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Evolutionary history of life

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Title: Evolutionary history of life  
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Subject: Evolutionary biology, Life, Macroevolution, Evolution, Mutation
Collection: Earth Sciences, Evolutionary Biology, Geochronology, Geologic Time Scale, History by Topic, Paleontology
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Evolutionary history of life

The evolutionary history of life on common ancestor from which all known species have diverged through the process of evolution.[1]

The earliest evidence for differentiated cells performing specialised functions.[9] Bilateria, animals with a front and a back, appeared by 555 million years ago.[10]

The earliest

  • The Complete Work of Charles Darwin Online
  • Understanding Evolution: History, Theory, Evidence, and Implications

History of evolutionary thought

  • General information on evolution- Fossil Museum nav.
  • Understanding Evolution from University of California, Berkeley
  • National Academies Evolution Resources
  • Evolution poster- PDF format "tree of life"
  • New ScientistEverything you wanted to know about evolution by
  • — How Evolution Works
  • Synthetic Theory Of Evolution: An Introduction to Modern Evolutionary Concepts and Theories

General information

External links

  • Cowen, R. (2004). History of Life (4th ed.). Blackwell Publishing Limited.  

Further reading

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  1. ^ Name given as in Butterfield's paper "Bangiomorpha pubescens ..." (2000). A fossil fish, also from China, has also been named Qingshania. The name of one of these will have to change.
  2. ^  


See also

shows a different trend: a fairly swift rise from ; a slight decline from , in which the devastating Permian–Triassic extinction event is an important factor; and a swift rise from to the present.[213]

"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"[213]

Biodiversity in the fossil record, which is

All genera
"Well-defined" genera
Trend line
"Big Five" mass extinctions
Other mass extinctions
Million years ago
Thousands of genera
  • The oceans may have become more hospitable to life over the last 500 Ma and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted.[211][212]
  • Reasonably complete [210]

The fossil record appears to show that the gaps between mass extinctions are becoming longer and the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways:[210]

Life on Earth has suffered occasional mass extinctions at least since . Although they were disasters at the time, mass extinctions have sometimes accelerated the evolution of [22][209]

Marine extinction intensity during the Phanerozoic
Millions of years ago
Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record. (Graph not meant to include recent epoch of Holocene extinction event)

Mass extinctions

The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by Robert Chambers in 1844 and taken up by Charles Darwin in 1871.[203] Modern humans evolved from a lineage of upright-walking apes that has been traced back over to Sahelanthropus.[204] The first known stone tools were made about , apparently by Australopithecus garhi, and were found near animal bones that bear scratches made by these tools.[205] The earliest hominines had chimp-sized brains, but there has been a fourfold increase in the last 3 Ma; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence.[206] There is a long-running debate about whether modern humans evolved all over the world simultaneously from existing advanced hominines or are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species.[207] There is also debate about whether anatomically modern humans had an intellectual, cultural and technological "Great Leap Forward" under 100,000 years ago and, if so, whether this was due to neurological changes that are not visible in fossils.[208]


The earliest fossils of insects have been found in Early Devonian rocks from about , which preserve only a few varieties of flightless insect. The Mazon Creek lagerstätten from the Late Carboniferous, about , include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Mid Cenozoic.[202]

The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual haplodiploid method of sex determination, which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed.[200] However Wilson and Hölldobler argue that this explanation is faulty: for example, it is based on kin selection, but there is no evidence of nepotism in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages through co-operative foraging. In support of this explanation they cite the appearance of eusociality in bathyergid mole rats,[199] which are not haplodiploid.[201]

The social insects are remarkable because the great majority of individuals in each colony are sterile. This appears contrary to basic concepts of evolution such as natural selection and the selfish gene. In fact there are very few eusocial insect species: only 15 out of approximately 2,600 living families of insects contain eusocial species, and it seems that eusociality has evolved independently only 12 times among arthropods, although some eusocial lineages have diversified into several families. Nevertheless social insects have been spectacularly successful; for example although ants and termites account for only about 2% of known insect species, they form over 50% of the total mass of insects. Their ability to control a territory appears to be the foundation of their success.[199]

These termite mounds have survived a bush fire.

