|Life (Biota / Vitae / Eobionti)|
|Plants in the Rwenzori Mountains, Uganda|
|Domains and kingdoms|
Life on Earth:
Life is a characteristic distinguishing physical entities having signaling and self-sustaining processes from those that do not, either because such functions have ceased (death), or because they lack such functions and are classified as inanimate. Biology is a science concerned with the study of life.
The smallest contiguous unit of life is called an organism. Organisms are composed of one, or more, cells, undergo metabolism, maintain homeostasis, can grow, respond to stimuli, reproduce and, through evolution, adapt to their environment in successive generations. A diverse array of living organisms can be found in the biosphere of Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information.
The Earth was formed about 4.54 billion years ago. The earliest life on Earth existed at least 3.5 billion years ago, during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence for life on Earth is biogenic graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Nevertheless, several studies suggest that life on Earth may have started even earlier, as early as 4.25 billion years ago according to one study, and even earlier yet, 4.4 billion years ago, according to another study. The mechanism by which life began on Earth is unknown, although many hypotheses have been formulated. Since emerging, life has evolved into a variety of forms, which biologists have classified into a hierarchy of taxa. Life can survive and thrive in a wide range of conditions.
Though life is confirmed only on the Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable. Other planets and moons in the Solar System have been examined for evidence of having once supported simple life, and projects such as SETI have attempted to detect radio transmissions from possible alien civilizations. According to the panspermia hypothesis, microscopic life exists throughout the Universe, and is distributed by meteoroids, asteroids and planetoids.
The meaning of life—its significance, origin, purpose, and ultimate fate—is a central concept and question in philosophy and religion. Both philosophy and religion have offered interpretations as to how life relates to existence and consciousness, and on related issues such as life stance, purpose, conception of a god or gods, a soul or an afterlife. Different cultures throughout history have had widely varying approaches to these issues.
- 1 Early theories
- 2 Definitions
- 3 Origin
- 4 Conditions
- 5 Form and function
- 6 Classification
- 7 Extraterrestrial life
- 8 Death
- 9 Artificial life
- 10 See also
- 11 Notes
- 12 References
- 13 Further reading
- 14 External links
Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. Empedocles (430 BC) argued that every thing in the universe is made up of a combination of four eternal "elements" or "roots of all": earth, water, air, and fire. All change is explained by the arrangement and rearrangement of these four elements. The various forms of life are caused by an appropriate mixture of elements.
Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche). Like other ancient writers, he was attempting to explain what makes something a living thing. His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing. He elaborates on fire because of the apparent connection between life and heat, and because fire moves.
Plato's world of eternal and unchanging Forms, imperfectly represented in matter by a divine Artisan, contrasts sharply with the various mechanistic Weltanschauungen, of which atomism was, by the fourth century at least, the most prominent... This debate persisted throughout the ancient world. Atomistic mechanism got a shot in the arm from Epicurus... while the Stoics adopted a divine teleology... The choice seems simple: either show how a structured, regular world could arise out of undirected processes, or inject intelligence into the system.—R. J. Hankinson, Cause and Explanation in Ancient Greek Thought
The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes, who held that animals and humans were assemblages of parts that together functioned as a machine. In the 19th century, the advances in cell theory in biological science encouraged this view. The evolutionary theory of Charles Darwin (1859) is a mechanistic explanation for the origin of species by means of natural selection.
Hylomorphism is a theory (originating with Aristotle (322 BC)) that all things are a combination of matter and form. Biology was one of his main interests, and there is extensive biological material in his extant writings. In this view, all things in the material universe have both matter and form, and the form of a living thing is its soul (Greek psyche, Latin anima). There are three kinds of souls: the vegetative soul of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the animal soul, which causes animals to move and feel; and the rational soul, which is the source of consciousness and reasoning, which (Aristotle believed) is found only in man. Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.
This account is consistent with teleological explanations of life, which account for phenomena in terms of purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality (from the future to the past) is in contradiction with the scientific evidence for natural selection, which explains the consequence in terms of a prior cause. Biological features are explained not by looking at future optimal results, but by looking at the past evolutionary history of a species, which led to the natural selection of the features in question.
Vitalism is the belief that the life-principle is non-material. This originated with Stahl (17th century), and held sway until the middle of the 19th century. It appealed to philosophers such as Henri Bergson, Nietzsche, Wilhelm Dilthey, anatomists like Bichat, and chemists like Liebig. Vitalism included the idea that there was a fundamental difference between organic and inorganic material, and the belief that organic material can only be derived from living things. This was disproved in 1828, when Friedrich Wöhler prepared urea from inorganic materials. This Wöhler synthesis is considered the starting point of modern organic chemistry. It is of historical significance because for the first time an organic compound was produced from inorganic reactants.
During the 1850s, Helmholtz, anticipated by Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle. These results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in pseudoscientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.
It is a challenge for scientists and philosophers to define life in unequivocal terms. This is difficult partly because life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all life with which we are familiar, and must be sufficiently general to include life that may be fundamentally different from life on Earth. Some may even consider that life is not real at all, but a concept instead.
Since there is no unequivocal definition of life, the current understanding is descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:
- Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
- Organization: Being structurally composed of one or more cells — the basic units of life.
- Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.,
- Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
- Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
- Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
- Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms. or "with an error rate below the sustainability threshold."
To reflect the minimum phenomena required, other biological definitions of life have been proposed, many of these are based upon chemical systems. Biophysicists have commented that living things function on negative entropy. In other words, living processes can be viewed as a delay of the spontaneous diffusion or dispersion of the internal energy of biological molecules towards more potential microstates. In more detail, according to physicists such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena that are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form. At a higher level, living beings are thermodynamic systems that have an organized molecular structure. That is, life is matter that can reproduce itself and evolve as survival dictates. Hence, life is a self-sustained chemical system capable of undergoing Darwinian evolution.
Others take a systemic viewpoint that does not necessarily depend on molecular chemistry. One systemic definition of life is that living things are self-organizing and autopoietic (self-producing). Variations of this definition include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.
Viruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life," since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and they require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.
Living systems theories
The idea that the Earth is alive is found in philosophy and religion, but the first scientific discussion of it was by the Scottish scientist James Hutton. In 1785, he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is considered the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century. The Gaia hypothesis, proposed in the 1960s by scientist James Lovelock, suggests that life on Earth functions as a single organism that defines and maintains environmental conditions necessary for its survival.
The first attempt at a general living systems theory for explaining the nature of life was in 1978, by American biologist James Grier Miller. Such a general theory, arising out of the ecological and biological sciences, attempts to map general principles for how all living systems work. Instead of examining phenomena by attempting to break things down into component parts, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment. Robert Rosen (1991) built on this by defining a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." From this and other starting concepts, he developed a "relational theory of systems" that attempts to explain the special properties of life. Specifically, he identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and "biological machines."
