings, edited by: McGinness, B., D. Reidel, Dordrecht, The Netherlands, 1974.
- Schrödinger, Erwin (1944) What is Life? The Physical Aspect of the Living Cell. Cambridge University Press
- Onsager, L. (1931) Reciprocal Relations in Irreversible Processes I and II, Phys. Rev. 37, 405; 38, 2265 (1931)
- Prigogine, I. (1967) An Introduction to the Thermodynamics of Irreversible Processes, Wiley, New York
- Dewar, R; Juretić, D.; Županović, P. (2006). "The functional design of the rotary enzyme ATP synthase is consistent with maximum entropy production". Chem. Phys. Lett. 430 (1): 177–182. Bibcode:2006CPL...430..177D. doi:10.1016/j.cplett.2006.08.095.
- Unrean, P., Srienc, F. (2011) Metabolic networks evolve towards states of maximum entropy production, Metabolic Engineering 13, 666–673.
- Zotin, A.I. (1984) "Bioenergetic trends of evolutionary progress of organisms", in: Thermodynamics and regulation of biological processes Lamprecht, I. and Zotin, A.I. (eds.), De Gruyter, Berlin, pp. 451–458.
- Schneider, E.D.; Kay, J.J. (1994). "Life as a Manifestation of the Second Law of Thermodynamics". Mathematical and Computer Modelling. 19 (6–8): 25–48. CiteSeerX 10.1.1.36.8381. doi:10.1016/0895-7177(94)90188-0.
- Michaelian, K. (2005). "Thermodynamic stability of ecosystems". Journal of Theoretical Biology. 237 (3): 323–335. Bibcode:2004APS..MAR.P9015M. doi:10.1016/j.jtbi.2005.04.019. PMID 15978624.
- Trifonov, Edward N. (17 March 2011). "Vocabulary of Definitions of Life Suggests a Definition". Journal of Biomolecular Structure and Dynamics. 29 (2): 259–266. doi:10.1080/073911011010524992. PMID 21875147. S2CID 38476092. Retrieved 15 December 2020.
- Gould, James L.; Keeton, William T. (1996). Biological Science (6 ed.). New York: W.W. Norton.
- Campbell, Neil A.; Reece, Jane B. (2005). Biology (7 ed.). Sn Feancisco: Benjamin.
- Casti, John L. (1989). Paradigms lost. Images of man in the mirror of science. New York: Morrow. Bibcode:1989plim.book.....C.
- Schulze-Makuch, Dirk; Irwin, Louis N. (2018). Life in the Universe. Expectations and Constraints (3 ed.). New York: Springer.
- Lehninger, Albert L. (1970). Biochemistry. The Molecular Basis of Cell Structure and Function. New York: Worth. p. 313.
- Anthonie W.J. Muller (1995). "Were the first organisms heat engines? A new model for biogenesis and the early evolution of biological energy conversion". Progress in Biophysics and Molecular Biology. 63 (2): 193–231. doi:10.1016/0079-6107(95)00004-7. PMID 7542789.
- Anthonie W.J. Muller and Dirk Schulze-Makuch (2006). "Thermal energy and the origin of life". Origins of Life and Evolution of Biospheres. 36 (2): 77–189. Bibcode:2006OLEB...36..177M. doi:10.1007/s11084-005-9003-4. PMID 16642267. S2CID 22179552.
- Wimberly, Brian T.; Brodersen, Ditlev E.; Clemons, William M. Jr.; et al. (21 September 2000). "Structure of the 30S ribosomal subunit". Nature. 407 (6802): 327–339. Bibcode:2000Natur.407..327W. doi:10.1038/35030006. PMID 11014182. S2CID 4419944.
- Zimmer, Carl (25 September 2014). "A Tiny Emissary From the Ancient Past". The New York Times. New York. Archived from the original on 27 September 2014. Retrieved 26 September 2014.
- Wade, Nicholas (4 May 2015). "Making Sense of the Chemistry That Led to Life on Earth". The New York Times. New York. Archived from the original on 9 July 2017. Retrieved 10 May 2015.
- Benner, S.A.; Bell, E.A.; Biondi, E.; Brasser, R.; Carell, T.; Kim, H.-J.; Mojzsis, S.J.; Omran, A.; Pasek, M.A.; Trail, D. (2020). "When Did Life Likely Emerge on Earth in an RNA‐First Process?". ChemSystemsChem. 2 (2). doi:10.1002/syst.201900035.
- Yarus, Michael (April 2011). "Getting Past the RNA World: The Initial Darwinian Ancestor". Cold Spring Harbor Perspectives in Biology. 3 (4): a003590. doi:10.1101/cshperspect.a003590. PMC 3062219. PMID 20719875.
- Fox, George.E. (9 June 2010). "Origin and evolution of the ribosome". Cold Spring Harbor Perspectives in Biology. 2 (9(a003483)): a003483. doi:10.1101/cshperspect.a003483. PMC 2926754. PMID 20534711.
- Neveu, Marc; Kim, Hyo-Joong; Benner, Steven A. (22 April 2013). "The 'Strong' RNA World Hypothesis: Fifty Years Old". Astrobiology. 13 (4): 391–403. Bibcode:2013AsBio..13..391N. doi:10.1089/ast.2012.0868. PMID 23551238.
- Gilbert, Walter (20 February 1986). "Origin of life: The RNA world". Nature. 319 (6055): 618. Bibcode:1986Natur.319..618G. doi:10.1038/319618a0. S2CID 8026658.
- Gough, Evan (10 March 2020). "Life Could be Common Across the Universe, Just Not in Our Region". Universe Today. Retrieved 15 March 2020.
- Totani, Tomonori (3 February 2020). "Emergence of life in an inflationary universe". Scientific Reports. 10 (1671): 1671. arXiv:1911.08092. Bibcode:2020NatSR..10.1671T. doi:10.1038/s41598-020-58060-0. PMC 6997386. PMID 32015390.
- Boone, David R.; Castenholz, Richard W.; Garrity, George M., eds. (2001). The Archaea and the Deeply Branching and Phototrophic Bacteria. Bergey's Manual of Systematic Bacteriology. Springer. ISBN 978-0-387-21609-6. Archived from the original on 25 December 2014.[page needed]
- Woese CR, Fox GE (1977). "Phylogenetic structure of the prokaryotic domain: the primary kingdoms". Proc Natl Acad Sci U S A. 74 (11): 5088–5090. Bibcode:1977PNAS...74.5088W. doi:10.1073/pnas.74.11.5088. PMC 432104. PMID 270744.
- Valas RE, Bourne PE (2011). "The origin of a derived superkingdom: how a gram-positive bacterium crossed the desert to become an archaeon". Biology Direct. 6: 16. doi:10.1186/1745-6150-6-16. PMC 3056875. PMID 21356104.
- Cavalier-Smith T (2006). "Rooting the tree of life by transition analyses". Biology Direct. 1: 19. doi:10.1186/1745-6150-1-19. PMC 1586193. PMID 16834776.
- Ward, Peter Douglas (2005). Life as We Do Not Know it: The NASA Search for (and Synthesis Of) Alien Life. Viking Books. ISBN 978-0670034581.
- Wade, Nicholas (25 July 2016). "Meet Luca, the Ancestor of All Living Things". The New York Times. Archived from the original on 28 July 2016.
- Weiss, M.C.; Sousa, F.L.; Mrnjavac, N.; Neukirchen, S.; Roettger, M.; Nelson-Sathi, S.; Martin, W.F. (2016). "The physiology and habitat of the last universal common ancestor". Nature Microbiology. 1 (9): 16116. doi:10.1038/NMICROBIOL.2016.116. PMID 27562259. S2CID 2997255.
- Nature Vol 535, 28 July 2016,"Early Life Liked it Hot", p.468
- Noller, Harry F. (April 2012). "Evolution of protein synthesis from an RNA world". Cold Spring Harbor Perspectives in Biology. 4 (4): a003681. doi:10.1101/cshperspect.a003681. PMC 3312679. PMID 20610545.
- Koonin, Eugene V. (31 May 2007). "The cosmological model of eternal inflation and the transition from chance to biological evolution in the history of life". Biology Direct. 2: 15. doi:10.1186/1745-6150-2-15. PMC 1892545. PMID 17540027.
- Hoffmann, Geoffrey W. (25 June 1974). "On the origin of the genetic code and the stability of the translation apparatus". Journal of Molecular Biology. 86 (2): 349–362. doi:10.1016/0022-2836(74)90024-2. PMID 4414916.
- Orgel, Leslie E. (April 1963). "The Maintenance of the Accuracy of Protein Synthesis and its Relevance to Ageing". Proc. Natl. Acad. Sci. U.S.A. 49 (4): 517–521. Bibcode:1963PNAS...49..517O. doi:10.1073/pnas.49.4.517. PMC 299893. PMID 13940312.
- Hoffmann, Geoffrey W. (October 1975). "The Stochastic Theory of the Origin of the Genetic Code". Annual Review of Physical Chemistry. 26: 123–144. Bibcode:1975ARPC...26..123H. doi:10.1146/annurev.pc.26.100175.001011.
- Chaichian, Rojas & Tureanu 2014, pp. 353–364
- Plasson, Raphaël; Kondepudi, Dilip K.; Bersini, Hugues; et al. (August 2007). "Emergence of homochirality in far-from-equilibrium systems: Mechanisms and role in prebiotic chemistry". Chirality. 19 (8): 589–600. doi:10.1002/chir.20440. PMID 17559107. "Special Issue: Proceedings from the Eighteenth International Symposium on Chirality (ISCD-18), Busan, Korea, 2006"
- Jafarpour, Farshid; Biancalani, Tommaso; Goldenfeld, Nigel (2017). "Noise-induced symmetry breaking far from equilibrium and the emergence of biological homochirality" (PDF). Physical Review E. 95 (3): 032407. Bibcode:2017PhRvE..95c2407J. doi:10.1103/PhysRevE.95.032407. PMID 28415353.
- Jafarpour, Farshid; Biancalani, Tommaso; Goldenfeld, Nigel (2015). "Noise-induced mechanism for biological homochirality of early life self-replicators". Physical Review Letters. 115 (15): 158101. arXiv:1507.00044. Bibcode:2015PhRvL.115o8101J. doi:10.1103/PhysRevLett.115.158101. PMID 26550754. S2CID 9775791.
- Frank, F.C. (1953). "On spontaneous asymmetric synthesis". Biochimica et Biophysica Acta. 11 (4): 459–463. doi:10.1016/0006-3002(53)90082-1. PMID 13105666.
- Clark, Stuart (July–August 1999). "Polarized Starlight and the Handedness of Life". American Scientist. 87 (4): 336. Bibcode:1999AmSci..87..336C. doi:10.1511/1999.4.336.
- Shibata, Takanori; Morioka, Hiroshi; Hayase, Tadakatsu; et al. (17 January 1996). "Highly Enantioselective Catalytic Asymmetric Automultiplication of Chiral Pyrimidyl Alcohol". Journal of the American Chemical Society. 118 (2): 471–472. doi:10.1021/ja953066g.
- Soai, Kenso; Sato, Itaru; Shibata, Takanori (2001). "Asymmetric autocatalysis and the origin of chiral homogeneity in organic compounds". The Chemical Record. 1 (4): 321–332. doi:10.1002/tcr.1017. PMID 11893072.