Social insects

The first flowering plants appeared around 130 million years ago.[198] The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from ,[196][197] and that their rise was associated with that of pollinating insects.[197] Among modern flowering plants Magnolias are thought to be close to the common ancestor of the group.[196] However paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants.[196][197]






(flowering plants)

One possible family tree of flowering plants.[196]

(flowering plants)






Another possible family tree.[197]

Flowering plants

[195] to the sea within 15 Ma.cetaceans and [194] taking to the air within 13 Ma,bats with [193][192], sizes and shapes, but increased rapidly in size and diversity after the extinction,taxa Mammals throughout the time of the dinosaurs had been restricted to a narrow range of [191] During the Late Jurassic,

The Permian-Triassic extinction wiped out almost all land vertebrates,[179] as well as the great majority of other life.[180] During the slow recovery from this catastrophe, estimated to have taken 30 million years,[181] a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of archosauriformes ("ruling lizard forms") have been found in Late Permian rocks,[182] but, by the Mid Triassic, archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods ().[183]

Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period (). The earliest fossils of the two surviving amniote groups, synapsids and sauropsids, date from around .[176][177] The synapsid pelycosaurs and their descendants the therapsids are the most common land vertebrates in the best-known Permian () fossil beds. However at the time these were all in temperate zones at middle latitudes, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians.[178]


Early synapsids (extinct)


Extinct pelycosaurs


Extinct therapsids

    Extinct mammaliaforms



Anapsids; whether turtles belong here is debated[175]


Captorhinidae and Protorothyrididae


Araeoscelidia (extinct)

    Squamata (lizards and snakes)


Extinct archosaurs



Pterosaurs (extinct)





    Ornithischians (extinct)

Possible family tree of dinosaurs, birds and mammals[176][177]

Dinosaurs, birds and mammals

Later discoveries revealed earlier transitional forms between Acanthostega and completely fish-like animals.[174] Unfortunately there is then a gap (Romer's gap) of about 30 Ma between the fossils of ancestral tetrapods and Mid Carboniferous fossils of vertebrates that look well-adapted for life on land. Some of these look like early relatives of modern amphibians, most of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the amniotes, whose waterproof skin enables them to live and breed far from water.[172]

The Devonian proliferation of land plants may help to explain why air breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.[171]

Tetrapods, vertebrates with four limbs, evolved from other rhipidistian fish over a relatively short timespan during the Late Devonian ().[173] The early groups are grouped together as Labyrinthodontia. They retained aquatic, fry-like tadpoles, a system still seen in modern amphibians. From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of Acanthostega from about showed that this Late Devonian transitional animal had legs and both lungs and gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that Acanthostega, which was about 1 metre (3.3 ft) long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the shoulder girdle and it has a distinct neck.[171]

Acanthostega changed views about the early evolution of tetrapods.[171]

Early land vertebrates

The earliest confirmed fossils of flying insects date from the Late Carboniferous, but it is thought that insects developed the ability to fly in the Early Carboniferous or even Late Devonian. This gave them a wider range of ecological niches for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment.[169] About 99% of modern insect species fly or are descendants of flying species.[170]

The fossil record of other major invertebrate groups on land is poor: none at all for non-parasitic flatworms, nematodes or nemerteans; some parasitic nematodes have been fossilized in amber; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of gastropods on land date from the Late Carboniferous, and this group may have had to wait until leaf litter became abundant enough to provide the moist conditions they need.[148]

The oldest known air-breathing animal is Pneumodesmus, an archipolypodan millipede from the Mid Silurian, about .[162][163] Its air-breathing, terrestrial nature is evidenced by the presence of spiracles, the openings to tracheal systems.[164] However, some earlier trace fossils from the Cambrian-Ordovician boundary about are interpreted as the tracks of large amphibious arthropods on coastal sand dunes, and may have been made by euthycarcinoids,[165] which are thought to be evolutionary "aunts" of myriapods.[166] Other trace fossils from the Late Ordovician a little over probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and alluvial plains shortly before the Silurian-Devonian boundary, about , including signs that some arthropods ate plants.[167] Arthropods were well pre-adapted to colonise land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.[168]