A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence", and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. As Harold J. Morowitz (1992) explains it, life is a property of an ecological system rather than a single organism or species. He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. Robert Ulanowicz (2009) highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.
Complex systems biology (CSB) is a field of science that studies the emergence of complexity in functional organisms from the viewpoint of dynamic systems theory. The latter is often called also systems biology and aims to understand the most fundamental aspects of life. A closely related approach to CSB and systems biology, called relational biology, is concerned mainly with understanding life processes in terms of the most important relations, and categories of such relations among the essential functional components of organisms; for multicellular organisms, this has been defined as "categorical biology", or a model representation of organisms as a category theory of biological relations, and also an algebraic topology of the functional organization of living organisms in terms of their dynamic, complex networks of metabolic, genetic, epigenetic processes and signaling pathways.
It has also been argued that the evolution of order in living systems and certain physical systems obey a common fundamental principle termed the Darwinian dynamic. The Darwinian dynamic was formulated by first considering how macroscopic order is generated in a simple non-biological system far from thermodynamic equilibrium, and then extending consideration to short, replicating RNA molecules. The underlying order generating process for both types of system was concluded to be basically similar.
Another systemic definition, called the Operator theory, proposes that 'life is a general term for the presence of the typical closures found in organisms; the typical closures are a membrane and an autocatalytic set in the cell', and also proposes that an organism is 'any system with an organisation that complies with an operator type that is at least as complex as the cell. Life can also be modeled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a superior positive feedback formed by the potential of expansion and reproduction.
Evidence suggests that life on Earth has existed for at least 3.5 billion years, with the oldest physical traces of life dating back 3.7 billion years. All known life forms share fundamental molecular mechanisms, reflecting their common descent; based on these observations, hypotheses on the origin of life attempt to find a mechanism explaining the formation of a universal common ancestor, from simple organic molecules via pre-cellular life to protocells and metabolism. Models have been divided into "genes-first" and "metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine both categories.
There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the following observations:
- The Miller–Urey experiment, and the work of Sidney Fox, show that conditions on the primitive Earth favored chemical reactions that synthesize amino acids and other organic compounds from inorganic precursors.
- Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
Living organisms synthesize proteins, which are polymers of amino acids using instructions encoded by deoxyribonucleic acid (DNA). Protein synthesis entails intermediary ribonucleic acid (RNA) polymers. One possibility for how life began is that genes originated first, followed by proteins; the alternative being that proteins came first and then genes.
However, since genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently.
Therefore, a possibility, first suggested by Francis Crick, is that the first life was based on RNA, which has the DNA-like properties of information storage and the catalytic properties of some proteins. This is called the RNA world hypothesis, and it is supported by the observation that many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed, but they were confirmed by Thomas Cech in 1986.
One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules. One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA. However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction. This study makes the RNA world hypothesis more plausible.
Geological findings in 2013 showed that reactive phosphorus species (like phosphite) were in abundance in the ocean before 3.5 Ga, and that Schreibersite easily reacts with aqueous glycerol to generate phosphite and glycerol 3-phosphate. It is hypothesized that Schreibersite-containing meteorites from the Late Heavy Bombardment could have provided early reduced phosphorus, which could react with prebiotic organic molecules to form phosphorylated biomolecules, like RNA.
In 2009, experiments demonstrated Darwinian evolution of a two-component system of RNA enzymes (ribozymes) in vitro. The work was performed in the laboratory of Gerald Joyce, who stated, "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system."
Prebiotic compounds may have extraterrestrial origin. NASA findings in 2011, based on studies with meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules) may be formed in outer space.
The diversity of life on Earth is a result of the dynamic interplay between genetic opportunity, metabolic capability, environmental challenges, and symbiosis. For most of its existence, Earth's habitable environment has been dominated by microorganisms and subjected to their metabolism and evolution. As a consequence of these microbial activities, the physical-chemical environment on Earth has been changing on a geologic time scale, thereby affecting the path of evolution of subsequent life. For example, the release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Since oxygen was toxic to most life on Earth at the time, this posed novel evolutionary challenges, and ultimately resulted in the formation of our planet's major animal and plant species. This interplay between organisms and their environment is an inherent feature of living systems.
All life forms require certain core chemical elements needed for biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the elemental macronutrients for all organisms—often represented by the acronym CHNOPS. Together these make up nucleic acids, proteins and lipids, the bulk of living matter. Five of these six elements comprise the chemical components of DNA, the exception being sulfur. The latter is a component of the amino acids cysteine and methionine. The most biologically abundant of these elements is carbon, which has the desirable attribute of forming multiple, stable covalent bonds. This allows carbon-based (organic) molecules to form an immense variety of chemical arrangements. Alternative hypothetical types of biochemistry have been proposed that eliminate one or more of these elements, swap out an element for one not on the list, or change required chiralities or other chemical properties.
Range of tolerance
The inert components of an ecosystem are the physical and chemical factors necessary for life — energy (sunlight or chemical energy), water, temperature, atmosphere, gravity, nutrients, and ultraviolet solar radiation protection. In most ecosystems, the conditions vary during the day and from one season to the next. To live in most ecosystems, then, organisms must be able to survive a range of conditions, called the "range of tolerance." Outside that are the "zones of physiological stress", where the survival and reproduction are possible but not optimal. Beyond these zones are the "zones of intolerance", where survival and reproduction of that organism is unlikely or impossible. Organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance.
To survive, selected microorganisms can assume forms that enable them to withstand freezing, complete desiccation, starvation, high levels of radiation exposure, and other physical or chemical challenges. These microorganisms may survive exposure to such conditions for weeks, months, years, or even centuries. Extremophiles are microbial life forms that thrive outside the ranges where life is commonly found. They excel at exploiting uncommon sources of energy. While all organisms are composed of nearly identical molecules, evolution has enabled such microbes to cope with this wide range of physical and chemical conditions. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing.
On 17 March 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth. Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States. According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."
Investigation of the tenacity and versatility of life on Earth, as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, is important for the search for life beyond Earth. In April 2012, scientists reported that lichen could survive for a month in a simulated Martian environment.
Form and function
Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division. Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted. The activity of an organism depends on the total activity of its cells, with energy flow occurring within and between them. Cells contain hereditary information that is carried forward as a genetic code during cell division.
There are two primary types of cells. Prokaryotes lack a nucleus and other membrane-bound organelles, although they have circular DNA and ribosomes. Bacteria and Archaea are two domains of prokaryotes. The other primary type of cells are the eukaryotes, which have distinct nuclei bound by a nuclear membrane and membrane-bound organelles, including mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, and vacuoles. In addition, they possess organized chromosomes that store genetic material. All species of large complex organisms are eukaryotes, including animals, plants and fungi, though most species of eukaryote are protist microorganisms. The conventional model is that eukaryotes evolved from prokaryotes, with the main organelles of the eukaryotes forming through endosymbiosis between bacteria and the progenitor eukaryotic cell.