- Hazen 2005, p. 184
- Meierhenrich, Uwe (2008). Amino acids and the asymmetry of life caught in the act of formation. Berlin: Springer. pp. 76–79. ISBN 978-3540768869.
- Mullen, Leslie (5 September 2005). "Building Life from Star-Stuff". Astrobiology Magazine. Archived from the original on 14 July 2015. Retrieved 15 June 2015.
- Rabie, Passant (6 July 2020). "Astronomers Have Found The Source Of Life In The Universe". Inverse. Retrieved 7 July 2020.
- Marigo, Paola; et al. (6 July 2020). "Carbon star formation as seen through the non-monotonic initial–final mass relation". Nature Astronomy. 152 (11): 1102–1110. arXiv:2007.04163. Bibcode:2020NatAs...4.1102M. doi:10.1038/s41550-020-1132-1. S2CID 220403402. Retrieved 7 July 2020.
- "WMAP- Life in the Universe".
- Formation of Solar Systems: Solar Nebular Theory. University of Massachusetts Amherst, Department of Astronomy. Accessed on 27 September 2019.
- Kasting, James F. (12 February 1993). "Earth's Early Atmosphere" (PDF). Science. 259 (5097): 920–926. Bibcode:1993Sci...259..920K. doi:10.1126/science.11536547. PMID 11536547. S2CID 21134564. Archived from the original (PDF) on 10 October 2015. Retrieved 28 July 2015.
- Fesenkov 1959, p. 9
- Morse, John (September 1998). "Hadean Ocean Carbonate Geochemistry". Aquatic Geochemistry. 4 (3/4): 301–319. Bibcode:1998MinM...62.1027M. doi:10.1023/A:1009632230875. S2CID 129616933.
- Morse, John W.; MacKenzie, Fred T. (1998). "Hadean Ocean Carbonate Geochemistry". Aquatic Geochemistry. 4 (3–4): 301–319. Bibcode:1998MinM...62.1027M. doi:10.1023/A:1009632230875. S2CID 129616933.
- Wilde, Simon A.; Valley, John W.; Peck, William H.; Graham, Colin M. (11 January 2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago" (PDF). Nature. 409 (6817): 175–178. Bibcode:2001Natur.409..175W. doi:10.1038/35051550. PMID 11196637. S2CID 4319774. Archived (PDF) from the original on 5 June 2015. Retrieved 3 June 2015.
- Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; et al. (22 March 2006). "The rise of continents – An essay on the geologic consequences of photosynthesis". Palaeogeography, Palaeoclimatology, Palaeoecology. 232 (2–4): 99–113. Bibcode:2006PPP...232...99R. doi:10.1016/j.palaeo.2006.01.007. Archived (PDF) from the original on 14 July 2015. Retrieved 8 June 2015.
- Sleep, Norman H.; Zahnle, Kevin J.; Kasting, James F.; et al. (9 November 1989). "Annihilation of ecosystems by large asteroid impacts on early Earth". Nature. 342 (6246): 139–142. Bibcode:1989Natur.342..139S. doi:10.1038/342139a0. PMID 11536616. S2CID 1137852.
- Gomes, Rodney; Levison, Hal F.; Tsiganis, Kleomenis; Morbidelli, Alessandro (26 May 2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets". Nature. 435 (7041): 466–469. Bibcode:2005Natur.435..466G. doi:10.1038/nature03676. PMID 15917802.
- Davies 2007, pp. 61–73
- Maher, Kevin A.; Stevenson, David J. (18 February 1988). "Impact frustration of the origin of life". Nature. 331 (6157): 612–614. Bibcode:1988Natur.331..612M. doi:10.1038/331612a0. PMID 11536595. S2CID 4284492.
- Chyba, Christopher; Sagan, Carl (9 January 1992). "Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life". Nature. 355 (6356): 125–132. Bibcode:1992Natur.355..125C. doi:10.1038/355125a0. PMID 11538392. S2CID 4346044.
- Furukawa, Yoshihiro; Sekine, Toshimori; Oba, Masahiro; et al. (January 2009). "Biomolecule formation by oceanic impacts on early Earth". Nature Geoscience. 2 (1): 62–66. Bibcode:2009NatGe...2...62F. doi:10.1038/NGEO383.
- Davies 1999, p. 155
- Bock & Goode 1996
- Mortillaro, Nicole (1 March 2017). "Oldest traces of life on Earth found in Quebec, dating back roughly 3.8 Gya". CBC News. Archived from the original on 1 March 2017. Retrieved 2 March 2017.
- Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; et al. (January 2014). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. 7 (1): 25–28. Bibcode:2014NatGe...7...25O. doi:10.1038/ngeo2025.
- Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Archived from the original on 29 June 2015. Retrieved 2 June 2015.
- Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (16 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Gyo Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12): 1103–1124. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812.
- Wade, Nicholas (31 August 2016). "World's Oldest Fossils Found in Greenland". The New York Times. Archived from the original on 31 August 2016. Retrieved 31 August 2016.
- Davies 1999
- Hassenkam, T.; Andersson, M.P.; Dalby, K.N.; Mackenzie, D.M.A.; Rosing, M.T. (2017). "Elements of Eoarchean life trapped in mineral inclusions". Nature. 548 (7665): 78–81. Bibcode:2017Natur.548...78H. doi:10.1038/nature23261. PMID 28738409. S2CID 205257931.
- Pearlman, Jonathan (13 November 2013). "Oldest signs of life on Earth found". The Daily Telegraph. London. Archived from the original on 16 December 2014. Retrieved 15 December 2014.
- O'Donoghue, James (21 August 2011). "Oldest reliable fossils show early life was a beach". New Scientist. 211: 13. doi:10.1016/S0262-4079(11)62064-2. Archived from the original on 30 June 2015.
- Wacey, David; Kilburn, Matt R.; Saunders, Martin; et al. (October 2011). "Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia". Nature Geoscience. 4 (10): 698–702. Bibcode:2011NatGe...4..698W. doi:10.1038/ngeo1238.
- Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". AP News. Associated Press. Retrieved 9 October 2018.
- Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark; et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon". Proc. Natl. Acad. Sci. U.S.A. 112 (47): 14518–14521. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. PMC 4664351. PMID 26483481. Early edition, published online before print.
- Wolpert, Stuart (19 October 2015). "Life on Earth likely started at least 4.1 billion years ago – much earlier than scientists had thought". ULCA. Archived from the original on 20 October 2015. Retrieved 20 October 2015.
- Baumgartner, Rafael; Van Kranendonk, Martin; Wacey, David; Fiorentini, Marco; Saunders, Martin; Caruso, Caruso; Pages, Anais; Homann, Martin; Guagliardo, Paul (2019). "Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life" (PDF). Geology. 47 (11): 1039–1043. Bibcode:2019Geo....47.1039B. doi:10.1130/G46365.1.
- Djokic, Tara; Van Kranendonk, Martin; Cambell, Kathleen; Walter, Malcolm (2017). "Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits". Nature Communications. 3.
- Wickramasinghe, Chandra (2011). "Bacterial morphologies supporting cometary panspermia: a reappraisal". International Journal of Astrobiology. 10 (1): 25–30. Bibcode:2011IJAsB..10...25W. CiteSeerX 10.1.1.368.4449. doi:10.1017/S1473550410000157.
- Rampelotto, P. H. (2010). "Panspermia: A promising field of research". In: Astrobiology Science Conference. Abs 5224.
- Chang, Kenneth (12 September 2016). "Visions of Life on Mars in Earth's Depths". The New York Times. Archived from the original on 12 September 2016. Retrieved 12 September 2016.
- Clark, Stuart (25 September 2002). "Tough Earth bug may be from Mars". New Scientist. Archived from the original on 2 December 2014. Retrieved 21 June 2015.
- Horneck, Gerda; Klaus, David M.; Mancinelli, Rocco L. (March 2010). "Space Microbiology". Microbiology and Molecular Biology Reviews. 74 (1): 121–156. Bibcode:2010MMBR...74..121H. doi:10.1128/MMBR.00016-09. PMC 2832349. PMID 20197502.
- Rabbow, Elke; Horneck, Gerda; Rettberg, Petra; et al. (December 2009). "EXPOSE, an Astrobiological Exposure Facility on the International Space Station – from Proposal to Flight". Origins of Life and Evolution of Biospheres. 39 (6): 581–598. Bibcode:2009OLEB...39..581R. doi:10.1007/s11084-009-9173-6. PMID 19629743. S2CID 19749414.
- Onofri, Silvano; de la Torre, Rosa; de Vera, Jean-Pierre; et al. (May 2012). "Survival of Rock-Colonizing Organisms After 1.5 Years in Outer Space". Astrobiology. 12 (5): 508–516. Bibcode:2012AsBio..12..508O. doi:10.1089/ast.2011.0736. PMID 22680696.
- Strickland, Ashley (26 August 2020). "Bacteria from Earth can survive in space and could endure the trip to Mars, according to new study". CNN News. Retrieved 26 August 2020.
- Kawaguchi, Yuko; et al. (26 August 2020). "DNA Damage and Survival Time Course of Deinococcal Cell Pellets During 3 Years of Exposure to Outer Space". Frontiers in Microbiology. 11: 2050. doi:10.3389/fmicb.2020.02050. PMC 7479814. PMID 32983036. S2CID 221300151.
- Loeb, Abraham (2014). "The habitable epoch of the early universe". International Journal of Astrobiology. 13 (4): 337–339. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. CiteSeerX 10.1.1.748.4820. doi:10.1017/S1473550414000196. S2CID 2777386.
- Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back". The New York Times. New York. p. D2. Archived from the original on 3 December 2014. Retrieved 3 December 2014.
- Zimmer, Carl (12 September 2013). "A Far-Flung Possibility for the Origin of Life". The New York Times. New York. Archived from the original on 8 July 2015. Retrieved 15 June 2015.
- Webb, Richard (29 August 2013). "Primordial broth of life was a dry Martian cup-a-soup". New Scientist. Archived from the original on 24 April 2015. Retrieved 16 June 2015.
- Wentao Ma; Chunwu Yu; Wentao Zhang; et al. (November 2007). "Nucleotide synthetase ribozymes may have emerged first in the RNA world". RNA. 13 (11): 2012–2019. doi:10.1261/rna.658507. PMC 2040096. PMID 17878321.
- Sheldon 2005
- Lennox 2001, pp. 229–258
- Vartanian 1973, pp. 307–312
- Bernal 1967
- Balme, D.M. (1962). "Development of Biology in Aristotle and Theophrastus: Theory of Spontaneous Generation". Phronesis. 7 (1–2): 91–104. doi:10.1163/156852862X00052.
- Ross 1652
- Dobell 1960
- Bondeson 1999
- Levine R, Evers C. "The Slow Death of Spontaneous Generation (1668-1859)". Archived from the original on 26 April 2008. Retrieved 18 April 2013.
- Oparin 1953, p. 196
- Tyndall 1905, IV, XII (1876), XIII (1878)
- "Biogenesis". Hmolpedia. Ancaster, Ontario, Canada: WikiFoundry, Inc. Archived from the original on 20 May 2014. Retrieved 19 May 2014.
