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History of life

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Part of the Biology series on
Evolution
Mechanisms and processes
Research and history
Evolutionary biology fields

The evolutionary history of life and origin of life are fields on ongoing geological and biological research. Although not necessary conditions for the acceptance of evolution by natural selection,[1] the origin of life and its evolutionary history can nonetheless help shed light on evolutionary processes. The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred.[2] Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any last universal common ancestor or ancestral gene pool.[3][4] Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as RNA,[5] and the assembly of simple cells.[6]

The first simple, sea dwelling organic structures appeared about 3,400 million years ago. It is considered that they may have formed when certain chemical (organic) molecules joined together. Prokaryotes, single-celled micro-organisms like blue green algae, were able to photosynthesize and produce oxygen. Around thousand million years later, sufficient oxygen had built up in the atmosphere and hence it allow multicellular organisms to proliferate in the Precambrian seas.Soft-bodied jellyfish , corals, and sea worms flourished about 700 Million years ago. Trilobites, the first animals with hard body frames, developed during the Cambrian period. About 363 million years ago amphibians came into existence. It was only after the appearence of reptiles that were independent of water.

Early signs of life

The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.
Main article: Origin of life

The details of the origin of life are unknown, though the broad principles have been established. It has been proposed that life, or at least organic components, may have arrived on Earth from space (see “Panspermia”), while others argue that terrestrial origins are more probable. The mechanisms by which life would initially arise are nevertheless held to be similar.[7] The timing of the origin of life is highly speculative—perhaps it arose around 4 billion years ago.[8] Somehow, in the energetic chemistry of early Earth, a molecule (or even something else) gained the ability to make copies of itself–the replicator. The nature of this molecule is unknown, its function having long since been superseded by life’s current replicator, DNA. In making copies of itself, the replicator did not always perform accurately: some copies contained an “error.” If the change destroyed the copying ability of the molecule, there could be no more copies, and the line would “die out.” On the other hand, a few rare changes might make the molecule replicate faster or better: those “strains” would become more numerous and “successful.” As choice raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.[9]

Several different models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins, nucleic acids, phospholipids, crystals,[10] or even quantum systems.[11] There is currently no method of determining which of these models, if any, closely fits the origin of life on Earth. One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur.

The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia.[12] Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this “organic soup” increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material.[13] The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance there arose a new molecule: the replicator. This had the bizarre property of promoting the chemical reactions which produced a copy of itself, and evolution began properly.

Other theories posit a different replicator. In any case, DNA took over the function of the replicator at some point; all known life (with the exception of some viruses and prions) use DNA as their replicator, in an almost identical manner (see genetic code). The first simple, sea dwelling organic structures appeared about 3,400 million years ago. It is considered that they may have formed when certain chemical (organic) molecules joined together. Prokaryotes, single-celled micro-organisms like blue green algae, were able to photosynthesize and produce oxygen. Around thousand million years later, sufficient oxygen had built up in the atmosphere and hence it allow multicellular organisms to proliferate in the Precambrian seas.

Common descent

Further information: Evidence of common descent, Common descent, and Homology (biology)
The hominoids are descendants of a common ancestor.

All organisms on Earth are descended from a common ancestor or ancestral gene pool.[14] Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.[15] The common descent of organisms was first deduced from four simple facts about organisms: Firstly, they have geographic distributions that cannot be explained by local adaptation. Secondly, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Thirdly, vestigial traits with no clear purpose resemble functional ancestral traits, and finally, that organisms can be classified using these similarities into a hierarchy of nested groups.