The molecular mechanisms of cell biology are based on proteins. Most of these are synthesized by the ribosomes through an enzyme-catalyzed process called protein biosynthesis. A sequence of amino acids is assembled and joined together based upon gene expression of the cell's nucleic acid. In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination.
Cells reproduce through a process of cell division in which the parent cell divides into two or more daughter cells. For prokaryotes, cell division occurs through a process of fission in which the DNA is replicated, then the two copies are attached to parts of the cell membrane. In eukaryotes, a more complex process of mitosis is followed. However, the end result is the same; the resulting cell copies are identical to each other and to the original cell (except for mutations), and both are capable of further division following an interphase period.
Multicellular organisms may have first evolved through the formation of colonies of like cells. These cells can form group organisms through cell adhesion. The individual members of a colony are capable of surviving on their own, whereas the members of a true multi-cellular organism have developed specialties, making them dependent on the remainder of the organism for survival. Such organisms are formed clonally or from a single germ cell that is capable of forming the various specialized cells that form the adult organism. This specialization allows multicellular organisms to exploit resources more efficiently than single cells.
Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability. Cell signaling coordinates cellular activities, and hence governs the basic functions of multicellular organisms. Signaling between cells can occur through direct cell contact using juxtacrine signalling, or indirectly through the exchange of agents as in the endocrine system. In more complex organisms, coordination of activities can occur through a dedicated nervous system.
The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle (384–322 BC), who classified all living organisms known at that time as either a plant or an animal, based mainly on their ability to move. He also distinguished animals with blood from animals without blood (or at least without red blood), which can be compared with the concepts of vertebrates and invertebrates respectively, and divided the blooded animals into five groups: viviparous quadrupeds (mammals), oviparous quadrupeds (reptiles and amphibians), birds, fishes and whales. The bloodless animals were also divided into five groups: cephalopods, crustaceans, insects (which included the spiders, scorpions, and centipedes, in addition to what we define as insects today), shelled animals (such as most molluscs and echinoderms) and "zoophytes." Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death.
The exploration of the American continent revealed large numbers of new plants and animals that needed descriptions and classification. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. In the late 1740s, Carolus Linnaeus introduced his system of binomial nomenclature for the classification of species. Linnaeus attempted to improve the composition and reduce the length of the previously used many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and precisely defining their meaning. By consistently using this system, Linnaeus separated nomenclature from taxonomy.
The fungi were originally treated as plants. For a short period Linnaeus had classified them in the taxon Vermes in Animalia, but later placed them back in Plantae. Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledging their special status. The problem was eventually solved by Whittaker, when he gave them their own kingdom in his five-kingdom system. Evolutionary history shows that the fungi are more closely related to animals than to plants.
As new discoveries enabled detailed study of cells and microorganisms, new groups of life were revealed, and the fields of cell biology and microbiology were created. These new organisms were originally described separately in protozoa as animals and protophyta/thallophyta as plants, but were united by Haeckel in the kingdom Protista; later, the prokaryotes were split off in the kingdom Monera, which would eventually be divided into two separate groups, the Bacteria and the Archaea. This led to the six-kingdom system and eventually to the current three-domain system, which is based on evolutionary relationships. However, the classification of eukaryotes, especially of protists, is still controversial.
As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids. Whether these are considered alive has been a matter of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability to grow or respond to their environments. Viruses can still be classed into "species" based on their biology and genetics, but many aspects of such a classification remain controversial.
|Woese et al.
|2 kingdoms||3 kingdoms||2 empires||4 kingdoms||5 kingdoms||3 domains||6 kingdoms|
Earth is the only planet known to harbor life. Other locations within the Solar System that may host life include subsurface Mars, the atmosphere of Venus, and subsurface oceans on some of the moons of the gas giant planets. The variables of the Drake equation are used to discuss the conditions in solar systems where civilization is most likely to exist.
The region around a main sequence star that could support Earth-like life on an Earth-like planet is known as the habitable zone. The inner and outer radii of this zone vary with the luminosity of the star, as does the time interval during which the zone survives. Stars more massive than the Sun have a larger habitable zone, but remain on the main sequence for a shorter time interval. Small red dwarf stars have the opposite problem, with a smaller habitable zone that is subject to higher levels of magnetic activity and the effects of tidal locking from close orbits. Hence, stars in the intermediate mass range such as the Sun may have a greater likelihood for Earth-like life to develop. The location of the star within a galaxy may also have an impact on the likelihood of life forming. Stars in regions with a greater abundance of heavier elements that can form planets, in combination with a low rate of potentially habitat-damaging supernova events, are predicted to have a higher probability of hosting planets with complex life.
Panspermia, also called exogenesis, is the hypothesis that life originated elsewhere in the universe and subsequently transferred to Earth in the form of spores via meteorites, comets, or cosmic dust. Conversely, terrestrial life may be seeded in other solar systems through directed panspermia, to secure and expand some terrestrial life forms. Astroecology experiments with meteorites show that Martian asteroids and cometary materials are rich in inorganic elements and may be fertile soils for microbial, algal and plant life, for past and future life in our and other solar systems.
In 2004, scientists reported detecting the spectral signatures of anthracene and pyrene in the ultraviolet light emitted by the Red Rectangle nebula (no other such complex molecules had ever been found before in outer space). This discovery was considered a confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, and radiate outward. As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. The scientists inferred that since they discovered polycyclic aromatic hydrocarbons (PAHs) — which may have been vital in the formation of early life on Earth — in a nebula, by necessity they must originate in nebulae.
In 2010, fullerenes (or "buckyballs") were detected in nebulae. Fullerenes have been implicated in the origin of life; according to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth."
In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space.
In October 2011, scientists found using spectroscopy that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created naturally, and rapidly, by stars. The compounds are so complex that their chemical structures resemble the makeup of coal and petroleum; such chemical complexity was previously thought to arise only from living organisms. These observations suggest that organic compounds introduced on Earth by interstellar dust particles could serve as basic ingredients for life due to their surface-catalytic activities. One of the scientists suggested that these compounds may have been related to the development of life on Earth and 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."
In August 2012, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.
In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) 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."
In 2013, the Atacama Large Millimeter Array (ALMA Project) confirmed that researchers have discovered an important pair of prebiotic molecules in the icy particles in interstellar space (ISM). The chemicals, found in a giant cloud of gas about 25,000 light-years from Earth in ISM, may be a precursor to a key component of DNA and the other may have a role in the formation of an important amino acid. Researchers found a molecule called cyanomethanimine, which produces adenine, one of the four nucleobases that form the "rungs" in the ladder-like structure of DNA. The other molecule, called ethanamine, is thought to play a role in forming alanine, one of the twenty amino acids in the genetic code. Previously, scientists thought such processes took place in the very tenuous gas between the stars. The new discoveries, however, suggest that the chemical formation sequences for these molecules occurred not in gas, but on the surfaces of ice grains in interstellar space. NASA ALMA scientist Anthony Remijan stated that finding these molecules in an interstellar gas cloud means that important building blocks for DNA and amino acids can 'seed' newly formed planets with the chemical precursors for life.