- Huxley 1968
- Bastian 1871
- Bastian 1871, p. xi–xii
- Abiogenesis – Definition. 20 April 2015. Encyclopedia of Astrobiology. doi:10.1007/978-3-642-27833-4_2-4
- Bahadur, Krishna (1973). "Photochemical Formation of Self–sustaining Coacervates" (PDF). Proceedings of the Indian National Science Academy. 39B (4): 455–467. doi:10.1016/S0044-4057(75)80076-1. PMID 1242552. Archived from the original (PDF) on 19 October 2013.
- Kasting 1993, p. 922
- Kasting 1993, p. 920
- Bernal 1967, The Origin of Life (A.I. Oparin, 1924), pp. 199–234
- Oparin 1953
- Bryson 2004, pp. 300–302
- Shapiro 1987, p. 110
- Bernal 1951
- Martin, William F. (January 2003). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Phil. Trans. R. Soc. Lond. A. 358 (1429): 59–83. doi:10.1098/rstb.2002.1183. PMC 1693102. PMID 12594918.
- Bernal, John Desmond (September 1949). "The Physical Basis of Life". Proceedings of the Physical Society, Section A. 62 (9): 537–558. Bibcode:1949PPSA...62..537B. doi:10.1088/0370-1298/62/9/301.
- Kauffman 1995
- Miller, Stanley L. (15 May 1953). "A Production of Amino Acids Under Possible Primitive Earth Conditions". Science. 117 (3046): 528–529. Bibcode:1953Sci...117..528M. doi:10.1126/science.117.3046.528. PMID 13056598.
- Parker, Eric T.; Cleaves, Henderson J.; Dworkin, Jason P.; et al. (5 April 2011). "Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment". Proc. Natl. Acad. Sci. U.S.A. 108 (14): 5526–5531. Bibcode:2011PNAS..108.5526P. doi:10.1073/pnas.1019191108. PMC 3078417. PMID 21422282.
- Zurich, Eth (29 November 2020). "Uncovering Mysteries of Earth's Primeval Atmosphere 4.5 Billion Years Ago and the Emergence of Life". Retrieved 30 November 2020.
- Landau, Elizabeth (12 October 2016). "Building Blocks of Life's Building Blocks Come From Starlight". NASA. Archived from the original on 13 October 2016. Retrieved 13 October 2016.
- Gawlowicz, Susan (6 November 2011). "Carbon-based organic 'carriers' in interstellar dust clouds? Newly discovered diffuse interstellar bands". Science Daily. Rockville, MD: ScienceDaily, LLC. Archived from the original on 11 July 2015. Retrieved 8 June 2015. Post is reprinted from materials provided by the Rochester Institute of Technology.
- Klyce 2001
- "biological abundance of elements". Encyclopedia of Science. Dundee, Scotland: David Darling Enterprises. Archived from the original on 4 February 2012. Retrieved 9 October 2008.
- Hoover, Rachel (21 February 2014). "Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That". Ames Research Center. Mountain View, CA: NASA. Archived from the original on 6 September 2015. Retrieved 22 June 2015.
- Chang, Kenneth (18 August 2009). "From a Distant Comet, a Clue to Life". The New York Times. New York. p. A18. Archived from the original on 23 June 2015. Retrieved 22 June 2015.
- Goncharuk, Vladislav V.; Zui, O. V. (February 2015). "Water and carbon dioxide as the main precursors of organic matter on Earth and in space". Journal of Water Chemistry and Technology. 37 (1): 2–3. doi:10.3103/S1063455X15010026. S2CID 97965067.
- Abou Mrad, Ninette; Vinogradoff, Vassilissa; Duvernay, Fabrice; et al. (2015). "Laboratory experimental simulations: Chemical evolution of the organic matter from interstellar and cometary ice analogs". Bulletin de la Société Royale des Sciences de Liège. 84: 21–32. Bibcode:2015BSRSL..84...21A. Archived from the original on 13 April 2015. Retrieved 6 April 2015.
- Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Salt Lake City, UT: Purch. Archived from the original on 14 August 2012. Retrieved 30 March 2012.
- "'Life chemical' detected in comet". BBC News. London. 18 August 2009. Archived from the original on 25 May 2015. Retrieved 23 June 2015.
- Thompson, William Reid; Murray, B. G.; Khare, Bishun Narain; Sagan, Carl (30 December 1987). "Coloration and darkening of methane clathrate and other ices by charged particle irradiation: Applications to the outer solar system". Journal of Geophysical Research. 92 (A13): 14933–14947. Bibcode:1987JGR....9214933T. doi:10.1029/JA092iA13p14933. PMID 11542127.
- Stark, Anne M. (5 June 2013). "Life on Earth shockingly comes from out of this world". Livermore, CA: Lawrence Livermore National Laboratory. Archived from the original on 16 September 2015. Retrieved 23 June 2015.
- Goldman, Nir; Tamblyn, Isaac (20 June 2013). "Prebiotic Chemistry within a Simple Impacting Icy Mixture". Journal of Physical Chemistry A. 117 (24): 5124–5131. Bibcode:2013JPCA..117.5124G. doi:10.1021/jp402976n. PMID 23639050.
- Carey, Bjorn (18 October 2005). "Life's Building Blocks 'Abundant in Space'". Space.com. Watsonville, CA: Imaginova. Archived from the original on 26 June 2015. Retrieved 23 June 2015.
- Hudgins, Douglas M.; Bauschlicher, Charles W. Jr.; Allamandola, Louis J. (10 October 2005). "Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population". The Astrophysical Journal. 632 (1): 316–332. Bibcode:2005ApJ...632..316H. CiteSeerX 10.1.1.218.8786. doi:10.1086/432495.
- Des Marais, David J.; Allamandola, Louis J.; Sandford, Scott; et al. (2009). "Cosmic Distribution of Chemical Complexity". Ames Research Center. Mountain View, CA: NASA. Archived from the original on 27 February 2014. Retrieved 24 June 2015. See the Ames Research Center 2009 annual team report to the NASA Astrobiology Institute here "Archived copy". Archived from the original on 1 March 2013. Retrieved 24 June 2015.CS1 maint: archived copy as title (link).
- García-Hernández, Domingo. A.; Manchado, Arturo; García-Lario, Pedro; et al. (20 November 2010). "Formation of Fullerenes in H-Containing Planetary Nebulae". The Astrophysical Journal Letters. 724 (1): L39–L43. arXiv:1009.4357. Bibcode:2010ApJ...724L..39G. doi:10.1088/2041-8205/724/1/L39. S2CID 119121764.
- Witt, Adolf N.; Vijh, Uma P.; Gordon, Karl D. (January 2004). Discovery of Blue Fluorescence by Polycyclic Aromatic Hydrocarbon Molecules in the Red Rectangle. American Astronomical Society Meeting 203. Atlanta, GA: American Astronomical Society. Bibcode:2003AAS...20311017W. Archived from the original on 19 December 2003. Retrieved 16 January 2019.
- d'Ischia, Marco; Manini, Paola; Moracci, Marco; Saladino, Raffaele; Ball, Vincent; Thissen, Helmut; Evans, Richard A.; Puzzarini, Cristina; Barone, Vincenzo (21 August 2019). "Astrochemistry and Astrobiology: Materials Science in Wonderland?". International Journal of Molecular Sciences. 20 (17): 4079. doi:10.3390/ijms20174079. ISSN 1422-0067. PMC 6747172. PMID 31438518.
- "NASA Cooks Up Icy Organics to Mimic Life's Origins". Space.com. Ogden, UT: Purch. 20 September 2012. Archived from the original on 25 June 2015. Retrieved 26 June 2015.
- Gudipati, Murthy S.; Rui Yang (1 September 2012). "In-situ Probing of Radiation-induced Processing of Organics in Astrophysical Ice Analogs – Novel Laser Desorption Laser Ionization Time-of-flight Mass Spectroscopic Studies". The Astrophysical Journal Letters. 756 (1): L24. Bibcode:2012ApJ...756L..24G. doi:10.1088/2041-8205/756/1/L24.
- "NASA Ames PAH IR Spectroscopic Database". NASA. Archived from the original on 29 June 2015. Retrieved 17 June 2015.
- Gallori, Enzo (June 2011). "Astrochemistry and the origin of genetic material". Rendiconti Lincei. 22 (2): 113–118. doi:10.1007/s12210-011-0118-4. S2CID 96659714. "Paper presented at the Symposium 'Astrochemistry: molecules in space and time' (Rome, 4–5 November 2010), sponsored by Fondazione 'Guido Donegani', Accademia Nazionale dei Lincei."
- Martins, Zita (February 2011). "Organic Chemistry of Carbonaceous Meteorites". Elements. 7 (1): 35–40. doi:10.2113/gselements.7.1.35.
- Martins, Zita; Botta, Oliver; Fogel, Marilyn L.; et al. (15 June 2008). "Extraterrestrial nucleobases in the Murchison meteorite". Earth and Planetary Science Letters. 270 (1–2): 130–136. arXiv:0806.2286. Bibcode:2008E&PSL.270..130M. doi:10.1016/j.epsl.2008.03.026. S2CID 14309508.
- "We may all be space aliens: study". Sydney: Australian Broadcasting Corporation. Agence France-Presse. 14 June 2008. Archived from the original on 23 June 2015. Retrieved 22 June 2015.
- Callahan, Michael P.; Smith, Karen E.; Cleaves, H. James, II; et al. (23 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". Proc. Natl. Acad. Sci. U.S.A. 108 (34): 13995–13998. Bibcode:2011PNAS..10813995C. doi:10.1073/pnas.1106493108. PMC 3161613. PMID 21836052.
- Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". Goddard Space Flight Center. Greenbelt, MD: NASA. Archived from the original on 23 June 2015. Retrieved 23 June 2015.
- Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Ogden, UT: Purch. Archived from the original on 14 July 2015. Retrieved 23 June 2015.
- The University of Hong Kong (27 October 2011). "Astronomers discover complex organic matter exists throughout the universe". Rockville, MD: ScienceDaily, LLC. Archived from the original on 3 July 2015.
- Sun Kwok; Yong Zhang (3 November 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature. 479 (7371): 80–83. Bibcode:2011Natur.479...80K. doi:10.1038/nature10542. PMID 22031328. S2CID 4419859.
- Clemence, Lara; Cohen, Jarrett (7 February 2005). "Space Sugar's a Sweet Find". Goddard Space Flight Center. Greenbelt, MD: NASA. Archived from the original on 5 March 2016. Retrieved 23 June 2015.
- Than, Ker (30 August 2012). "Sugar Found in Space: A Sign of Life?". National Geographic News. Washington, D.C.: National Geographic Society. Archived from the original on 14 July 2015. Retrieved 23 June 2015.
- "Sweet! Astronomers spot sugar molecule near star". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. 29 August 2012. Archived from the original on 14 July 2015. Retrieved 23 June 2015.
- "Building blocks of life found around young star". News & Events. Leiden, the Netherlands: Leiden University. 30 September 2012. Archived from the original on 13 December 2013. Retrieved 11 December 2013.