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.[16] By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Furthermore, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same nucleic acids and amino acids.[17] The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations.[18] For example, these DNA sequence comparisons have revealed the close genetic similarity between humans and chimpanzees and shed light on when the common ancestor of these species existed.[19]

Evolution of life

Evolutionary tree showing the divergence of modern species from their common ancestor in the center.[20] The three domains are colored, with bacteria blue, archaea green, and eukaryotes red.
Further information: Timeline of evolution

Despite the uncertainty on how life began, it is clear that prokaryotes were the first organisms to inhabit Earth,[21] approximately 3–4 billion years ago.[22] No obvious changes in morphology or cellular organization occurred in these organisms over the next few billion years.[23]

The eukaryotes were the next major innovation in evolution. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in an cooperative association called endosymbiosis.[24][25] The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes.[26] An independent second engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.[27]

As these early eukaryotes and prokaryotes were microscopic, most of the history of life describes simple microorganisms. It is only about a billion years ago in the Ediacaran period that complex multicellular organisms began to appear in the oceans.[21][28] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.[29]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals evolved, as well as unique lineages that subsequently became extinct.[30] Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.[31] About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals.[32] Amphibians first appeared around 300 million years ago, followed by early amniotes, then mammals around 200 million years ago and birds around 100 million years ago (both from "reptile"-like lineages). However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.

Life before Cambrian

Main article: Precambrian

The Precambrian extends from 4.6 billion years ago to the beginning of the Cambrian Period (about 570 Ma). It is not known when life originated, but carbon in 3800 million year old rocks from islands of western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian. [33]

Excepting a few contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan.

A very diverse collection of forms appeared around 544 Ma, starting in the latest Precambrian with a poorly understood small shelly fauna and ending in the very early Cambrian with a very diverse, and quite modern Burgess fauna, the rapid radiation of forms called the Cambrian explosion of life.

Life during Phanerozoic

Paleozoic life

Main article: Paleozoic
Trilobites flourished throughout the lower Paleozoic era until becoming extinct in the Permian period

The Paleozoic spanned from roughly 542 mya to roughly 251 mya (ICS 2004). The Paleozoic covers the time from the first appearance of abundant, hard-shelled fossils to the time when the continents were beginning to be dominated by large, relatively sophisticated reptiles and relatively modern plants. The lower (oldest) boundary was classically set at the first appearance of creatures known as trilobites and archeocyathids. The upper (youngest) boundary is set at a major extinction event 300 million years later, known as the Permian extinction. Modern practice sets the older boundary at the first appearance of a distinctive trace fossil called Trichophycus pedum.

At the start of the era, life was confined to Ediacaran fauna. There is some evidence that simple life may already have invaded the land at the start of the Paleozoic, but substantial plants and animals did not take to the land until the Silurian and did not thrive until the Devonian. Although primitive vertebrates are known near the start of the Paleozoic, animal forms were dominated by invertebrates until the mid-Paleozoic. Fish populations exploded in the Devonian. During the late Paleozoic, great forests of primitive plants thrived on land forming the great coal beds of Europe and eastern North America. By the end of the era, the first large, sophisticated reptiles and the first modern plants (conifers) had developed.[34]

Mesozoic life

Main article: Mesozoic

The Mesozoic extended roughly from 251 Ma to 65 Ma (ICS 2004). The extinction of nearly all animal species at the end of the Permian period allowed for the radiation of many new lifeforms. In particular, the extinction of the large herbivorous and carnivorous dinocephalia left those ecological niches empty. Some were filled by the surviving cynodonts and dicynodonts, the latter of which subsequently became extinct. Animal life during the Mesozoic was dominated, however, by large archosaurian reptiles that appeared a few million years after the Permian extinction: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs.

The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and placental mammals also appeared. Angiosperms radiated sometime in the early Cretaceous, first in the tropics, but the even temperature gradient allowed them to spread toward the poles throughout the period. By the end of the Cretaceous, angiosperms dominated tree floras in many areas, although some evidence suggests that biomass was still dominated by cycad and ferns until after the KT extinction.

Some have argued that insects diversified with angiosperms because insect anatomy, especially the mouth parts, seems particularly well-suited for flowering plants. However, all major insect mouth parts preceded angiosperms and insect diversification actually slowed when they arrived, so their anatomy originally must have been suited for some other purpose.