In January 2014, NASA reported that current studies on the planet Mars by the Curiosity and Opportunity rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.
In February 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, 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.
Death is the permanent termination of all vital functions or life processes in an organism or cell. It can occur as a result of an accident, medical conditions, biological interaction, malnutrition, poisoning, senescence, or suicide. After death, the remains of an organism re-enter the biogeochemical cycle. Organisms may be consumed by a predator or a scavenger and leftover organic material may then be further decomposed by detritivores, organisms that recycle detritus, returning it to the environment for reuse in the food chain.
One of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment life ends, or when the state that follows life begins. However, determining when death has occurred requires drawing precise conceptual boundaries between life and death. This is problematic, however, because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world's religious traditions and of philosophical inquiry. Many religions maintain faith in either a kind of afterlife or reincarnation for the soul, or resurrection of the body at a later date.
Extinction is the process by which a group of taxa or species dies out, reducing biodiversity. The moment of extinction is generally considered the death of the last individual of that species. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively after a period of apparent absence. Species become extinct when they are no longer able to survive in changing habitat or against superior competition. In Earth's history, over 99% of all the species that have ever lived have gone extinct; however, mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.
Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossil-containing rock formations and sedimentary layers (strata) is known as the fossil record. A preserved specimen is called a fossil if it is older than the arbitrary date of 10,000 years ago. Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest from the Archaean Eon, up to 3.4 billion years old.
Artificial life is a field of study that examines systems related to life, its processes, and its evolution through simulations using computer models, robotics, and biochemistry. The study of artificial life imitates traditional biology by recreating some aspects of biological phenomena. Scientists study the logic of living systems by creating artificial environments—seeking to understand the complex information processing that defines such systems. While life is, by definition, alive, artificial life is generally referred to as data confined to a digital environment and existence.
Synthetic biology is a new area of biological research and technology that combines science and biological engineering. The common goal is the design and construction of new biological functions and systems not found in nature. Synthetic biology includes the broad redefinition and expansion of biotechnology, with the ultimate goals of being able to design and build engineered biological systems that process information, manipulate chemicals, fabricate materials and structures, produce energy, provide food, and maintain and enhance human health and our environment.
- Koshland, Jr., Daniel E. (March 22, 2002). "The Seven Pillars of Life". Science 295 (5563): 2215–2216. doi:10.1126/science.1068489. PMID 11910092. Retrieved 2009-05-25.
- The American Heritage Dictionary of the English Language, 4th edition, published by Houghton Mifflin Company, via Answers.com:
- "The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment originating from within the organism."
- "The characteristic state or condition of a living organism."
- Definition of inanimate. WordNet Search by Princeton University.
- "Merriam-Webster Dictionary". Merriam-Webster Dictionary. Retrieved 2009-06-21.
- "organism". Chambers 21st Century Dictionary (online ed.). Chambers Publishers Ltd. 1999. Retrieved 2012-05-26.
- Schopf, JW, Kudryavtsev, AB, Czaja, AD, and Tripathi, AB. (2007). Evidence of Archean life: Stromatolites and microfossils. Precambrian Research 158:141-155.
- Schopf, JW (2006). Fossil evidence of Archaean life. Philos Trans R Soc Lond B Biol Sci 29;361(1470) 869-85.
- Hamilton Raven, Peter; Brooks Johnson, George (2002). Biology. McGraw-Hill Education. p. 68. ISBN 978-0-07-112261-0. Retrieved 2013-07-07.
- Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; Nagase, Toshiro; Rosing, Minik T. (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. doi:10.1038/ngeo2025.
- Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News.
- Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology (journal). Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030.
- Tenenbaum, David (October 14, 2002). "When Did Life on Earth Begin? Ask a Rock". Astrobiology Magazine. Retrieved April 13, 2014.
- Courtland, Rachel (July 2, 2008). "Did newborn Earth harbour life?". New Scientist. Retrieved April 13, 2014.
- Steenhuysen, Julie (May 20, 2009). "Study turns back clock on origins of life on Earth". Reuters. Retrieved April 13, 2014.
- Race, Margaret S.; Randolph, Richard O. (2002). "The need for operating guidelines and a decision making framework applicable to the discovery of non-intelligent extraterrestrial life". Advances in Space Research 30 (6): 1583–1591. Bibcode:2002AdSpR..30.1583R. doi:10.1016/S0273-1177(02)00478-7. ISSN 0273-1177. "There is growing scientific confidence that the discovery of extraterrestrial life in some form is nearly inevitable"
- Cantor, Matt (15 Feb 2009). "Alien Life 'Inevitable': Astronomer". newser. Archived from the original on 2013-05-03. Retrieved 2013-05-03. "Scientists now believe there could be as many habitable planets in the cosmos as there are stars, and that makes life's existence elsewhere "inevitable" over billions of years, says one."
- Rampelotto, P. H. (2010). Panspermia: A promising field of research. In: Astrobiology Science Conference. Abs 5224.
- Parry, Richard (March 4, 2005). "Empedocles". Stanford Encyclopedia of Philosophy. Retrieved 2012-05-25.
- Parry, Richard (August 25, 2010). "Democritus". Stanford Encyclopedia of Philosophy. Retrieved 2012-05-25.
- Hankinson, R. J. (1997). Cause and Explanation in Ancient Greek Thought. Oxford University Press. p. 125. ISBN 978-0-19-924656-4.
- Thagard, Paul (2012). The Cognitive Science of Science: Explanation, Discovery, and Conceptual Change. MIT Press. pp. 204–205. ISBN 0262017288.
- Aristotle, De Anima, Book II
- Marietta, Don (1998). Introduction to ancient philosophy. M. E. Sharpe. p. 104. ISBN 0765602164.
- Stewart-Williams, Steve (2010). Darwin, God and the meaning of life: how evolutionary theory undermines everything you thought you knew of life. Cambridge University Press. pp. 193–194. ISBN 0521762782
- Schwartz, Sanford (2009). C. S. Lewis on the Final Frontier: Science and the Supernatural in the Space Trilogy. Oxford University Press. p. 56. ISBN 0199888396.
- Wilkinson, Ian (1998). "History of Clinical Chemistry – Wöhler & the Birth of Clinical Chemistry". The Journal of the International Federation of Clinical Chemistry and Laboratory Medicine 13 (4). Retrieved 0212-06-12.
- Friedrich Wöhler (1828). "Ueber künstliche Bildung des Harnstoffs". Annalen der Physik und Chemie 88 (2): 253–256. Bibcode:1828AnP....88..253W. doi:10.1002/andp.18280880206.