- Jørgensen, Jes K.; Favre, Cécile; Bisschop, Suzanne E.; et al. (2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA" (PDF). The Astrophysical Journal Letters. 757 (1): L4. arXiv:1208.5498. Bibcode:2012ApJ...757L...4J. doi:10.1088/2041-8205/757/1/L4. S2CID 14205612. Archived (PDF) from the original on 24 September 2015. Retrieved 23 June 2015.
- Furukawa, Yoshihiro; Chikaraishi, Yoshito; Ohkouchi, Naohiko; Ogawa, Nanako O.; Glavin, Daniel P.; Dworkin, Jason P.; Abe, Chiaki; Nakamura, Tomoki (13 November 2019). "Extraterrestrial ribose and other sugars in primitive meteorites". Proceedings of the National Academy of Sciences. 116 (49): 24440–24445. Bibcode:2019PNAS..11624440F. doi:10.1073/pnas.1907169116. ISSN 0027-8424. PMC 6900709. PMID 31740594.
- Brown, Michael R. W.; Kornberg, Arthur (16 November 2004). "Inorganic polyphosphate in the origin and survival of species". Proc. Natl. Acad. Sci. U.S.A. 101 (46): 16085–16087. Bibcode:2004PNAS..10116085B. doi:10.1073/pnas.0406909101. PMC 528972. PMID 15520374.
- Clark, David P. (3 August 1999). "The Origin of Life". Microbiology 425: Biochemistry and Physiology of Microorganism (Lecture). Carbondale, IL: College of Science; Southern Illinois University Carbondale. Archived from the original on 2 October 2000. Retrieved 26 June 2015.
- Pasek, Matthew A. (22 January 2008). "Rethinking early Earth phosphorus geochemistry". Proc. Natl. Acad. Sci. U.S.A. 105 (3): 853–858. Bibcode:2008PNAS..105..853P. doi:10.1073/pnas.0708205105. PMC 2242691. PMID 18195373.
- Ciesla, F.J.; Sandford, S.A. (29 March 2012). "Organic Synthesis via Irradiation and Warming of Ice Grains in the Solar Nebula". Science. 336 (6080): 452–454. Bibcode:2012Sci...336..452C. doi:10.1126/science.1217291. hdl:2060/20120011864. PMID 22461502. S2CID 25454671.
- Steigerwald, Bill; Jones, Nancy; Furukawa, Yoshihiro (18 November 2019). "First Detection of Sugars in Meteorites Gives Clues to Origin of Life". NASA. Retrieved 18 November 2019.
- Walsh, J. Bruce (1995). "Part 4: Experimental studies of the origins of life". Origins of life (Lecture notes). Tucson, AZ: University of Arizona. Archived from the original on 13 January 2008. Retrieved 8 June 2015.
- Saladino, Raffaele; Crestini, Claudia; Pino, Samanta; et al. (March 2012). "Formamide and the origin of life" (PDF). Physics of Life Reviews. 9 (1): 84–104. Bibcode:2012PhLRv...9...84S. doi:10.1016/j.plrev.2011.12.002. hdl:2108/85168. PMID 22196896.
- Saladino, Raffaele; Botta, Giorgia; Pino, Samanta; et al. (July 2012). "From the one-carbon amide formamide to RNA all the steps are prebiotically possible". Biochimie. 94 (7): 1451–1456. doi:10.1016/j.biochi.2012.02.018. PMID 22738728.
- Marlaire, Ruth, ed. (3 March 2015). "NASA Ames Reproduces the Building Blocks of Life in Laboratory". Ames Research Center. Moffett Field, CA: NASA. Archived from the original on 5 March 2015. Retrieved 5 March 2015.
- Ferus, Martin; Nesvorný, David; Šponer, Jiří; Kubelík, Petr; Michalčíková, Regina; Shestivská, Violetta; Šponer, Judit E.; Civiš, Svatopluk (2015). "High-energy chemistry of formamide: A unified mechanism of nucleobase formation". Proc. Natl. Acad. Sci. U.S.A. 112 (3): 657–662. Bibcode:2015PNAS..112..657F. doi:10.1073/pnas.1412072111. PMC 4311869. PMID 25489115.
- Oró, Joan (16 September 1961). "Mechanism of Synthesis of Adenine from Hydrogen Cyanide under Possible Primitive Earth Conditions". Nature. 191 (4794): 1193–1194. Bibcode:1961Natur.191.1193O. doi:10.1038/1911193a0. PMID 13731264. S2CID 4276712.
- Basile, Brenda; Lazcano, Antonio; Oró, Joan (1984). "Prebiotic syntheses of purines and pyrimidines". Advances in Space Research. 4 (12): 125–131. Bibcode:1984AdSpR...4..125B. doi:10.1016/0273-1177(84)90554-4. PMID 11537766.
- Orgel, Leslie E. (August 2004). "Prebiotic Adenine Revisited: Eutectics and Photochemistry". Origins of Life and Evolution of Biospheres. 34 (4): 361–369. Bibcode:2004OLEB...34..361O. doi:10.1023/B:ORIG.0000029882.52156.c2. PMID 15279171. S2CID 4998122.
- Robertson, Michael P.; Miller, Stanley L. (29 June 1995). "An efficient prebiotic synthesis of cytosine and uracil". Nature. 375 (6534): 772–774. Bibcode:1995Natur.375..772R. doi:10.1038/375772a0. PMID 7596408. S2CID 4351012.
- Fox, Douglas (February 2008). "Did Life Evolve in Ice?". Discover. Archived from the original on 30 June 2008. Retrieved 3 July 2008.
- Levy, Matthew; Miller, Stanley L.; Brinton, Karen; Bada, Jeffrey L. (June 2000). "Prebiotic Synthesis of Adenine and Amino Acids Under Europa-like Conditions". Icarus. 145 (2): 609–613. Bibcode:2000Icar..145..609L. doi:10.1006/icar.2000.6365. PMID 11543508.
- Menor-Salván, César; Ruiz-Bermejo, Marta; Guzmán, Marcelo I.; Osuna-Esteban, Susana; Veintemillas-Verdaguer, Sabino (20 April 2009). "Synthesis of Pyrimidines and Triazines in Ice: Implications for the Prebiotic Chemistry of Nucleobases". Chemistry: A European Journal. 15 (17): 4411–4418. doi:10.1002/chem.200802656. PMID 19288488.
- Roy, Debjani; Najafian, Katayoun; von Ragué Schleyer, Paul (30 October 2007). "Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions". Proc. Natl. Acad. Sci. U.S.A. 104 (44): 17272–17277. Bibcode:2007PNAS..10417272R. doi:10.1073/pnas.0708434104. PMC 2077245. PMID 17951429.
- Cleaves, H. James; Chalmers, John H.; Lazcano, Antonio; et al. (April 2008). "A Reassessment of Prebiotic Organic Synthesis in Neutral Planetary Atmospheres". Origins of Life and Evolution of Biospheres. 38 (2): 105–115. Bibcode:2008OLEB...38..105C. doi:10.1007/s11084-007-9120-3. PMID 18204914. S2CID 7731172.
- Chyba, Christopher F. (13 May 2005). "Rethinking Earth's Early Atmosphere". Science. 308 (5724): 962–963. doi:10.1126/science.1113157. PMID 15890865. S2CID 93303848.
- Barton et al. 2007, pp. 93–95
- Bada & Lazcano 2009, pp. 56–57
- Bada, Jeffrey L.; Lazcano, Antonio (2 May 2003). "Prebiotic Soup – Revisiting the Miller Experiment" (PDF). Science. 300 (5620): 745–746. doi:10.1126/science.1085145. PMID 12730584. S2CID 93020326. Archived (PDF) from the original on 4 March 2016. Retrieved 13 June 2015.
- Service, Robert F. (16 March 2015). "Researchers may have solved origin-of-life conundrum". Science (News). Washington, D.C.: American Association for the Advancement of Science. Archived from the original on 12 August 2015. Retrieved 26 July 2015.
- Patel, Bhavesh H.; Percivalle, Claudia; Ritson, Dougal J.; Duffy, Colm D.; Sutherland, John D. (April 2015). "Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism". Nature Chemistry. 7 (4): 301–307. Bibcode:2015NatCh...7..301P. doi:10.1038/nchem.2202. PMC 4568310. PMID 25803468.
- Patel et al. 2015, p. 302
- Oró, Joan; Kimball, Aubrey P. (February 1962). "Synthesis of purines under possible primitive earth conditions: II. Purine intermediates from hydrogen cyanide". Archives of Biochemistry and Biophysics. 96 (2): 293–313. doi:10.1016/0003-9861(62)90412-5. PMID 14482339.
- Ahuja, Mukesh, ed. (2006). "Origin of Life". Life Science. 1. Delhi: Isha Books. p. 11. ISBN 978-81-8205-386-1. OCLC 297208106.[unreliable source?]
- Cleaves II, Henderson (2010). "The origin of the biologically coded amino acids". Journal of Theoretical Biology. 263 (4): 490–498. doi:10.1016/j.jtbi.2009.12.014. PMID 20034500.
- Frenkel-Pinter, Moran; Mousumi, Samanta; Ashkenasy, Gonen; Leman, Luke (2020). "Prebiotic Peptides: Molecular Hubs in the Origin of Life". Chemical Reviuews. 263 (4): 490–498.
- Paul, Natasha; Joyce, Gerald F. (December 2004). "Minimal self-replicating systems". Current Opinion in Chemical Biology. 8 (6): 634–639. doi:10.1016/j.cbpa.2004.09.005. PMID 15556408.
- Bissette, Andrew J.; Fletcher, Stephen P. (2 December 2013). "Mechanisms of Autocatalysis". Angewandte Chemie International Edition. 52 (49): 12800–12826. doi:10.1002/anie.201303822. PMID 24127341.
- Kauffman 1993, chpt. 7
- Dawkins 2004
- Tjivikua, T.; Ballester, Pablo; Rebek, Julius Jr. (January 1990). "Self-replicating system". Journal of the American Chemical Society. 112 (3): 1249–1250. doi:10.1021/ja00159a057.
- Browne, Malcolm W. (30 October 1990). "Chemists Make Molecule With Hint of Life". The New York Times. New York. Archived from the original on 21 July 2015. Retrieved 14 July 2015.
- Jia, Tony Z.; Chandru, Kuhan; Hongo, Yayoi; Afrin, Rehana; Usui, Tomohiro; Myojo, Kunihiro; Cleaves, H. James (22 July 2019). "Membraneless polyester microdroplets as primordial compartments at the origins of life". Proceedings of the National Academy of Sciences. 116 (32): 15830–15835. doi:10.1073/pnas.1902336116. PMC 6690027. PMID 31332006.
- Chandru, Kuhan; Mamajanov, Irena; Cleaves, H. James; Jia, Tony Z. (January 2020). "Polyesters as a Model System for Building Primitive Biologies from Non-Biological Prebiotic Chemistry". Life. 10 (1): 6. doi:10.3390/life10010006. PMC 7175156. PMID 31963928.
- Marc, Kaufman (18 July 2019). "NASA Astrobiology". astrobiology.nasa.gov.
- Guttenberg, Nicholas; Virgo, Nathaniel; Chandru, Kuhan; Scharf, Caleb; Mamajanov, Irena (13 November 2017). "Bulk measurements of messy chemistries are needed for a theory of the origins of life". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 375 (2109): 20160347. Bibcode:2017RSPTA.37560347G. doi:10.1098/rsta.2016.0347. PMC 5686404. PMID 29133446.