Animals had to change their feeding and excretory systems, and most land animals developed internal fertilization of their eggs. The difference in refractive index between water and air required changes in their eyes. On the other hand, in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in air encouraged the development of hearing.[148]

Land invertebrates

  • They removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus causing an ice age in the Carboniferous period.[160] In later ecosystems the carbon dioxide "locked up" in wood is returned to the atmosphere by decomposition of dead wood. However, the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian.[161]
  • The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused algal blooms whose high consumption of oxygen caused anoxic events in deeper waters, increasing the extinction rate among deep-water animals.[160]

By the late Devonian , trees such as Archaeopteris were so abundant that they changed river systems from mostly braided to mostly meandering, because their roots bound the soil firmly.[158] In fact they caused a "Late Devonian wood crisis",[159] because:

Spores of land plants, possibly rather like liverworts, have been found in Mid Ordovician rocks dated to about . In Mid Silurian rocks there are fossils of actual plants including clubmosses such as Baragwanathia; most were under 10 centimetres (3.9 in) high, and some appear closely related to vascular plants, the group that includes trees.[157]

In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land required plants to become internally more complex and specialized: photosynthesis was most efficient at the top; roots were required in order to extract water from the ground; the parts in between became supports and transport systems for water and nutrients.[147][157]

Fossilized trees from the Mid-Devonian Gilboa fossil forest
Reconstruction of Cooksonia, a vascular plant from the Silurian

Plants and the Late Devonian wood crisis

[156][150] ("worms") or arthropods.annelids sediments, and are attributed to Ordovician Burrows have been found in [150] Soil formation would have been very slow until the appearance of burrowing animals, which mix the mineral and organic components of soil and whose

Films of Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to tidal zones and then to land.[150] Lichens, which are symbiotic combinations of a fungus (almost always an ascomycete) and one or more photosynthesizers (green algae or cyanobacteria),[155] are also important colonizers of lifeless environments,[150] and their ability to break down rocks contributes to soil formation in situations where plants cannot survive.[155] The earliest known ascomycete fossils date from in the Silurian.[150]

Lichens growing on concrete

Before the colonization of land, weathering. Water and any nutrients in it would have drained away very quickly.[150]

Evolution of soil

[154] In fact,

When plants and animals began to transfer from the sea to rivers and land about 500 Ma ago, environmental deficiency of these marine mineral antioxidants and iodine, was a challenge to the evolution of terrestrial life.[151][152] Terrestrial plants slowly optimized the production of “new” endogenous antioxidants such as ascorbic acid, polyphenols, flavonoids, tocopherols etc. A few of these appeared more recently, in last 200-50 Ma ago, in fruits and flowers of angiosperm plants.

zinc, iron, copper, molybdenum, selenium and iodine which is concentrated more than 30,000 times the concentration of this element in seawater. Protective endogenous antioxidant enzymes and exogenous dietary antioxidants helped to prevent oxidative damage. Most marine mineral antioxidants act in the cells as essential trace-elements in redox and antioxidant metallo-enzymes.

Evolution of terrestrial antioxidants

[150], about .Devonian only appeared in the late ecosystems modern land [149] Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size must create special structures to withstand gravity;

Colonization of land

Most of the animals at the heart of the Cambrian explosion debate are protostomes, one of the two main groups of complex animals. The other major group, the deuterostomes, contains invertebrates such as sea stars and urchins (echinoderms), as well as chordates (see below). Many echinoderms have hard calcite "shells", which are fairly common from the Early Cambrian small shelly fauna onwards.[135] Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the Chengjiang fauna, a lagerstätte in China.[142] The chordates are another major deuterostome group: animals with a distinct dorsal nerve cord. Chordates include soft-bodied invertebrates such as tunicates as well as vertebrates- animals with a backbone. While tunicate fossils predate the Cambrian explosion,[143] the Chengjiang fossils Haikouichthys and Myllokunmingia appear to be true vertebrates,[144] and Haikouichthys had distinct vertebrae, which may have been slightly mineralized.[145] Vertebrates with jaws, such as the Acanthodians, first appeared in the Late Ordovician.[146]