- Rabinbach, Anson (1992). The Human Motor: Energy, Fatigue, and the Origins of Modernity. University of California Press. pp. 124–125. ISBN 0520078276.
- "NCAHF position paper on Homeopathy". National Council Against Health Fraud. February 1994. Retrieved 2012-06-12.
- Mullen, Leslie (June 19, 2002). "Defining Life". Origin & Evolution of Life. Astrobiology. Retrieved 2012-05-25.
- Emmeche, Claus (1997). "Defining Life, Explaining Emergence". Niels Bohr Institute. Retrieved 2012-05-25.
- "Can We Define Life". Colorado Arts & Sciences. 2009. Retrieved 2009-06-22.
- Mautner, Michael N. (1997). "Directed panspermia. 3. Strategies and motivation for seeding star-forming clouds". Journal of the British Interplanetary Society 50: 93–102. Bibcode:1997JBIS...50...93M.
- Mautner, Michael N. (2000). Seeding the Universe with Life: Securing Our Cosmological Future. Washington D. C.: Legacy Books (www.amazon.com). ISBN 0-476-00330-X.
- Nealson, K. H.; Conrad, P. G. (December 1999). "Life: past, present and future". Philosophical Transactions of the Royal Society B 354 (1392): 1923–39. doi:10.1098/rstb.1999.0532. PMC 1692713. PMID 10670014.
- McKay, Chris P. (September 14, 2004). "What Is Life—and How Do We Search for It in Other Worlds?". Public Library of Science – Biology 2 (2(9)): 302. doi:10.1371/journal.pbio.0020302. PMC 516796. PMID 15367939.
- Mautner, Michael N. (2009). "Life-centered ethics, and the human future in space". Bioethics 23 (8): 433–440. doi:10.1111/j.1467-8519.2008.00688.x. PMID 19077128.
- Jabr, Ferris (March 12, 2014). "Why Nothing Is Truly Alive". New York Times. Retrieved March 12, 2014.
- Davison, Paul G. "How to Define Life". The University of North Alabama. Retrieved 2008-10-17.
- "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Retrieved 2013-06-06.
- Trifonov, Edward N. (2012). "Definition of Life: Navigation through Uncertainties". Journal of Biomolecular Structure & Dynamics (Adenine Press) 29 (4): 647–650. doi:10.1080/073911012010525017. ISSN 0739-1102. Retrieved 2012-01-12.
- Zimmer, Carl (January 11, 2012). "Can scientists define 'life' ... using just three words?". MSN. Retrieved 2012-01-12.
- Popa, Radu (March 2004). Between Necessity and Probability: Searching for the Definition and Origin of Life (Advances in Astrobiology and Biogeophysics). Springer. ISBN 3-540-20490-3.
- Schrödinger, Erwin (1944). What is Life?. Cambridge University Press. ISBN 0-521-42708-8.
- Margulis, Lynn; Sagan, Dorion (1995). What is Life?. University of California Press. ISBN 0-520-22021-8.
- Nahle, Nasif Sabag (September 26, 2006). "Astrobiology". Biology Cabinet Organization. Retrieved 2011-01-17.
- Lovelock, James (2000). Gaia – a New Look at Life on Earth. Oxford University Press. ISBN 0-19-286218-9.
- Avery, John (2003). Information Theory and Evolution. World Scientific. ISBN 981-238-399-9.
- Nahle, Nasif Sabag (September 29, 2006). "Biophysics: definition of life and brief explanation of each term". Exobiology. Biology Cabinet Organization. Retrieved 2012-05-27.
- Luttermoser, Donald G. "ASTR-1020: Astronomy II Course Lecture Notes Section XII" (PDF). East Tennessee State University. Retrieved 2011-08-28.
- Luttermoser, Donald G. (Spring 2008). "Physics 2028: Great Ideas in Science: The Exobiology Module" (PDF). East Tennessee State University. Retrieved 2011-08-28.
- Joyce, Gerald F. (1995). The RNA world: life before DNA and protein. Cambridge University Press. pp. 139–151. doi:10.1017/CBO9780511564970.017. Retrieved 2012-05-27.
- Kaufmann, Stuart (2004). "Autonomous agents". In Barrow, John D.; Davies, P. C. W.; Harper, Jr., C. L. Science and Ultimate Reality: Quantum Theory, Cosmology, and Complexity (Cambridge University Press): 654–666. ISBN 052183113X.
- Rybicki, EP (1990). "The classification of organisms at the edge of life, or problems with virus systematics". S Aft J Sci 86: 182–186.
- Holmes, E. C. (October 2007). "Viral evolution in the genomic age". PLoS Biol. 5 (10): e278. doi:10.1371/journal.pbio.0050278. PMC 1994994. PMID 17914905. Retrieved 2008-09-13.
- Forterre, Patrick (3 March 2010). "Defining Life: The Virus Viewpoint". Orig Life Evol Biosph. 40 (2): 151–160. Bibcode:2010OLEB...40..151F. doi:10.1007/s11084-010-9194-1. PMC 2837877. PMID 20198436. Retrieved 2013-04-27.
- Koonin, E. V.; Senkevich, T. G.; Dolja, V. V. (2006). "The ancient Virus World and evolution of cells". Biology Direct 1: 29. doi:10.1186/1745-6150-1-29. PMC 1594570. PMID 16984643. Retrieved 2008-09-14.
- Rybicki, Ed (November 1997). "Origins of Viruses". Retrieved 2009-04-12.
- "Giant Viruses Shake Up Tree of LIfe". journal BMC Evolutionary Biology (Astrobiology Magazine). September 15, 2012. Retrieved 2012-09-18.
- Lovecock, James (1979). GAIA – A new look at life on Earth. Oxford University Press. p. 10. ISBN 0-19-286030-5.
- Lovelock, J. E. (1965). "A physical basis for life detection experiments". Nature 207 (7): 568–570. Bibcode:1965Natur.207..568L. doi:10.1038/207568a0. PMID 5883628.
- Lovelock, James. "Geophysiology". Papers by James Lovelock.
- Lovelock, James (1979). GAIA – A new look at life on Earth. Oxford University Press. ISBN 0-19-286030-5.
- Woodruff, T. Sullivan; John Baross (October 8, 2007). Planets and Life: The Emerging Science of Astrobiology. Cambridge University Press. ISBN 0521824214. Cleland and Chyba wrote a chapter in Planets and Life: "In the absence of such a theory, we are in a position analogous to that of a 16th-century investigator trying to define 'water' in the absence of molecular theory." [...] "Without access to living things having a different historical origin, it is difficult and perhaps ultimately impossible to formulate an adequately general theory of the nature of living systems".
- Brown, Molly Young (2002). "Patterns, flows, and interrelationship". Psychosynthesis and Ecopsychology. Retrieved 2012-05-27.
- Robert, Rosen (November 1991). Life Itself: A Comprehensive Inquiry into the Nature, Origin, and Fabrication of Life. ISBN 978-0-231-07565-7.