- Woodward 1969, p. 287
- Lancet, Doron (30 December 2014). "Systems Prebiology-Studies of the origin of Life". The Lancet Lab. Rehovot, Israel: Department of Molecular Genetics; Weizmann Institute of Science. Archived from the original on 26 June 2015. Retrieved 26 June 2015.
- Segré, Daniel; Ben-Eli, Dafna; Deamer, David W.; Lancet, Doron (February 2001). "The Lipid World" (PDF). Origins of Life and Evolution of the Biosphere. 31 (1–2): 119–145. Bibcode:2001OLEB...31..119S. doi:10.1023/A:1006746807104. PMID 11296516. S2CID 10959497. Archived (PDF) from the original on 26 June 2015. Retrieved 11 September 2008.
- Chen, Irene A.; Walde, Peter (July 2010). "From Self-Assembled Vesicles to Protocells". Cold Spring Harbor Perspectives in Biology. 2 (7): a002170. doi:10.1101/cshperspect.a002170. PMC 2890201. PMID 20519344.
- Eigen, Manfred; Schuster, Peter (November 1977). "The Hypercycle. A Principle of Natural Self-Organization. Part A: Emergence of the Hypercycle" (PDF). Naturwissenschaften. 64 (11): 541–65. Bibcode:1977NW.....64..541E. doi:10.1007/bf00450633. PMID 593400. Archived from the original (PDF) on 3 March 2016. Retrieved 13 June 2015.
- Eigen, Manfred; Schuster, Peter (1978). "The Hypercycle. A Principle of Natural Self-Organization. Part B: The Abstract Hypercycle" (PDF). Naturwissenschaften. 65 (1): 7–41. Bibcode:1978NW.....65....7E. doi:10.1007/bf00420631. S2CID 1812273. Archived from the original (PDF) on 3 March 2016. Retrieved 13 June 2015.
- Eigen, Manfred; Schuster, Peter (July 1978). "The Hypercycle. A Principle of Natural Self-Organization. Part C: The Realistic Hypercycle" (PDF). Naturwissenschaften. 65 (7): 341–369. Bibcode:1978NW.....65..341E. doi:10.1007/bf00439699. S2CID 13825356. Archived from the original (PDF) on 16 June 2016. Retrieved 13 June 2015.
- Markovitch, Omer; Lancet, Doron (Summer 2012). "Excess Mutual Catalysis Is Required for Effective Evolvability". Artificial Life. 18 (3): 243–266. doi:10.1162/artl_a_00064. PMID 22662913. S2CID 5236043.
- Tessera, Marc (2011). "Origin of Evolution versus Origin of Life: A Shift of Paradigm". International Journal of Molecular Sciences. 12 (6): 3445–3458. doi:10.3390/ijms12063445. PMC 3131571. PMID 21747687. Special Issue: "Origin of Life 2011"
- "Exploring Life's Origins: Protocells". Exploring Life's Origins: A Virtual Exhibit. Arlington County, VA: National Science Foundation. Archived from the original on 28 February 2014. Retrieved 18 March 2014.
- Chen, Irene A. (8 December 2006). "The Emergence of Cells During the Origin of Life". Science. 314 (5805): 1558–1559. doi:10.1126/science.1137541. PMID 17158315.
- Zimmer, Carl (26 June 2004). "What Came Before DNA?". Discover. Archived from the original on 19 March 2014.
- Shapiro, Robert (June 2007). "A Simpler Origin for Life". Scientific American. 296 (6): 46–53. Bibcode:2007SciAm.296f..46S. doi:10.1038/scientificamerican0607-46. PMID 17663224. Archived from the original on 14 June 2015. Retrieved 15 June 2015.
- Chang 2007
- Marshall, Michael (14 December 2020). "He may have found the key to the origins of life. So why have so few heard of him? - Hungarian biologist Tibor Gánti is an obscure figure. Now, more than a decade after his death, his ideas about how life began are finally coming to fruition". National Geographic Society. Retrieved 15 December 2020.
- Hugues Bersini (2011). "Minimal cell: the computer scientist's point of view". In Muriel Gargaud; Purificación López-Garcìa; Hervé Martin (eds.). Origins and Evolution of Life: An Astrobiological Perspective. Cambridge University Press. pp. 60–61. ISBN 9781139494595.
- Van Segbroeck S, Nowé A, Lenaerts T (2009). "Stochastic simulation of the chemoton". Artif Life. 15 (2): 213–226. CiteSeerX 10.1.1.398.8949. doi:10.1162/artl.2009.15.2.15203. PMID 19199383. S2CID 10634307.
- Hoenigsberg HF (2007). "From geochemistry and biochemistry to prebiotic evolution...we necessarily enter into Gánti's fluid automata". Genet Mol Res. 6 (2): 358–373. PMID 17624859.
- Switek, Brian (13 February 2012). "Debate bubbles over the origin of life". Nature. London: Nature Publishing Group. doi:10.1038/nature.2012.10024.
- z3530495 (5 May 2020). "'When chemistry became biology': looking for the origins of life in hot springs". UNSW Newsroom. Retrieved 12 October 2020.
- Damer, Bruce; Deamer, David (13 March 2015). "Coupled Phases and Combinatorial Selection in Fluctuating Hydrothermal Pools: A Scenario to Guide Experimental Approaches to the Origin of Cellular Life". Life. 5 (1): 872–887. doi:10.3390/life5010872. PMC 4390883. PMID 25780958.
- Grote, Mathias (September 2011). "Jeewanu, or the 'particles of life'" (PDF). Journal of Biosciences. 36 (4): 563–570. doi:10.1007/s12038-011-9087-0. PMID 21857103. S2CID 19551399. Archived (PDF) from the original on 24 September 2015. Retrieved 15 June 2015.
- Gupta, V.K.; Rai, R.K. (August 2013). "Histochemical localisation of RNA-like material in photochemically formed self-sustaining, abiogenic supramolecular assemblies 'Jeewanu'". International Research Journal of Science & Engineering. 1 (1): 1–4. Archived from the original on 28 June 2017. Retrieved 15 June 2015.
- Welter, Kira (10 August 2015). "Peptide glue may have held first protocell components together". Chemistry World (News). London: Royal Society of Chemistry. Archived from the original on 5 September 2015. Retrieved 29 August 2015.
- Kamat, Neha P.; Tobé, Sylvia; Hill, Ian T.; Szostak, Jack W. (29 July 2015). "Electrostatic Localization of RNA to Protocell Membranes by Cationic Hydrophobic Peptides". Angewandte Chemie International Edition. 54 (40): 11735–11739. doi:10.1002/anie.201505742. PMC 4600236. PMID 26223820.
- Martin, William; Russell, Michael J. (29 January 2003). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Philosophical Transactions of the Royal Society B. 358 (1429): 59–83, discussion 83–85. doi:10.1098/rstb.2002.1183. PMC 1693102. PMID 12594918.
- "Letter no. 7471, Charles Darwin to Joseph Dalton Hooker, 1 February (1871)". Darwin Correspondence Project. Retrieved 7 July 2020.
- Priscu, John C. "Origin and Evolution of Life on a Frozen Earth". Arlington County, VA: National Science Foundation. Archived from the original on 18 December 2013. Retrieved 1 March 2014.
- Marshall, Michael (11 November 2020). "Charles Darwin's hunch about early life was probably right - In a few scrawled notes to a friend, biologist Charles Darwin theorised how life began. Not only was it probably correct, his theory was a century ahead of its time". BBC News. Retrieved 11 November 2020.
- Johnston, Ian (2 October 2017). "Life first emerged in 'warm little ponds' almost as old as the Earth itself – Darwin's famous idea backed by new scientific study". The Independent. Archived from the original on 3 October 2017. Retrieved 2 October 2017.
- M.D> Brasier (2012), "Secret Chambers: The Inside Story of Cells and Complex Life" (Oxford Uni Press), p.298
- Ward, Peter & Kirschvink, Joe, op cit, p. 42
- Colín-García, M.; A. Heredia; G. Cordero; A. Camprubí; A. Negrón-Mendoza; F. Ortega-Gutiérrez; H. Berald; S. Ramos-Bernal (2016). "Hydrothermal vents and prebiotic chemistry: a review". Boletín de la Sociedad Geológica Mexicana. 68 (3): 599–620. doi:10.18268/BSGM2016v68n3a13. Archived from the original on 18 August 2017.
- Schirber, Michael (24 June 2014). "Hydrothermal Vents Could Explain Chemical Precursors to Life". NASA Astrobiology: Life in the Universe. NASA. Archived from the original on 29 November 2014. Retrieved 19 June 2015.
- Lane 2009
- Ignatov, Ignat; Mosin, Oleg V. (2013). "Possible Processes for Origin of Life and Living Matter with modeling of Physiological Processes of Bacterium Bacillus Subtilis in Heavy Water as Model System". Journal of Natural Sciences Research. 3 (9): 65–76.
- Ignatov, Ignat; Valcheva, Nedyalka; Mihaylova, Sashka; Dinkov, Georgi (2020). "Physicochemical and Microbiological Results of Hyperthemal (Hot) Mineral Water in Rupite, Bulgaria as Model System for Origin of Life". Pradesh Journal of Zoology. 41 (24): 16–22.
- Derekova, Anna; Sjoholm, Carsten; Mandeva, Rossica; Kambourova, Margarita (2007). "Anoxybacillus rupiences sp. Nov. a novel thermophylic bacterium isolated from Rupi basin (Bulgaria)". Extremophiles. 11 (4): 577–583. doi:10.1007/s00792-007-0071-4. PMID 17505776. S2CID 22582377.
- Strunecký, Otakar; Kopejtka, Karel; Goecke, Franz; Tomasch, Juergen; Lukavský, Jaromir; Neori, Amir; Kahe, Silke; Pieper, Dietmar; Pilarski, Plamen; Kaftan, David; Koblížek, Michal (2019). "High diversity of thermophilic cyanobacteria in Rupite hot spring identified by microscopy, Cultivation, Single-cell PCR and Amplicon sequencing". Extremophiles. 23 (9): 35–48. doi:10.1007/s00792-018-1058-z. PMID 30284641. S2CID 52917273.
- Calvin 1969
- Schirber, Michael (1 March 2010). "First Fossil-Makers in Hot Water". Astrobiology Magazine. Archived from the original on 14 July 2015. Retrieved 19 June 2015.
- Kurihara, Kensuke; Tamura, Mieko; Shohda, Koh-ichiroh; et al. (October 2011). "Self-Reproduction of supramolecular giant vesicles combined with the amplification of encapsulated DNA". Nature Chemistry. 3 (10): 775–781. Bibcode:2011NatCh...3..775K. doi:10.1038/nchem.1127. PMID 21941249.
- Usher, Oli (27 April 2015). "Chemistry of seabed's hot vents could explain emergence of life" (Press release). University College London. Archived from the original on 20 June 2015. Retrieved 19 June 2015.
- Roldan, Alberto; Hollingsworth, Nathan; Roffey, Anna; Islam, Husn-Ubayda; et al. (May 2015). "Bio-inspired CO2 conversion by iron sulfide catalysts under sustainable conditions". Chemical Communications. 51 (35): 7501–7504. doi:10.1039/C5CC02078F. PMID 25835242. Archived from the original on 20 June 2015. Retrieved 19 June 2015.