As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear while smaller animals of all kinds, including lizards, snakes, and perhaps the ancestor mammals to primates, evolved. The KT extinction exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today.

Cenozoic life

Main article: Cenozoic

The Cenozoic era covers the 65.5 million years since the Cretaceous-Tertiary extinction event. The Cenozoic is the age of new life. During this era, mammals diverged from a few small, simple, generalized forms into a diverse collection of terrestrial, marine, and flying animals. As the dinosaurs perished at the end of the Cretaceous period, the mammals took center stage. The mammals are the largest land animals of the Era, as the dinosaurs had been during the Mesozoic. [35] The Cenozoic is just as much the age of savannas, or the age of co-dependent flowering plants and insects. Birds also evolved substantially in the Cenozoic. Monte Bolca is an important lagerstätte near Verona, Italy, containing excellently preserved fish and other fossils of Eocene age.

Major geological extinction events

Cambrian explosion

Main article: Cambrian explosion
File:Hallucigenia ru.jpg
The iconic Cambrian organism Hallucigenia

The Cambrian explosion (542-530 Ma) describes the profound diversification in life on Earth. Prior to around 580 million years ago, organisms were on the whole simple, comprising of individual cells occasionally organised into colonies. Over the subsequent 70-80 million years, evolution would accelerate by an order of magnitude,[36] and the diversity of life would begin to resemble today's.[37]

The fossil record provides us with a few cases of exceptional preservation around the end of this explosion, with famous units such as the Burgess shale offering a glimpse into a bustling yet strikingly different world, in stark contrast to the microbe-dominated seas of just 80 million years before.

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the "Primordial Strata" was noted as early as the mid 19th century;[38] Charles Darwin saw it as one of the principal objections that could be lodged against his theory of evolution by natural selection.[39]

The Cambrian explosion has proven difficult to study, partly because of the problems involved in matching up rocks of the same age across disparate continents. It should be borne in mind that absolute radiometric dates for much of the Cambrian, obtained by detailed analysis of radioactive elements contained within rocks, have only rather recently become available[40] and that, especially for the Lower Cambrian, detailed biostratigraphic correlation — using widespread but short-lived species to match the ages of rocks — remains rather tenuous, particularly around the internationally-defined Precambrian/Cambrian boundary section in Newfoundland. Dating of important boundaries, and description of faunal successions, should thus be regarded with some degree of caution until better data become available.


Ordovician-Silurian extinction events

Main article: Ordovician-Silurian extinction events
The Ordovician-Silurian extinction event, labeled "End O" here.

The extinctions occurred approximately 444–447 million years ago and mark the boundary between the Ordovician and the following Silurian Periods. During this extinction event, which may have been composed of several distinct closely spaced events, there were several marked changes in biologically responsive carbon and oxygen isotopes, which may indicate separate events or particular phases within one event.

At that time most complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49%[41] of genera of fauna (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a sharp increase in extinctions than by a decrease in speciation.[42]

Permian-Triassic extinction event

Main article: Permian-Triassic extinction event

The Permian-Triassic (P-Tr) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred 251.4 million years ago (mya),[43] forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species[44] and 70 percent of terrestrial vertebrate species becoming extinct.

The extinction rate of marine organisms was catastrophic; [45][46][47][48][49] it is harder to produce such detailed statistics for land, river, swamp and lake environments because good Permian-Triassic rock sequences from terrestrial environments are extremely rare (the Karoo basin is by far the best).

Even so, there is enough evidence to indicate that:-

Late Devonian extinction

Main article: Late Devonian extinction

The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids, and the rugose and tabulate corals. The reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian corals) did not recover until the Mesozoic era.