- "The Ecosystemic Life Hypothesis". Bulletin of the Ecological Society of America. April 2002. Retrieved 2009-08-28.
- Morowitz, Harold J. (1992). Beginnings of cellular life: metabolism recapitulates biogenesis. Yale University Press. ISBN 0-300-05483-1.
- Ulanowicz, Robert W.; Ulanowicz, Robert E. (2009). A third window: natural life beyond Newton and Darwin. Templeton Foundation Press. ISBN 1-59947-154-X.
- Baianu, I. C.: 2006, "Robert Rosen's Work and Complex Systems Biology", Axiomathes 16(1-2):25-34.
- * Rosen, R.: 1958a, "A Relational Theory of Biological Systems". Bulletin of Mathematical Biophysics 20: 245-260.
- * Rosen, R.: 1958b, "The Representation of Biological Systems from the Standpoint of the Theory of Categories.", Bulletin of Mathematical Biophysics 20: 317-341.
- Bernstein H, Byerly HC, Hopf FA, Michod RE, and Vemulapalli GK. (1983) The Darwinian D ynamic. The Quarterly Review of Biology 58(2): 185-207. Published by: The University of Chicago Press
- Michod RE. (1999) Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality. Princeton University Press, Princeton, New Jersey ISBN 0691050112, 9780691050119
- Vol 58, No. 2, Harris Bernstein, Henry C. Byerly, Frederick A. Hopf, Richard A. Michod and G. Krishna Vemulapalli (June 1983). "The Darwinian Dynamic". The Quarterly Review of Biology (The University of Chicago Press). JSTOR 2828805.
- The pursuit of complexity KNNV Publishing, Zeist, The Netherlands, (2012) pages: 27-29, 87-88 and 94-96.
- Towards a hierarchical definition of life, the organism, and death. Jagers op Akkerhuis G.A.J.M. (2010). Foundations of Science 15: 245-262.
- Explaining the origin of life is not enough for a definition of life. Jagers op Akkerhuis G.A.J.M. (2010). Foundations of Science 16: 327-329.
- The Role of Logic and Insight in the Search for a Definition of Life. Jagers op Akkerhuis G.A.J.M. (2012). J. Biomol Struct Dyn 29(4), 619-620 (2012).
- Contributions of the Operator Hierarchy to the field of biologically driven mathematics and computation. Jagers op Akkerhuis G.A.J.M. (2012). In: Integral Biomathics: Tracing the Road to Reality.
- Korzeniewski, Bernard (April 7, 2001). "Cybernetic formulation of the definition of life". Journal of Theoretical Biology 209 (3): 275–86. doi:10.1006/jtbi.2001.2262. PMID 11312589.
- Milsom, Clare; Rigby, Sue (2009). Fossils at a Glance (2nd ed.). John Wiley & Sons. p. 134. ISBN 1405193360.
- Coveney, Peter V.; Fowler, Philip W. (2005). "Modelling biological complexity: a physical scientist's perspective". Journal of the Royal Society Interface 2 (4): 267–280. doi:10.1098/rsif.2005.0045.
- "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Retrieved 2013-06-06.
- Senapathy, Periannan (1994). Independent birth of organisms. Madison, Wisconsin: Genome Press. ISBN 0964130408.
- Eigen, Manfred; Winkler, Ruthild (1992). Steps towards life: a perspective on evolution (German edition, 1987). Oxford University Press. p. 31. ISBN 019854751X.
- Barazesh, Solmaz (May 13, 2009). "How RNA Got Started: Scientists Look for the Origins of Life". Science News. Retrieved 2012-05-25.
- Watson, James D. (1993). "Prologue: early speculations and facts about RNA templates". In Gesteland, R. F.; Atkins, J. F. The RNA World (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press). pp. xv–xxiii.
- Gilbert, Walter (February 20, 1986). "Origin of life: The RNA world". Nature 319 (618): 618. Bibcode:1986Natur.319..618G. doi:10.1038/319618a0.
- Cech, Thomas R. (1986). "A model for the RNA-catalyzed replication of RNA". Proceedings of the National Academy of Science USA 83 (12): 4360–4363. Bibcode:1986PNAS...83.4360C. doi:10.1073/pnas.83.12.4360. Retrieved 2012-05-25.
- Cech, T.R. (2011). The RNA Worlds in Context. Source: Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215. Cold Spring Harb Perspect Biol. 2011 Feb 16. pii: cshperspect.a006742v1. doi:10.1101/cshperspect.a006742.
- Powner, Matthew W.; Gerland, Béatrice; Sutherland, John D. (May 14, 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.
- Szostak, Jack W. (May 14, 2009). "Origins of life: Systems chemistry on early Earth". Nature 459 (7244): 171–172. Bibcode:2009Natur.459..171S. doi:10.1038/459171a. PMID 19444196.
- Pasek, Matthew A.; et at. (18 June 2013). "Evidence for reactive reduced phosphorus species in the early Archean ocean". PNAS 110 (25): 10089–10094. doi:10.1073/pnas.1303904110. Retrieved 16 July 2013.
- Lincoln, Tracey A.; Joyce, Gerald F. (February 27, 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 Symposium on Quantitative Biology 74: 17–23. doi:10.1101/sqb.2009.74.004. PMC 2891321. PMID 19667013.
- Callahan; Smith, K.E.; Cleaves, H.J.; Ruzica, J.; Stern, J.C.; Glavin, D.P.; House, C.H.; Dworkin, J.P. (11 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". PNAS. doi:10.1073/pnas.1106493108. Retrieved 15 August 2011.
- Steigerwald, John (August 8, 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". NASA. Retrieved 2011-08-10.
- ScienceDaily Staff (August 9, 2011). "DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests". ScienceDaily. Retrieved 2011-08-09.
- Gallori, Enzo (November 2010). "Astrochemistry and the origin of genetic material". Rendiconti Lincei 22 (2): 113–118. doi:10.1007/s12210-011-0118-4. Retrieved 2011-08-11.
- Rothschild, Lynn (September 2003). "Understand the evolutionary mechanisms and environmental limits of life". NASA. Retrieved 2009-07-13.
- King, G.A.M. (April 1977). "Symbiosis and the origin of life". Origins of Life and Evolution of Biospheres 8 (1): 39–53. Bibcode:1977OrLi....8...39K. doi:10.1007/BF00930938. Retrieved 2010-02-22.
- Margulis, Lynn (2001). The Symbiotic Planet: A New Look at Evolution. London, England: Orion Books Ltd. ISBN 0-7538-0785-8.
- Douglas J. Futuyma; Janis Antonovics (1992). Oxford surveys in evolutionary biology: Symbiosis in evolution 8. London, England: Oxford University Press. pp. 347–374. ISBN 0-19-507623-0.
- Hotz, Robert Lee (December 3, 2010). "New link in chain of life". Wall Street Journal (Dow Jones & Company, Inc). "Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA."