- Baross, J.A.; Hoffman, S.E. (1985). "Submarine hydrothermal vents and associated gradient environments as sites for the origin and evolution of life". Origins LifeEvol. B. 15 (4): 327–345. Bibcode:1985OrLi...15..327B. doi:10.1007/bf01808177. S2CID 4613918.
- Russell, M.J.; Hall, A.J. (1997). "The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front". Journal of the Geological Society. 154 (3): 377–402. Bibcode:1997JGSoc.154..377R. doi:10.1144/gsjgs.154.3.0377. PMID 11541234. S2CID 24792282.
- Amend, J.P.; LaRowe, D.E.; McCollom, T.M.; Shock, E.L. (2013). "The energetics of organic synthesis inside and outside the cell". Phil. Trans. R. Soc. Lond. B. 368 (1622): 20120255. doi:10.1098/rstb.2012.0255. PMC 3685458. PMID 23754809.
- Shock, E.L.; Boyd, E.S. (2015). "Geomicrobiology and microbial geochemistry:principles of geobiochemistry". Elements. 11: 389–394. doi:10.2113/gselements.11.6.395.
- Martin, W.; Russell, M.J. (2007). "On the origin of biochemistry at an alkaline hydrothermal vent". Phil. Trans. R. Soc. Lond. B. 362 (1486): 1887–1925. doi:10.1098/rstb.2006.1881. PMC 2442388. PMID 17255002.
- Nature, Vol 535, 28 July 2016. p.468
- Chandru, Kuhan; Guttenberg, Nicholas; Giri, Chaitanya; Hongo, Yayoi; Butch, Christopher; Mamajanov, Irena; Cleaves, H. James (31 May 2018). "Simple prebiotic synthesis of high diversity dynamic combinatorial polyester libraries". Communications Chemistry. 1 (1). doi:10.1038/s42004-018-0031-1.
- Forsythe, Jay G; Yu, Sheng-Sheng; Mamajanov, Irena; Grover, Martha A; Krishnamurthy, Ramanarayanan; Fernández, Facundo M; Hud, Nicholas V (17 August 2015). "Ester-Mediated Amide Bond Formation Driven by Wet–Dry Cycles: A Possible Path to Polypeptides on the Prebiotic Earth". Angewandte Chemie International Edition in English. 54 (34): 9871–9875. doi:10.1002/anie.201503792. PMC 4678426. PMID 26201989.
- Mulkidjanian, Armid; Bychkov, Andrew; Dibrova, Daria; Galperin, Michael; Koonin, Eugene (3 April 2012). "Origin of first cells at terrestrial, anoxic geothermal fields". PNAS. 109 (14): E821–E830. Bibcode:2012PNAS..109E.821M. doi:10.1073/pnas.1117774109. PMC 3325685. PMID 22331915.
- Hoffmann, Geoffrey William (24 December 2016). "A network theory of the origin of life". bioRxiv 10.1101/096701.
- Dartnell, Lewis (12 January 2008). "Did life begin on a radioactive beach?". New Scientist (2638): 8. Archived from the original on 27 June 2015. Retrieved 26 June 2015.
- Adam, Zachary (2007). "Actinides and Life's Origins". Astrobiology. 7 (6): 852–872. Bibcode:2007AsBio...7..852A. doi:10.1089/ast.2006.0066. PMID 18163867.
- Parnell, John (December 2004). "Mineral Radioactivity in Sands as a Mechanism for Fixation of Organic Carbon on the Early Earth". Origins of Life and Evolution of Biospheres. 34 (6): 533–547. Bibcode:2004OLEB...34..533P. CiteSeerX 10.1.1.456.8955. doi:10.1023/B:ORIG.0000043132.23966.a1. PMID 15570707. S2CID 6067448.
- Davies, Paul (December 2007). "Are Aliens Among Us?" (PDF). Scientific American. 297 (6): 62–69. Bibcode:2007SciAm.297f..62D. doi:10.1038/scientificamerican1207-62. Archived (PDF) from the original on 4 March 2016. Retrieved 16 July 2015.
...if life does emerge readily under terrestrial conditions, then perhaps it formed many times on our home planet. To pursue this possibility, deserts, lakes and other extreme or isolated environments have been searched for evidence of "alien" life-forms—organisms that would differ fundamentally from known organisms because they arose independently.
- Hartman, Hyman (1998). "Photosynthesis and the Origin of Life". Origins of Life and Evolution of Biospheres. 28 (4–6): 515–521. Bibcode:1998OLEB...28..515H. doi:10.1023/A:1006548904157. PMID 11536891. S2CID 2464.
- Senthilingam, Meera (25 April 2014). "Metabolism May Have Started in Early Oceans Before the Origin of Life" (Press release). Wellcome Trust. EurekAlert!. Archived from the original on 17 June 2015. Retrieved 16 June 2015.
- Perry, Caroline (7 February 2011). "Clay-armored bubbles may have formed first protocells" (Press release). Cambridge, MA: Harvard University. EurekAlert!. Archived from the original on 14 July 2015. Retrieved 20 June 2015.
- Dawkins 1996, pp. 148–161
- Wenhua Huang; Ferris, James P. (12 July 2006). "One-Step, Regioselective Synthesis of up to 50-mers of RNA Oligomers by Montmorillonite Catalysis". Journal of the American Chemical Society. 128 (27): 8914–8919. doi:10.1021/ja061782k. PMID 16819887.
- Moore, Caroline (16 July 2007). "Crystals as genes?". Highlights in Chemical Science. Archived from the original on 14 July 2015. Retrieved 21 June 2015.
- Yue-Ching Ho, Eugene (July–September 1990). "Evolutionary Epistemology and Sir Karl Popper's Latest Intellectual Interest: A First-Hand Report". Intellectus. 15: 1–3. OCLC 26878740. Archived from the original on 11 March 2012. Retrieved 13 August 2012.
- Wade, Nicholas (22 April 1997). "Amateur Shakes Up Ideas on Recipe for Life". The New York Times. New York. Archived from the original on 17 June 2015. Retrieved 16 June 2015.
- Popper, Karl R. (29 March 1990). "Pyrite and the origin of life". Nature. 344 (6265): 387. Bibcode:1990Natur.344..387P. doi:10.1038/344387a0. S2CID 4322774.
- Huber, Claudia; Wächtershäuser, Günter (31 July 1998). "Peptides by Activation of Amino Acids with CO on (Ni,Fe)S Surfaces: Implications for the Origin of Life". Science. 281 (5377): 670–672. Bibcode:1998Sci...281..670H. doi:10.1126/science.281.5377.670. PMID 9685253.
- Musser, George (23 September 2011). "How Life Arose on Earth, and How a Singularity Might Bring It Down". Observations (Blog). Archived from the original on 17 June 2015. Retrieved 17 June 2015.
- Carroll, Sean (10 March 2010). "Free Energy and the Meaning of Life". Cosmic Variance (Blog). Discover. Archived from the original on 14 July 2015. Retrieved 17 June 2015.
- Wolchover, Natalie (22 January 2014). "A New Physics Theory of Life". Quanta Magazine. Archived from the original on 13 June 2015. Retrieved 17 June 2015.
- England, Jeremy L. (28 September 2013). "Statistical physics of self-replication" (PDF). Journal of Chemical Physics. 139 (12): 121923. arXiv:1209.1179. Bibcode:2013JChPh.139l1923E. doi:10.1063/1.4818538. hdl:1721.1/90392. PMID 24089735. S2CID 478964. Archived (PDF) from the original on 4 June 2015. Retrieved 18 June 2015.
- Orgel, Leslie E. (7 November 2000). "Self-organizing biochemical cycles". Proc. Natl. Acad. Sci. U.S.A. 97 (23): 12503–12507. Bibcode:2000PNAS...9712503O. doi:10.1073/pnas.220406697. PMC 18793. PMID 11058157.
- Chandru, Kuhan; Gilbert, Alexis; Butch, Christopher; Aono, Masashi; Cleaves, Henderson James II (21 July 2016). "The Abiotic Chemistry of Thiolated Acetate Derivatives and the Origin of Life". Scientific Reports. 6 (29883): 29883. Bibcode:2016NatSR...629883C. doi:10.1038/srep29883. PMC 4956751. PMID 27443234.
- Vallee, Yannick; Shalayel, Ibrahim; Ly, Kieu-Dung; Rao, K. V. Raghavendra; Paëpe, Gael De; Märker, Katharina; Milet, Anne (8 November 2017). "At the very beginning of life on Earth: the thiol-rich peptide (TRP) world hypothesis". International Journal of Developmental Biology. 61 (8–9): 471–478. doi:10.1387/ijdb.170028yv. PMID 29139533.
- Mulkidjanian, Armen Y. (24 August 2009). "On the origin of life in the zinc world: 1. Photosynthesizing, porous edifices built of hydrothermally precipitated zinc sulfide as cradles of life on Earth". Biology Direct. 4: 26. doi:10.1186/1745-6150-4-26. PMC 3152778. PMID 19703272.
- Wächtershäuser, Günter (December 1988). "Before Enzymes and Templates: Theory of Surface Metabolism". Microbiological Reviews. 52 (4): 452–484. doi:10.1128/MMBR.52.4.452-484.1988. PMC 373159. PMID 3070320.
- Mulkidjanian, Armen Y.; Galperin, Michael Y. (24 August 2009). "On the origin of life in the zinc world. 2. Validation of the hypothesis on the photosynthesizing zinc sulfide edifices as cradles of life on Earth". Biology Direct. 4: 27. doi:10.1186/1745-6150-4-27. PMC 2749021. PMID 19703275.
- Macallum, A. B. (1 April 1926). "The Paleochemistry of the body fluids and tissues". Physiological Reviews. 6 (2): 316–357. doi:10.1152/physrev.19188.8.131.526.
- Mulkidjanian, Armen Y.; Bychkov, Andrew Yu.; Dibrova, Daria V.; et al. (3 April 2012). "Origin of first cells at terrestrial, anoxic geothermal fields". Proc. Natl. Acad. Sci. U.S.A. 109 (14): E821–E830. Bibcode:2012PNAS..109E.821M. doi:10.1073/pnas.1117774109. PMC 3325685. PMID 22331915.
- For a deeper integrative version of this hypothesis, see in particular Lankenau 2011, pp. 225–286, interconnecting the "Two RNA worlds" concept and other detailed aspects; and Davidovich, Chen; Belousoff, Matthew; Bashan, Anat; Yonath, Ada (September 2009). "The evolving ribosome: from non-coded peptide bond formation to sophisticated translation machinery". Research in Microbiology. 160 (7): 487–492. doi:10.1016/j.resmic.2009.07.004. PMID 19619641.
- Starr, Michelle (3 October 2020). "A New Chemical 'Tree of The Origins of Life' Reveals Our Possible Molecular Evolution". ScienceAlert. Retrieved 3 October 2020.