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: a recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated, the category of families offering a broad range of real structural diversity. Some 57 percent of the genera went extinct, and—the estimate most likely to be adjusted—at least 75 percent of the species did not survive into the following Carboniferous. The estimates of species loss depend on surveys of marine taxa that are perhaps not well enough known to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases. Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms. Freshwater species, including our tetrapod ancestors, were less affected.

Triassic-Jurassic extinction event

Main article: Triassic-Jurassic extinction event
Ranges of families tetrapods through the Triassic and Early Jurassic.

The Triassic-Jurassic extinction event occurred 200 million years ago and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. 20% of all marine families and all large Crurotarsi (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event opened an ecological niche allowing the dinosaurs to assume the dominant roles in the Jurassic period.

Cretaceous–Tertiary extinction event

Main article: Cretaceous–Tertiary extinction event
Artist's impression of a major impact event. The collision between Earth and an asteroid a few kilometers in diameter may release as much energy as several million nuclear weapons detonating.

The Cretaceous–Tertiary extinction event was the catastrophic mass extinction of extant animal species in a comparatively short period of time, approximately 65.5 million years ago. Most, if not all, non-avian dinosaurs became extinct during or possibly a few years after the event.[53] Non-avian dinosaur fossils are not found later than the K–T boundary, except for a few controversial exceptions. Many other groups of animals and plants, including mosasaurs, plesiosaurs, pterosaurs, and some invertebrates, also became extinct at the K–T boundary. The event marks the end of the Mesozoic Era, and the beginning of the Cenozoic Era.

Most of the extinctions occurred in a relatively short time because extensive weather changes reduced photosynthesis, thereby decreasing the amount of plant material available to herbivorous animals. This change in food supply caused a massive disruption in Earth's ecology.

Despite the severity of the K–T extinction event, there was significant variability in the rate of extinction of different classes of organisms. Organisms which depended on photosynthesis became extinct or suffered heavy losses due to reduced sunlight. Photosynthesizing organisms, from plankton (e.g. coccolithophorids) to land plants, formed the primary part of the food chain in the late Cretaceous. Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, top predators such as Tyrannosaurus rex also began dying.[54][55] Animals which built calcium carbonate shells (for example, coccolithophorids along with many groups of molluscs, including ammonites, rudists, freshwater snails and mussels), as well as organisms whose food chain depended on these calcium carbonate shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary.[56]

Most omnivores, insectivores and carrion-eaters appear to have survived quite well. At the end of the Cretaceous there seem to have been no purely herbivorous or carnivorous mammals. Many mammals, and the birds which survived the extinction, fed on insects, larvae, worms, snails, etc., which in turn fed on dead plant matter. They may have survived the collapse of plant-based food chains because they lived in "detritus-based" food chains.[57]

In stream communities few groups of animals became extinct. Stream communities tend to rely less on food from living plants and more on detritus that washes in from land. This may have buffered them from extinction.[58] Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column, than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while many animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.[59]

The largest air-breathing survivors, crocodilians and champsosaurs, were semi-aquatic. Modern crocodilians can live as scavengers and can survive for months without food. Modern crocodilian young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. This has been linked to crocodilian survival at the end of the Cretaceous.[57]

Holocene extinction event

Main article: Holocene extinction event
The Dodo, a bird of Mauritius, became extinct during the mid-late 17th century after humans destroyed the forests where the birds made their homes and introduced animals that ate their eggs.

The Holocene extinction event is a name customarily given to the widespread, ongoing mass extinction of species during the modern Holocene epoch. The large number of extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods; a sizeable fraction of these extinctions are occurring in the rainforests. Because the rate of this extinction event appears to be much more rapid than the "Big Five" mass extinctions, it is also known as the Sixth Extinction. Since 1500 AD, 784 extinctions have been documented by the International Union for Conservation of Nature and Natural Resources.[60] However, since most extinctions are likely to go undocumented, scientists estimate that during the last century, between 20,000 and two million species have become extinct, but the precise total cannot be determined more accurately within the limits of present knowledge. Up to 140,000 species per year (based on Species-area theory)[61] may be the present rate of extinction based upon upper bound estimating.