- Neuhaus, Scott (2005). Handbook for the Deep Ecologist: What Everyone Should Know About Self, the Environment, And the Planet. iUniverse. pp. 23–50. ISBN 059535789X
- Committee on the Limits of Organic Life in Planetary Systems; Committee on the Origins and Evolution of Life; National Research Council (2007). The Limits of Organic Life in Planetary Systems. National Academy of Sciences. ISBN 0-309-66906-5. Retrieved 2012-06-03.
- Benner, Steven A.; Ricardo, Alonso; Carrigan, Matthew A. (December 2004). "Is there a common chemical model for life in the universe?". Current Opinion in Chemical Biology 8 (6): 672–689. doi:10.1016/j.cbpa.2004.10.003. PMID 15556414. Retrieved 2012-06-03.
- "Essential requirements for life". CMEX-NASA. Retrieved 2009-07-14.
- Chiras, Daniel C. (2001). Environmental Science – Creating a Sustainable Future (6th ed.). ISBN 0763713163.
- Rampelotto, Pabulo Henrique (2010). "Resistance of microorganisms to extreme environmental conditions and its contribution to astrobiology". Sustainability 2 (6): 1602–1623. Bibcode:2010Sust....2.1602R. doi:10.3390/su2061602.
- Choi, Charles Q. (17 March 2013). "Microbes Thrive in Deepest Spot on Earth". LiveScience. Retrieved 17 March 2013.
- Glud, Ronnie; Wenzhöfer, Frank; Middleboe, Mathias; Oguri, Kazumasa; Turnewitsch, Robert; Canfield, Donald E.; Kitazato, Hiroshi (17 March 2013). "High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth". Nature Geoscience. Bibcode:2013NatGe...6..284G. doi:10.1038/ngeo1773. Retrieved 17 March 2013.
- Oskin, Becky (14 March 2013). "Intraterrestrials: Life Thrives in Ocean Floor". LiveScience. Retrieved 17 March 2013.
- Baldwin, Emily (April 26, 2012). "Lichen survives harsh Mars environment". Skymania News. Retrieved 27 April 2012.
- de Vera, J.-P.; Kohler, Ulrich (April 26, 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars". European Geosciences Union. Retrieved 27 April 2012.
- Sapp, Jan (2003). Genesis: The Evolution of Biology. Oxford University Press. pp. 75–78. ISBN 0195156196.
- Lintilhac, P. M. (Jan 1999). "Thinking of biology: toward a theory of cellularity--speculations on the nature of the living cell". BioScience 49 (1): 59–68. doi:10.2307/1313494. PMID 11543344. Retrieved 2012-06-02.
- Whitman, W.; Coleman, D.; Wiebe, W. (1998). "Prokaryotes: The unseen majority". Proceedings of the National Academy of Science of the United States of America 95 (12): 6578–83. Bibcode:1998PNAS...95.6578W. doi:10.1073/pnas.95.12.6578. PMC 33863. PMID 9618454.
- Pace, Norman R. (May 18, 2006). "Concept Time for a change". Nature 441 (7091): 289. Bibcode:2006Natur.441..289P. doi:10.1038/441289a. PMID 16710401. Retrieved 2012-06-02.
- "Scientific background". The Nobel Prize in Chemistry 2009. Royal Swedish Academy of Sciences. Retrieved 2012-06-10.
- Panno, Joseph (2004). The Cell. Facts on File science library. Infobase Publishing. pp. 60–70. ISBN 0816067368.
- Alberts, Bruce et al. (1994). "From Single Cells to Multicellular Organisms". Molecular Biology of the Cell (3rd ed.). New York: Garland Science. ISBN 0-8153-1620-8. Retrieved 2012-06-12.
- Alberts, Bruce et al. (2002). "General Principles of Cell Communication". Molecular Biology of the Cell. New York: Garland Science. ISBN 0-8153-3218-1. Retrieved 2012-06-12.
- "Aristotle -biography". University of California Museum of Paleontology. Retrieved 2008-10-20.
- Knapp S, Lamas G, Lughadha EN, Novarino G (April 2004). "Stability or stasis in the names of organisms: the evolving codes of nomenclature". Philosophical Transactions of the Royal Society B 359 (1444): 611–22. doi:10.1098/rstb.2003.1445. PMC 1693349. PMID 15253348.
- Copeland, H.F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology 13 (4): 383. doi:10.1086/394568. JSTOR 2808554.
- Whittaker, R. H. (January 1969). "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science 163 (3863): 150–60. Bibcode:1969Sci...163..150W. doi:10.1126/science.163.3863.150. PMID 5762760.
- Woese, C.; Kandler, O.; Wheelis, M. (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya.". Proceedings of the National Academy of Sciences of the United States of America 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
- Adl SM, Simpson AG, Farmer MA, et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". J. Eukaryot. Microbiol. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873.
- Van Regenmortel MH (January 2007). "Virus species and virus identification: past and current controversies". Infection, Genetics and Evolution 7 (1): 133–44. doi:10.1016/j.meegid.2006.04.002. PMID 16713373.
- Pennisi E (March 2001). "Taxonomy. Linnaeus's last stand?". Science (New York, N.Y.) 291 (5512): 2304–7. doi:10.1126/science.291.5512.2304. PMID 11269295.
- Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.
- Haeckel, E. (1866). Generelle Morphologie der Organismen. Reimer, Berlin.
- Chatton, É. (1925). "Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires". Annales des Sciences Naturelles - Zoologie et Biologie Animale. 10-VII: 1–84.
- Whittaker, R. H. (January 1969). "New concepts of kingdoms of organisms". Science 163 (3863): 150–60. Bibcode:1969Sci...163..150W. doi:10.1126/science.163.3863.150. PMID 5762760.
- Cavalier-Smith, T. (1998). "A revised six-kingdom system of life". Biological Reviews 73 (03): 203–66. doi:10.1111/j.1469-185X.1998.tb00030.x. PMID 9809012.
- Schulze-Makuch, Dirk; Dohm, James M.; Fairén, Alberto G.; Baker, Victor R.; Fink, Wolfgang; Strom, Robert G. (December 2005). "Venus, Mars, and the Ices on Mercury and the Moon: Astrobiological Implications and Proposed Mission Designs". Astrobiology 5 (6). pp. 778–795. Bibcode:2005AsBio...5..778S. doi:10.1089/ast.2005.5.778.
- Strain, Daniel (December 14, 2009). "Icy moons of Saturn and Jupiter may have conditions needed for life". The University of Santa Cruz. Retrieved 2012-07-04.