- Wolos, Agnieszka; et al. (25 September 2020). "Synthetic connectivity, emergence, and self-regeneration in the network of prebiotic chemistry". Science. 369 (6511). doi:10.1126/science.aaw1955 (inactive 19 January 2021). PMID 32973002. Retrieved 3 October 2020.CS1 maint: DOI inactive as of January 2021 (link)
- Eigen & Schuster 1979
- Michaelian, K (2009). "Thermodynamic Origin of Life". Earth System Dynamics. 0907 (2011): 37–51. arXiv:0907.0042. Bibcode:2011ESD.....2...37M. doi:10.5194/esd-2-37-2011. S2CID 14574109.
- Sagan, C. (1973) Ultraviolet Selection Pressure on the Earliest Organisms, J. Theor. Biol., 39, 195–200.
- Michaelian, K; Simeonov, A (2015). "Fundamental molecules of life are pigments which arose and co-evolved as a response to the thermodynamic imperative of dissipating the prevailing solar spectrum". Biogeosciences. 12 (16): 4913–4937. arXiv:1405.4059. Bibcode:2015BGeo...12.4913M. doi:10.5194/bg-12-4913-2015.
- Michaelian, K (2013). "A non-linear irreversible thermodynamic perspective on organic pigment proliferation and biological evolution". Journal of Physics: Conference Series. 475 (1): 012010. arXiv:1307.5924. Bibcode:2013JPhCS.475a2010M. doi:10.1088/1742-6596/475/1/012010. S2CID 118564759.
- Doglioni, C.; Pignatti, J.; Coleman, M. (2016). "Why did life develop on the surface of the Earth in the Cambrian?" (PDF). Geoscience Frontiers. 7 (6): 865–873. doi:10.1016/j.gsf.2016.02.001.
- Michaelian, Karo; Santillán, Norberto (June 2019). "UVC photon-induced denaturing of DNA: A possible dissipative route to Archean enzyme-less replication". Heliyon. 5 (6): e01902. doi:10.1016/j.heliyon.2019.e01902. PMC 6584779. PMID 31249892.
- Michaelian, Karo (June 2018). "Homochirality through Photon-Induced Denaturing of RNA/DNA at the Origin of Life". Life. 8 (2): 21. doi:10.3390/life8020021. PMC 6027432. PMID 29882802.
- Morales, Julián Mejía; Michaelian, Karo (September 2020). "Photon Dissipation as the Origin of Information Encoding in RNA and DNA". Entropy. 22 (9): 940. Bibcode:2020Entrp..22..940M. doi:10.3390/e22090940. PMC 7597208. PMID 33286709.
- Maury, CP (2009). "Self-proagating beta-sheet polypeptide structures as prebiotic informational entities:The amyloid world". Origins of Life and Evolution of Biospheres. 39 (2): 141–150. doi:10.1007/s11084-009-9165-6. PMID 19301141. S2CID 20073536.
- Maury, CP (2015). "Origin of Life.Primordial genetics: Information transfer in a pre-RNA world based on self-replicating beta-sheet amyloid conformers". Journal of Theoretical Biology. 382: 292–297. doi:10.1016/j.jtbi.2015.07.008. PMID 26196585.
- Nanda, J; Rubinov, B; Ivnitski, D; Mukherjee, R; Shtelman, E; Motro, Y; Miller, Y; Wagner, N; Cohen-Luria, R; Ashkenasy, G (2017). "Emergence of native peptide seuqences in prebiotic replication networks". Nature Communications. 8 (1): 343. Bibcode:2017NatCo...8..434N. doi:10.1038/s41467-017-00463-1. PMC 5585222. PMID 28874657.
- Rout, SK; Friedmann, MP; Riek, R; Greenwald, J (2018). "A prebiotic templated-directed synthesis based on amyloids". Nature Communications. 9 (1): 234–242. doi:10.1038/s41467-017-02742-3. PMC 5770463. PMID 29339755.
- Blum, H.F. (1957). On the origin of self-replicating systems. In Rhythmic and Synthetic Processes in Growth, ed. Rudnick, D., pp. 155–170. Princeton University Press, Princeton, NJ.
- Lathe, Richard (2004). "Fast tidal cycling and the origin of life". Icarus. 168 (1): 18–22. Bibcode:2004Icar..168...18L. doi:10.1016/j.icarus.2003.10.018.
- Lathe, Richard (2005). "Tidal chain reaction and the origin of replicating biopolymers". International Journal of Astrobiology. 4 (1): 19–31. Bibcode:2005IJAsB...4...19L. doi:10.1017/S1473550405002314.
- Varga, P.; Rybicki, K.; Denis, C. (2006). "Comment on the paper "Fast tidal cycling and the origin of life" by Richard Lathe". Icarus. 180 (1): 274–276. Bibcode:2006Icar..180..274V. doi:10.1016/j.icarus.2005.04.022.
- Lathe, R. (2006). "Early tides: Response to Varga et al". Icarus. 180 (1): 277–280. Bibcode:2006Icar..180..277L. doi:10.1016/j.icarus.2005.08.019.
- Flament, Nicolas; Coltice, Nicolas; Rey, Patrice F. (2008). "A case for late-Archaean continental emergence from thermal evolution models and hypsometry". Earth and Planetary Science Letters. 275 (3–4): 326–336. Bibcode:2008E&PSL.275..326F. doi:10.1016/j.epsl.2008.08.029.
- Muller, Anthonie W. J. (7 August 1985). "Thermosynthesis by biomembranes: Energy gain from cyclic temperature changes". Journal of Theoretical Biology. 115 (3): 429–453. doi:10.1016/S0022-5193(85)80202-2. PMID 3162066.
- Muller, Anthonie W. J. (1995). "Were the first organisms heat engines? A new model for biogenesis and the early evolution of biological energy conversion". Progress in Biophysics and Molecular Biology. 63 (2): 193–231. doi:10.1016/0079-6107(95)00004-7. PMID 7542789.
- Muller, Anthonie W. J.; Schulze-Makuch, Dirk (1 April 2006). "Sorption heat engines: Simple inanimate negative entropy generators". Physica A: Statistical Mechanics and its Applications. 362 (2): 369–381. arXiv:physics/0507173. Bibcode:2006PhyA..362..369M. doi:10.1016/j.physa.2005.12.003. S2CID 96186464.
- Orgel 1987, pp. 9–16
- Orgel, Leslie E. (17 November 2000). "A Simpler Nucleic Acid". Science. 290 (5495): 1306–1307. doi:10.1126/science.290.5495.1306. PMID 11185405. S2CID 83662769.
- Nelson, Kevin E.; Levy, Matthew; Miller, Stanley L. (11 April 2000). "Peptide nucleic acids rather than RNA may have been the first genetic molecule". Proc. Natl. Acad. Sci. U.S.A. 97 (8): 3868–3871. Bibcode:2000PNAS...97.3868N. doi:10.1073/pnas.97.8.3868. PMC 18108. PMID 10760258.
- Larralde, Rosa; Robertson, Michael P.; Miller, Stanley L. (29 August 1995). "Rates of Decomposition of Ribose and Other Sugars: Implications for Chemical Evolution". Proc. Natl. Acad. Sci. U.S.A. 92 (18): 8158–8160. Bibcode:1995PNAS...92.8158L. doi:10.1073/pnas.92.18.8158. PMC 41115. PMID 7667262.
- Lindahl, Tomas (22 April 1993). "Instability and decay of the primary structure of DNA". Nature. 362 (6422): 709–715. Bibcode:1993Natur.362..709L. doi:10.1038/362709a0. PMID 8469282. S2CID 4283694.
- Anastasi, Carole; Crowe, Michael A.; Powner, Matthew W.; Sutherland, John D. (18 September 2006). "Direct Assembly of Nucleoside Precursors from Two- and Three-Carbon Units". Angewandte Chemie International Edition. 45 (37): 6176–6179. doi:10.1002/anie.200601267. PMID 16917794.
- Powner, Matthew W.; Sutherland, John D. (13 October 2008). "Potentially Prebiotic Synthesis of Pyrimidine β-D-Ribonucleotides by Photoanomerization/Hydrolysis of α-D-Cytidine-2'-Phosphate". ChemBioChem. 9 (15): 2386–2387. doi:10.1002/cbic.200800391. PMID 18798212. S2CID 5704391.
- Powner, Matthew W.; Gerland, Béatrice; Sutherland, John D. (14 May 2009). "Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions". Nature. 459 (7244): 239–242. Bibcode:2009Natur.459..239P. doi:10.1038/nature08013. PMID 19444213. S2CID 4412117.
- Palasek, Stan (23 May 2013). "Primordial RNA Replication and Applications in PCR Technology". arXiv:1305.5581v1 [q-bio.BM].
- Koonin, Eugene V.; Senkevich, Tatiana G.; Dolja, Valerian V. (19 September 2006). "The ancient Virus World and evolution of cells". Biology Direct. 1: 29. doi:10.1186/1745-6150-1-29. PMC 1594570. PMID 16984643.
- Vlassov, Alexander V.; Kazakov, Sergei A.; Johnston, Brian H.; et al. (August 2005). "The RNA World on Ice: A New Scenario for the Emergence of RNA Information". Journal of Molecular Evolution. 61 (2): 264–273. Bibcode:2005JMolE..61..264V. doi:10.1007/s00239-004-0362-7. PMID 16044244. S2CID 21096886.
- Nussinov, Mark D.; Otroshchenko, Vladimir A.; Santoli, Salvatore (1997). "The emergence of the non-cellular phase of life on the fine-grained clayish particles of the early Earth's regolith". BioSystems. 42 (2–3): 111–118. doi:10.1016/S0303-2647(96)01699-1. PMID 9184757.
- Yates, Diana (25 September 2015). "Study adds to evidence that viruses are alive" (Press release). Champaign, IL: University of Illinois at Urbana–Champaign. Archived from the original on 19 November 2015. Retrieved 20 October 2015.
- Janjic, Aleksandar (2018). "The Need for Including Virus Detection Methods in Future Mars Missions". Astrobiology. 18 (12): 1611–1614. Bibcode:2018AsBio..18.1611J. doi:10.1089/ast.2018.1851.
- Katzourakis, A (2013). "Paleovirology: Inferring viral evolution from host genome sequence data". Philosophical Transactions of the Royal Society B: Biological Sciences. 368 (1626): 20120493. doi:10.1098/rstb.2012.0493. PMC 3758182. PMID 23938747.
- Arshan, Nasir; Caetano-Anollés, Gustavo (25 September 2015). "A phylogenomic data-driven exploration of viral origins and evolution". Science Advances. 1 (8): e1500527. Bibcode:2015SciA....1E0527N. doi:10.1126/sciadv.1500527. PMC 4643759. PMID 26601271.
- Nasir, Arshan; Naeem, Aisha; Jawad Khan, Muhammad; et al. (December 2011). "Annotation of Protein Domains Reveals Remarkable Conservation in the Functional Make up of Proteomes Across Superkingdoms". Genes. 2 (4): 869–911. doi:10.3390/genes2040869. PMC 3927607. PMID 24710297.
- Jalasvuori M, Mattila S, Hoikkala V (2015). "Chasing the Origin of Viruses: Capsid-Forming Genes as a Life-Saving Preadaptation within a Community of Early Replicators". PLOS ONE. 10 (5): e0126094. Bibcode:2015PLoSO..1026094J. doi:10.1371/journal.pone.0126094. PMC 4425637. PMID 25955384.