Acanthodians were among the earliest vertebrates with jaws.[141]

Deuterostomes and the first vertebrates

In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Pre-Cambrian animal fossils.[135] A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the "Cambrian explosion" and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.[137] Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups[138] – for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades.[139] Nevertheless there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.[140]

Opabinia made the largest single contribution to modern interest in the Cambrian explosion.[136]

The small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Mid Cambrian periods. The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates", Halkieria and Microdictyon, were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.[135]

The triploblastic bilaterian animal, in other words significantly more complex than cnidarians.[134]

The earliest widely accepted animal fossils are rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and hydras), possibly from around , although fossils from the Doushantuo Formation can only be dated approximately. Their presence implies that the cnidarian and bilaterian lineages had already diverged.[126]

Animals are multicellular eukaryotes,[note 2] and are distinguished from plants, algae, and fungi by lacking cell walls.[123] All animals are motile,[124] if only at certain life stages. All animals except sponges have bodies differentiated into separate tissues, including muscles, which move parts of the animal by contracting, and nerve tissue, which transmits and processes signals.[125]

Emergence of animals

The Horodyskia, found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan;[8] however it has also been interpreted as a colonial foraminiferan.[112]

Fossil evidence

[120] The available evidence indicates that eukaryotes evolved much earlier but remained inconspicuous until a rapid diversification around 1 Ga. The only respect in which eukaryotes clearly surpass

Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity.[120]

The initial advantages of multicellularity may have included: more efficient sharing of nutrients that are digested outside the cell,[119] increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis;[120] the ability to create an internal environment that gives protection against the external one;[117] and even the opportunity for a group of cells to behave "intelligently" by sharing information.[118] These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.[120]

A slime mold solves a maze. The mold (yellow) explored and filled the maze (left). When the researchers placed sugar (red) at two separate points, the mold concentrated most of its mass there and left only the most efficient connection between the two points (right).[118]

The simplest definitions of "multicellular", for example "having multiple cells", could include evolution of complexity could be regarded as "rather anthropocentric".[117]


The adaptive function of sex today remains a major unresolved issue in biology. The competing models to explain the adaptive function of sex were reviewed by Birdsell and Wills.[113] The hypotheses discussed above all depend on possible beneficial effects of random genetic variation produced by genetic recombination. An alternative view is that sex arose, and is maintained, as a process for repairing DNA damage, and that the genetic variation produced is an occasionally beneficial byproduct.[107][114]

The random nature of recombination causes the relative abundance of alternative traits to vary from one generation to another. This genetic drift is insufficient on its own to make sexual reproduction advantageous, but a combination of genetic drift and natural selection may be sufficient. When chance produces combinations of good traits, natural selection gives a large advantage to lineages in which these traits become genetically linked. On the other hand, the benefits of good traits are neutralized if they appear along with bad traits. Sexual recombination gives good traits the opportunities to become linked with other good traits, and mathematical models suggest this may be more than enough to offset the disadvantages of sexual reproduction.[109] Other combinations of hypotheses that are inadequate on their own are also being examined.[101]

Horodyskia may have been an early metazoan,[8] or a colonial foraminiferan.[112] It apparently re-arranged itself into fewer but larger main masses as the sediment grew deeper round its base.[8]

The Mutation Deterministic Hypothesis assumes that each organism has more than one harmful mutation and the combined effects of these mutations are more harmful than the sum of the harm done by each individual mutation. If so, sexual recombination of genes will reduce the harm that bad mutations do to offspring and at the same time eliminate some bad mutations from the gene pool by isolating them in individuals that perish quickly because they have an above-average number of bad mutations. However the evidence suggests that the MDH's assumptions are shaky, because many species have on average less than one harmful mutation per individual and no species that has been investigated shows evidence of synergy between harmful mutations.[101] Further criticisms of this hypothesis are discussed in the article Evolution of sexual reproduction#Removal of deleterious genes