In broad usage, the Holocene extinction event includes the notable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both. These extinctions, occurring near the Pleistocene / Holocene boundary, are sometimes referred to as the Pleistocene Extinction Event or Ice Age extinction event. However the Holocene extinction event continues through the events of the past several millennia and includes the present time.

The observed rate of extinction has accelerated dramatically in the last 50 years, to a pace greater than the rate seen during the Big Five. There is no general agreement on whether to consider more recent extinctions as a distinct event or merely part of a single escalating process. Only during these most recent parts of the extinction have plants also suffered large losses. Overall, the Holocene extinction event is most significantly characterised by the presence of man-made driving factors and its very short geological timescale (tens to thousands of years) compared to most other extinction events.

Tree of life

Fig. 1: Unrooted tree of the myosin supergene family[62]
Fig. 2: A highly resolved, automatically generated Tree Of Life, based on completely sequenced genomes [63][64].

Prokaryote

Main article: Prokaryote

It is generally accepted that the first living cells were some form of prokaryote and may have developed out of protobionts. Fossilized prokaryotes approximately 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant organism even today. In contrast the eukaryote only appeared between approximately 1.7 and 2.2 billion years ago.[65] While Earth is the only known place in the universe where life exist, some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes; this is open to considerable debate and skepticism.

Prokaryotes diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct types of prokaryotes. For example, in addition to using photosynthesis or organic compounds for energy like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as hydrogen sulfide.

Eukaryote

Main article: Eukaryote
The original, now outdated, five-kingdom system.

The origin of the eukaryotic cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms. The timing of this series of events is hard to determine; Knoll (1992) suggests they developed approximately 1.6 - 2.1 billion years ago. Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga.

rRNA trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true crown), which was usually divided by the form of the mitochondrial cristae. The few groups that lack mitochondria branched separately and so the absence was believed to be primitive, but this is now considered an artifact of long branch attraction and they are known to have lost them secondarily.

Trees based on actin and other molecules have painted a different and more complete picture. Most eukaryotes are now included in several supergroups:

Opisthokonts Animals, fungi, choanoflagellates, etc.
Amoebozoa Most lobose amoebae and slime moulds
Rhizaria Foraminifera, Radiolaria, and various other amoeboid protozoa
Excavates Various flagellate protozoa
Primoplantae (or Archaeplastida) Land plants, green algae, red algae, and glaucophytes
Chromists Brown algae, diatoms, water molds, etc.
Alveolates Ciliates, Apicomplexa, dinoflagellates, etc.

Several authorities recognize two larger clades, the unikonts and the bikonts, the unikonts deriving from an ancestral uniflagellar organism, and the bikonts deriving from an ancestral biflagellate. In this system, the opisthokonts and amoebozoans are considered unikonts, and the rest are considered bikonts. The chromists and alveolates may be part of a larger group that is ancestrally photosynthetic, called the chromalveolates, but this remains contentious. Some small protist groups have not been related to any of these supergroups, in particular the centrohelids. Eukaryotes are closely related to Archaea, at least in terms of nuclear DNA and genetic machinery, and are placed by some, along with the Archaea, in the clade Neomura. In other respects, such as membrane composition, they are similar to eubacteria. Three main explanations for this have been proposed:

  • Eukaryotes resulted from the complete fusion of two or more cells, the cytoplasm forming from a eubacterium and the nucleus from an archaeon (alternatively a virus).
  • Eukaryotes developed from Archaea, and acquired their eubacterial characteristics from the proto-mitochondrion.
  • Eukaryotes and Archaea developed separately from a modified eubacterium.