- Krupovic M, Dolja VV, Koonin EV (July 2019). "Origin of viruses: primordial replicators recruiting capsids from hosts" (PDF). Nature Reviews. Microbiology. 17 (7): 449–458. doi:10.1038/s41579-019-0205-6. PMID 31142823. S2CID 169035711.
- Orgel, Leslie E. (October 1994). "The origin of life on Earth". Scientific American. 271 (4): 76–83. Bibcode:1994SciAm.271d..76O. doi:10.1038/scientificamerican1094-76. PMID 7524147.
- Camprubí, E.; de Leeuw, J.W.; House, C.H.; Raulin, F.; Russell, M.J.; Spang, A.; Tirumalai, M.R.; Westall, F. (12 December 2019). "Emergence of Life". Space Sci. Rev. 215 (56): 56. Bibcode:2019SSRv..215...56C. doi:10.1007/s11214-019-0624-8.
- Johnston, Wendy K.; Unrau, Peter J.; Lawrence, Michael S.; et al. (18 May 2001). "RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension". Science. 292 (5520): 1319–1325. Bibcode:2001Sci...292.1319J. CiteSeerX 10.1.1.70.5439. doi:10.1126/science.1060786. PMID 11358999. S2CID 14174984.
- Szostak, Jack W. (5 February 2015). "The Origins of Function in Biological Nucleic Acids, Proteins, and Membranes". Chevy Chase (CDP), MD: Howard Hughes Medical Institute. Archived from the original on 14 July 2015. Retrieved 16 June 2015.
- Lincoln, Tracey A.; Joyce, Gerald F. (27 February 2009). "Self-Sustained Replication of an RNA Enzyme". Science. 323 (5918): 1229–1232. Bibcode:2009Sci...323.1229L. doi:10.1126/science.1167856. PMC 2652413. PMID 19131595.
- Joyce, Gerald F. (2009). "Evolution in an RNA world". Cold Spring Harbor Perspectives in Biology. 74 (Evolution: The Molecular Landscape): 17–23. doi:10.1101/sqb.2009.74.004. PMC 2891321. PMID 19667013.
- Bernstein, Harris; Byerly, Henry C.; Hopf, Frederick A.; et al. (June 1983). "The Darwinian Dynamic". The Quarterly Review of Biology. 58 (2): 185–207. doi:10.1086/413216. JSTOR 2828805. S2CID 83956410.
- Michod 1999
- Gibard, Clémentine; Bhowmik, Subhendu; Karki, Megha; Kim, Eun-Kyong; Krishnamurthy, Ramanarayanan (February 2018). "Phosphorylation, oligomerization and self-assembly in water under potential prebiotic conditions". Nature Chemistry. 10 (2): 212–217. doi:10.1038/nchem.2878. ISSN 1755-4349. PMC 6295206. PMID 29359747.
- Gibard, Clémentine; Gorrell, Ian B.; Jiménez, Eddy I.; Kee, Terence P.; Pasek, Matthew A.; Krishnamurthy, Ramanarayanan (2019). "Geochemical Sources and Availability of Amidophosphates on the Early Earth". Angewandte Chemie International Edition. 58 (24): 8151–8155. doi:10.1002/anie.201903808. ISSN 1521-3773. PMID 30989779.
- Krishnamurthy, Ramanarayanan; Jiménez, Eddy I.; Gibard, Clémentine (15 December 2020). "Prebiotic Phosphorylation and Concomitant Oligomerization of Deoxynucleosides to form DNA". Angewandte Chemie. doi:10.1002/anie.202015910. PMID 33325148. Retrieved 28 December 2020.
- The Scripps Research Institute (28 December 2020). "Discovery boosts theory that life on Earth arose from RNA-DNA mix". Phys.org. Retrieved 28 December 2020.
- Gavette, Jesse V.; Stoop, Matthias; Hud, Nicholas V.; Krishnamurthy, Ramanarayanan (2016). "RNA–DNA Chimeras in the Context of an RNA World Transition to an RNA/DNA World". Angewandte Chemie International Edition. 55 (42): 13204–13209. doi:10.1002/anie.201607919. ISSN 1521-3773. PMID 27650222.
- Bhowmik, Subhendu; Krishnamurthy, Ramanarayanan (November 2019). "The role of sugar-backbone heterogeneity and chimeras in the simultaneous emergence of RNA and DNA". Nature Chemistry. 11 (11): 1009–1018. Bibcode:2019NatCh..11.1009B. doi:10.1038/s41557-019-0322-x. ISSN 1755-4349. PMC 6815252. PMID 31527850.
- McCollom, Thomas; Mayhew, Lisa; Scott, Jim (7 October 2014). "NASA awards CU-Boulder-led team $7 million to study origins, evolution of life in universe" (Press release). Boulder, CO: University of Colorado Boulder. Archived from the original on 31 July 2015. Retrieved 8 June 2015.
- Oehlenschläger, Frank; Eigen, Manfred (December 1997). "30 Years Later – a New Approach to Sol Spiegelman's and Leslie Orgel's in vitro Evolutionary Studies Dedicated to Leslie Orgel on the occasion of his 70th birthday". Origins of Life and Evolution of Biospheres. 27 (5–6): 437–457. Bibcode:1997OLEB...27..437O. doi:10.1023/A:1006501326129. PMID 9394469. S2CID 26717033.
- Gibson, Daniel G.; Glass, John I.; Lartigue, Carole; et al. (2 July 2010). "Creation of a Bacterial Cell Controlled by a Chemically Synthesized Genome". Science. 329 (5987): 52–56. Bibcode:2010Sci...329...52G. CiteSeerX 10.1.1.167.1455. doi:10.1126/science.1190719. PMID 20488990. S2CID 7320517.
- Swaby, Rachel (20 May 2010). "Scientists Create First Self-Replicating Synthetic Life". Wired. New York. Archived from the original on 17 June 2015. Retrieved 8 June 2015.
- Coughlan, Andy (2016) "Smallest ever genome comes to life: Humans built it but we don't know what a third of its genes actually do" (New Scientist 2 April 2016 No 3067)p.6
- Balch, Erica (4 October 2018). "Ground-breaking lab poised to unlock the mystery of the origins of life on Earth and beyond". McMaster University. Retrieved 4 October 2018.
- Staff (4 October 2018). "Ground-breaking lab poised to unlock the mystery of the origins of life". EurekAlert!. Retrieved 14 October 2018.
- Staff (2018). "Planet Simulator". IntraVisionGroup.com. Retrieved 14 October 2018.
- Anderson, Paul Scott (14 October 2018). "New technology may help solve mystery of life's origins – How did life on Earth begin? A new technology, called Planet Simulator, might finally help solve the mystery". EarthSky. Retrieved 14 October 2018.
- Altermann, Wladyslaw (2009). "Introduction: A Roadmap to Fata Morgana?". In Seckbach, Joseph; Walsh, Maud (eds.). From Fossils to Astrobiology: Records of Life on Earth and the Search for Extraterrestrial Biosignatures. Cellular Origin, Life in Extreme Habitats and Astrobiology. 12. Dordrecht, the Netherlands; London: Springer Science+Business Media. ISBN 978-1-4020-8836-0. LCCN 2008933212.
- Bada, Jeffrey L.; Lazcano, Antonio (2009). "The Origin of Life". In Ruse, Michael; Travis, Joseph (eds.). Evolution: The First Four Billion Years. Foreword by Edward O. Wilson. Cambridge: Belknap Press of Harvard University Press. ISBN 978-0-674-03175-3. LCCN 2008030270. OCLC 225874308.
- Barton, Nicholas H.; Briggs, Derek E.G.; Eisen, Jonathan A.; et al. (2007). Evolution. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. ISBN 978-0-87969-684-9. LCCN 2007010767. OCLC 86090399.
- Bastian, H. Charlton (1871). The Modes of Origin of Lowest Organisms. London; New York: Macmillan and Company. LCCN 11004276. OCLC 42959303. Retrieved 6 June 2015.
- Bernal, J.D. (1951). The Physical Basis of Life. London: Routledge & Kegan Paul. LCCN 51005794.
- Bernal, J.D. (1960). "The Problem of Stages in Biopoesis". In Florkin, M. (ed.). Aspects of the Origin of Life. International Series of Monographs on Pure and Applied Biology. Oxford, UK; New York: Pergamon Press. ISBN 978-1-4831-3587-8. LCCN 60013823.
- Bernal, J.D. (1967) [Reprinted work by A.I. Oparin originally published 1924; Moscow: The Moscow Worker]. The Origin of Life. The Weidenfeld and Nicolson Natural History. Translation of Oparin by Ann Synge. London: Weidenfeld & Nicolson. LCCN 67098482.
- Bock, Gregory R.; Goode, Jamie A., eds. (1996). Evolution of Hydrothermal Ecosystems on Earth (and Mars?). Ciba Foundation Symposium. 202. Chichester, UK; New York: John Wiley & Sons. ISBN 978-0-471-96509-1. LCCN 96031351.
- Bondeson, Jan (1999). The Feejee Mermaid and Other Essays in Natural and Unnatural History. Ithaca, NY: Cornell University Press. ISBN 978-0-8014-3609-3. LCCN 98038295.
- Bryson, Bill (2004). A Short History of Nearly Everything. London: Black Swan. ISBN 978-0-552-99704-1. OCLC 55589795.
- Calvin, Melvin (1969). Chemical Evolution: Molecular Evolution Towards the Origin of Living Systems on the Earth and Elsewhere. Oxford, UK: Clarendon Press. ISBN 978-0-19-855342-7. LCCN 70415289. OCLC 25220.
- Cardwell, Donald S. L. (1971). From Watt to Clausius: The Rise of Thermodynamics in the Early Industrial Age. Cornell University Press.
- Chaichian, Masud; Rojas, Hugo Perez; Tureanu, Anca (2014). "Physics and Life". Basic Concepts in Physics: From the Cosmos to Quarks. Undergraduate Lecture Notes in Physics. Berlin; Heidelberg: Springer Berlin Heidelberg. doi:10.1007/978-3-642-19598-3_12. ISBN 978-3-642-19597-6. LCCN 2013950482. OCLC 900189038.
- Chang, Thomas Ming Swi (2007). Artificial Cells: Biotechnology, Nanomedicine, Regenerative Medicine, Blood Substitutes, Bioencapsulation, and Cell/Stem Cell Therapy. Regenerative Medicine, Artificial Cells and Nanomedicine. 1. Hackensack, NJ: World Scientific. ISBN 978-981-270-576-1. LCCN 2007013738. OCLC 173522612.
- Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". In Lewis, C.L.E.; Knell, S.J. (eds.). The Age of the Earth: from 4004 BC to AD 2002. Geological Society of London, Special Publications. Geological Society Special Publication. 190. London: Geological Society of London. pp. 205–221. Bibcode:2001GSLSP.190..205D. doi:10.1144/gsl.sp.2001.190.01.14. ISBN 978-1-86239-093-5. LCCN 2003464816. OCLC 48570033. S2CID 130092094.
- Darwin, Charles (1887). Darwin, Francis (ed.). The Life and Letters of Charles Darwin, Including an Autobiographical Chapter. 3 (3rd ed.). London: John Murray. OCLC