The Red Queen Hypothesis suggests that sexual reproduction provides protection against parasites, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical clones than those of sexual species that present moving targets, and there is some experimental evidence for this. However there is still doubt about whether it would explain the survival of sexual species if multiple similar clone species were present, as one of the clones may survive the attacks of parasites for long enough to out-breed the sexual species.[101] Furthermore, contrary to the expectations of the Red Queen Hypothesis, Hanley et al. found that the prevalence, abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat.[110] In addition, Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in the host.[111]

The disadvantages of sexual reproduction are well-known: the genetic reshuffle of recombination may break up favorable combinations of genes; and since males do not directly increase the number of offspring in the next generation, an asexual population can out-breed and displace in as little as 50 generations a sexual population that is equal in every other respect.[101] Nevertheless the great majority of animals, plants, fungi and protists reproduce sexually. There is strong evidence that sexual reproduction arose early in the history of eukaryotes and that the genes controlling it have changed very little since then.[108] How sexual reproduction evolved and survived is an unsolved puzzle.[109]

On the other hand, bacterial transformation is clearly an adaptation for transfer of DNA between bacteria of the same species. Bacterial transformation is a complex process involving the products of numerous bacterial genes and can be regarded as a bacterial form of sex.[104][105] This process occurs naturally in at least 67 prokaryotic species (in seven different phyla).[106] Sexual reproduction in eukaryotes may have evolved from bacterial transformation.[107] (Also see Evolution of sexual reproduction#Origin of sexual reproduction.)

The defining characteristics of sexual reproduction in eukaryotes are meiosis and fertilization. There is much genetic recombination in this kind of reproduction, in which offspring receive 50% of their genes from each parent,[101] in contrast with asexual reproduction, in which there is no recombination. Bacteria also exchange DNA by bacterial conjugation, the benefits of which include resistance to antibiotics and other toxins, and the ability to utilize new metabolites.[102] However conjugation is not a means of reproduction, and is not limited to members of the same species – there are cases where bacteria transfer DNA to plants and animals.[103]

Evolution of sexual reproduction

Sexual reproduction and multicellular organisms

Plastids are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1500 million years ago and enabled eukaryotes to carry out oxygenic photosynthesis.[100] Three evolutionary lineages have since emerged in which the plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in the glaucophytes.


[99] date from 1.43 Ga.fungi The earliest known fossils of [98] A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga.[97] There is a debate about when eukaryotes first appeared: the presence of

archaean, but the attack was neutralized, and the attacker took up residence and evolved into the first of the mitochondria; one of these chimeras later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of plants; and so on. After each endosymbiosis began, the partners would have eliminated unproductive duplication of genetic functions by re-arranging their genomes, a process which sometimes involved transfer of genes between them.[91][92][93] Another hypothesis proposes that mitochondria were originally sulfur- or hydrogen-metabolising endosymbionts, and became oxygen-consumers later.[94] On the other hand mitochondria might have been part of eukaryotes' original equipment.[95]

Chromatin, nucleus, endomembrane system, and mitochondria

Diversification of eukaryotes

Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient of the mat organisms and were well-adapted to strike out on their own both as floating mats and as the first of the phytoplankton, providing the basis of most marine food chains.[5]

[5] In modern underwater mats the top layer often consists of

Stromatolites are stubby pillars built as microbes in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.[77] There has been vigorous debate about the validity of alleged fossils from before 3 Ga,[79] with critics arguing that so-called stromatolites could have been formed by non-biological processes.[40] In 2006 another find of stromatolites was reported from the same part of Australia as previous ones, in rocks dated to 3.5 Ga.[80]

Microbial mats are multi-layered, multi-[78]

Modern stromatolites in Shark Bay, Western Australia.