The final hypothesis is currently the most accepted. The origin of the endomembrane system and mitochondria are also disputed. The phagotrophic hypothesis states the membranes originated with the development of endocytosis and later specialized; mitochondria were acquired by ingestion, like plastids. The syntrophic hypothesis states that the proto-eukaryote relied on the proto-mitochondrion for food, and so ultimately grew to surround it; the membranes originate later, in part thanks to mitochondrial genes (the hydrogen hypothesis is one particular version).

Viruses

Main article: Viruses

The origins of modern viruses are not entirely clear. It may be that no single mechanism can account for all viruses. They do not fossilize well, so molecular techniques have been the most useful means of hypothesising how they arose. Research in microfossil identification and molecular biology may yet discern fossil evidence dating to the Archean or Proterozoic eons. Two main hypotheses currently exist.[66]

Small viruses with only a few genes may be runaway stretches of nucleic acid originating from the genome of a living organism. Their genetic material could have been derived from transferable genetic elements such as plasmids or transposons, which are prone to moving within, leaving, and entering genomes.

Viruses with larger genomes, such as poxviruses, may have once been small cells which parasitised larger host cells. Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as retrograde-evolution or reverse-evolution. The bacteria Rickettsia and Chlamydia are living cells that, like viruses, can only reproduce inside host cells. They lend credence to the streamlining hypothesis, as their parasitic lifestyle is likely to have caused the loss of genes that enabled them to survive outside a host cell.

It is hypothetically possible that viruses represent a primitive form of self replicating DNA and are a precursor to life as it is presently defined.

Evolution of asymmetry

Features of the symmetry are determined by the type of environment under which the organism has evoloved. Maximal extent of organism symmetry corresponds to a completely isotropic ecological niche.

First organisms on Earth floating in the depths of water (unicellular and lower multicellular organisms) have the maximum possible spherical symmetry. They appeared approximately 3.5 billion years ago. Asymmetrization along the top–bottom axis occurred under the influence of gravity. This led to the appearance of the attached, low-mobility forms (plants and coelenterates) that had radial symmetry.

Asymmetrization along the front–back axis occurred due to the interaction with space, when rapid motion was required (to escape from the predator, or to chase a prey). As a result, the main receptors and the brain were moved to the front of the body. Organisms with bilateral symmetry were dominating last 650-800 million years. These are crustaceans, fish as well as the most progressive forms, i.e., mammals, birds, and insects.

Each transition changes one axis from symmetry to asymmetry with triaxial asymmetry placed at the end of the array (spherical → radial → bilateral → triaxial).

The trend towards asymmetrization can be followed in phylogeny of plant organs (flower, leaf, fruits, and seeds). It is known, that zygomorphic (bilateral symmetry) flowers are evolutionary more progressive, than actinomorphic (radial symmetry) flowers, but are less progressive, than triaxial asymmetric ones. The morphology of a leaf during evolution follows the same picture- spherical symmetry of chlorella, radial symmetry of pine needles, bilateral symmetry of Magnolia leafs, and triaxial asymmetry of Begonia or Elms leafs. The same trend can be found in embryogenesis—spherical zygote, radial gastrula, bilateral embryo and triaxial asymmetric child.

Evolution of sex

Main article: Evolution of sex

The evolution of sex is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years ago with DNA forming 3.5 - 4.6 billion years.[67]Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.

A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.

Organisms need to replicate their genetic material in an efficient and reliable manner. The necessity to repair genetic damage is one of the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a mutated section of its DNA via homologous recombination, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. A mutation in an haploid individual, on the other hand, is more likely to become resident, as the DNA repair machinery has no way of knowing what the original undamaged sequence was.[68] The most primitive form of sex may have been one organism with damaged DNA replicating an undamaged strand from a similar organism in order to repair itself.[69]