- Vakoch, Douglas A.; Harrison, Albert A. (2011). Civilizations beyond Earth: extraterrestrial life and society. Berghahn Series. Berghahn Books. pp. 37–41. ISBN 0857452118
- Selis, Frank (2006). "Habitability: the point of view of an astronomer". In Gargaud, Muriel; Martin, Hervé; Claeys, Philippe. Lectures in Astrobiology 2. Springer. pp. 210–214. ISBN 3540336923
- Lineweaver, Charles H.; Fenner, Yeshe; Gibson, Brad K. (January 2004). "The Galactic Habitable Zone and the age distribution of complex life in the Milky Way". Science 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode:2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421.
- Mautner, Michael N. (2002). "Planetary bioresources and astroecology. 1. Planetary microcosm bioessays of Martian and meteorite materials: soluble electrolytes, nutrients, and algal and plant responses". Icarus 158 (1): 72–86. Bibcode:2002Icar..158...72M. doi:10.1006/icar.2002.6841. PMID 12449855.
- Battersby, S. (2004). "Space molecules point to organic origins". New Scientist. Retrieved 11 December 2009.
- Mulas, G.; Malloci, G.; Joblin, C.; Toublanc, D. (2006). "Estimated IR and phosphorescence emission fluxes for specific polycyclic aromatic hydrocarbons in the Red Rectangle". Astronomy and Astrophysics 446 (2): 537. arXiv:astro-ph/0509586. Bibcode:2006A&A...446..537M. doi:10.1051/0004-6361:20053738.
- Staff (18 August 2009). "'Life chemical' detected in comet". NASA (BBC News). Retrieved 6 March 2010.
- García-Hernández, D. A.; Manchado, A.; García-Lario, P.; Stanghellini, L.; Villaver, E.; Shaw, R. A.; Szczerba, R.; Perea-Calderón, J. V. (2010-10-28). "Formation Of Fullerenes In H-Containing Planatary Nebulae". The Astrophysical Journal Letters 724 (1): L39–L43. arXiv:1009.4357. Bibcode:2010ApJ...724L..39G. doi:10.1088/2041-8205/724/1/L39.
- Atkinson, Nancy (2010-10-27). "Buckyballs Could Be Plentiful in the Universe". Universe Today. Retrieved 2010-10-28.
- Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Retrieved 2011-10-26.
- ScienceDaily Staff (26 October 2011). "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". ScienceDaily. Retrieved 2011-10-27.
- Kwok, Sun; Zhang, Yong (26 October 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature 479 (7371): 80. Bibcode:2011Natur.479...80K. doi:10.1038/nature10542. PMID 22031328.
- Martins, Zita (February 2011). "Organic Chemistry of Carbonaceous Meteorites". Elements 7 (1): 35–40. doi:10.2113/gselements.7.1.35. Retrieved 2011-08-11.
- Than, Ker (August 29, 2012). "Sugar Found In Space". National Geographic. Retrieved August 31, 2012.
- Staff (August 29, 2012). "Sweet! Astronomers spot sugar molecule near star". AP News. Retrieved August 31, 2012.
- Jørgensen, J. K.; Favre; Bisschop; Bourke; Van Dishoeck; Schmalzl; Favre, C.; Bisschop, S.; Bourke, T.; Dishoeck, E.; Schmalzl, M. (2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA". The Astrophysical Journal Letters. eprint 757: L4. arXiv:1208.5498. Bibcode:2012ApJ...757L...4J. doi:10.1088/2041-8205/757/1/L4.
- Staff (September 20, 2012). "NASA Cooks Up Icy Organics to Mimic Life's Origins". Space.com. Retrieved September 22, 2012.
- Gudipati, Murthy S.; Yang, Rui (September 1, 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. Retrieved September 22, 2012.
- López-Puertas, Manuel (June 6, 2013). "PAH's in Titan's Upper Atmosphere". CSIC. Retrieved June 6, 2013.
- Loomis, Ryan A.; Zaleski, Daniel P.; Steber, Amanda L.; Neill, Justin L.; Muckle, Matthew T.; Harris, Brent J.; Hollis, Jan M.; Jewell, Philip R.; Lattanzi, Valerio; Lovas, Frank J.; Martinez, Oscar; McCarthy, Michael C.; Remijan, Anthony J.; Pate, Brooks H.; Corby, Joanna F. (2013). "The Detection of Interstellar Ethanimine (Ch3Chnh) from Observations Taken During the Gbt Primos Survey". The Astrophysical Journal 765: L9. Bibcode:2013ApJ...765L...9L. doi:10.1088/2041-8205/765/1/L9.
- Finley, Dave, Discoveries Suggest Icy Cosmic Start for Amino Acids and DNA Ingredients, The National Radio Astronomy Observatory, Feb. 28, 2013
- Grotzinger, John P. (January 24, 2014). "Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science 343 (6169): 386–387. doi:10.1126/science.1249944. Retrieved January 24, 2014.
- Various (January 24, 2014). "Special Issue - Table of Contents - Exploring Martian Habitability". Science 343 (6169): 345–452. Retrieved 24 January 2014.
- Various (January 24, 2014). "Special Collection - Curiosity - Exploring Martian Habitability". Science. Retrieved January 24, 2014.
- Grotzinger, J.P. et al. (January 24, 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science 343 (6169). doi:10.1126/science.1242777. Retrieved January 24, 2014.
- Hoover, Rachel (February 21, 2014). "Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That". NASA. Retrieved February 22, 2014.
- Definition of death. Archived from the original on 2009-10-31.
- "Definition of death". Encyclopedia of Death and Dying. Advameg, Inc. Retrieved 2012-05-25.
- Extinction – definition. Archived from the original on 2009-10-31.
- "What is an extinction?". Late Triassic. Bristol University. Retrieved 2012-06-5-27.
- Van Valkenburgh, B. (1999). "Major patterns in the history of carnivorous mammals". Annual Review of Earth and Planetary Sciences 27: 463–493. Bibcode:1999AREPS..27..463V. doi:10.1146/annurev.earth.27.1.463.
- "Frequently asked questions". San Diego Natural History Museum. Retrieved 2012-05-25.
- Vastag, Brian (August 21, 2011). "Oldest 'microfossils' raise hopes for life on Mars". The Washington Post. Retrieved 2011-08-21.
- Wade, Nicholas (August 21, 2011). "Geological Team Lays Claim to Oldest Known Fossils". The New York Times. Retrieved 2011-08-21.
- "Dictionary.com definition". Retrieved 2007-01-19.
- Chopra, Paras; Akhil Kamma. "Engineering life through Synthetic Biology". In Silico Biology 6. Retrieved 2008-06-09.
- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman
- Seeding the Universe With Life Legacy Books, Washington D. C., 2000, ISBN 0-476-00330-X
- Walker, Martin G. LIFE! Why We Exist...And What We Must Do to Survive Dog Ear Publishing, 2006, ISBN 1-59858-243-7
- Adrian Bejan, Sylvie Lorente, The constructal law and the evolution of design in nature, Physics of Life Reviews, Volume 8, Issue 3, October 2011, Pages 209-240, ISSN 1571-0645, 
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