Another theory is that sexual reproduction originated from selfish parasitic genetic elements that exchange genetic material (that is: copies of their own genome) for their transmission and propagation. In some organisms, sexual reproduction has been shown to enhance the spread of parasitic genetic elements (e.g.: yeast, filamentous fungi).[70] Bacterial conjugation, a form of genetic exchange that some sources describe as sex, is not a form of reproduction. However, it does support the selfish genetic element theory, as it is propagated through such a "selfish gene", the F-plasmid.[69]

A third theory is that sex evolved as a form of cannibalism. One primitive organism ate another one, but rather than completely digesting it, some of the 'eaten' organism's DNA was incorporated into the 'eater' organism.[69]

A theory states that sexual reproduction evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids. [71]

A comprehensive origin of sex as vaccination theory proposes that eukaryan sex-as-syngamy (fusion sex) arose from prokaryan unilateral sex-as-infection when infected hosts began swapping nuclearized genomes containing coevolved, vertically transmitted symbionts that provided protection against horizontal superinfection by more virulent symbionts. Sex-as-meiosis (fission sex) then evolved as a host strategy to uncouple (and thereby emasculate) the acquired symbiont genomes.[72]

Human evolution

Further information: Human evolution
File:Pekingthr.jpg
Reconstructed skull of Peking Man, a representative of the extinct species believed to be the nearest ancestor of Homo sapiens, Homo erectus.

The scientific study of human evolution encompasses the development of the genus Homo, but usually involves studying other hominids and hominines as well, such as Australopithecus. "Modern humans" are defined as the Homo sapiens species, of which the only extant subspecies is Homo sapiens sapiens; Homo sapiens idaltu (roughly translated as "elder wise man"), the other known subspecies, is extinct.[73] Anatomically modern humans appear in the fossil record in Africa about 130,000 years ago.[74][75]

The closest living relatives of Homo sapiens are the two species of chimpanzee: the Bonobo (Pan paniscus) and the Common Chimpanzee (Pan troglodytes). These species are equally close (they share the same common ancestor), the main difference between them is the social organization: matriarchal for the Bonobo and patriarchal for the Common Chimpanzee. Full genome sequencing resulted in the conclusion that "after 6.5 million years of separate evolution, the differences between bonobo/chimpanzee and human are just 10 times greater than those between two unrelated people and 10 times less than those between rats and mice". In fact, 98.4% of the DNA sequence is identical between the two Pan species and human.[76][77][78][79] It has been estimated that the human lineage diverged from that of chimpanzees about five million years ago, and from gorillas about eight million years ago. However, a hominid skull discovered in Chad in 2001, classified as Sahelanthropus tchadensis, is approximately seven million years old, which may indicate an earlier divergence.[80]

There are two prominent scientific theories of the origins of contemporary humans. They concern the relationship between modern humans and other hominids. The single-origin, or "out-of-Africa", hypothesis proposes that modern humans evolved in Africa and later migrated outwards to replace hominids in other parts of the world. The multiregional hypothesis, on the other hand, proposes that modern humans evolved, at least in part, from independent hominid populations.[81]

Geneticists Lynn Jorde and Henry Harpending of the University of Utah proposed that the variation in human DNA is minute compared to that of other species, and that during the Late Pleistocene, the human population was reduced to a small number of breeding pairs — no more than 10,000 and possibly as few as 1,000 — resulting in a very small residual gene pool. Various reasons for this hypothetical bottleneck have been postulated, the most popular being the Toba catastrophe theory.

Human evolution is characterized by a number of important physiological trends, including the expansion of the brain cavity and brain itself, which is typically 1,400 cm³ in volume, over twice that of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from that of other apes (heterochrony), allowing for an extended period of social learning and language acquisition in juvenile humans. Physical anthropologists argue that a reorganization of the structure of the brain is more important than cranial expansion itself. Other significant evolutionary changes included a reduction of the canine tooth, development of bipedal locomotion, and the descent of the larynx and hyoid bone, making speech possible. How these trends are related and what their role is in the evolution of complex social organization and culture are matters of ongoing debate.[82][83]

See also

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