Environmental and evolutionary impact of microbial mats

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.[76]

Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes.[75]

RNA is complex and there are doubts about whether it can be produced non-biologically in the wild.[66] Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analog of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules.[73] Although this idea has not become the scientific consensus, it still has active supporters.[74]

The clay theory

It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.[71] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.[72]

     = water-attracting heads of lipid molecules
     = water-repellent tails
Cross-section through a liposome.

Membranes first: Lipid world

A series of experiments starting in 1997 showed that early stages in the formation of carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts. Most of the steps required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents.[70]

Metabolism first: Iron–sulfur world

In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores.[69]

Although short self-replicating RNA molecules have been artificially produced in laboratories,[65] doubts have been raised about where natural non-biological synthesis of RNA is possible.[66] The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA.[67][68]

[64], modern cells' "protein factories".ribosomes Ribozymes remain as the main components of [63][62][61] Even the simplest members of the

Replication first: RNA world

Research on how life might have emerged from non-living chemicals focuses on three possible starting points: metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.[57] Research on abiogenesis still has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other.[58][59]

Life on Earth is based on alternative biochemistry may however be possible on other planets.[56]

Independent emergence on Earth

The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher spores can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years.[52][53] Scientists are divided over the likelihood of life arising independently on Mars,[54] or on other planets in our galaxy.[50]

Life "seeded" from elsewhere


Evolutionary tree showing the divergence of modern species from their common ancestor in the center.[1] The three domains are colored, with bacteria blue, archaea green, and eukaryotes red.

Origins of life on Earth

The earliest identified organisms were minute and relatively featureless, and their fossils look like small rods, which are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga.[37] Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria,[38] with geochemical evidence also seeming to show the presence of life 3.8 Ga.[39] However these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.[40][41] While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life,[37][42] although these statements have not been thoroughly examined by critics.

Earliest evidence for life on Earth

Evidence from the Moon indicates that from 4 billion to 3.8 billion years ago it suffered a [30][33] While there is no direct evidence of conditions on Earth 4 billion to 3.8 billion years ago, there is no reason to think that the Earth was not also affected by this late heavy bombardment.[34] This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although volcanic outgassing on Earth would have supplied at least half.[35] However, if subsurface microbial life had evolved by this point, it would have survived the bombardment.[36]

Until 2001, the oldest rocks found on Earth were about 3.8 Ga.[26] [27] [28] [29] leading scientists to believe that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the [30] However analysis of zircons formed 4.4 billion years ago indicates that Earth's crust solidified about 100 Ma after the planet's formation and that the planet quickly acquired oceans and an atmosphere, which may have been capable of supporting life.[31] [32]

The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time.[24] The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 40 million years later a body the size of Mars struck the Earth, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity than the early Moon, attracted almost all the iron particles in the area.[25]

Earliest history of Earth


  • Earliest history of Earth 1
  • Earliest evidence for life on Earth 2
  • Origins of life on Earth 3
    • Life "seeded" from elsewhere 3.1
    • Independent emergence on Earth 3.2
      • Replication first: RNA world 3.2.1
      • Metabolism first: Iron–sulfur world 3.2.2
      • Membranes first: Lipid world 3.2.3
      • The clay theory 3.2.4
  • Environmental and evolutionary impact of microbial mats 4
  • Diversification of eukaryotes 5
    • Chromatin, nucleus, endomembrane system, and mitochondria 5.1
    • Plastids 5.2
  • Sexual reproduction and multicellular organisms 6
    • Evolution of sexual reproduction 6.1
    • Multicellularity 6.2
    • Fossil evidence 6.3
  • Emergence of animals 7
    • Deuterostomes and the first vertebrates 7.1
  • Colonization of land 8
    • Evolution of terrestrial antioxidants 8.1
    • Evolution of soil 8.2
    • Plants and the Late Devonian wood crisis 8.3
    • Land invertebrates 8.4
    • Early land vertebrates 8.5
  • Dinosaurs, birds and mammals 9
  • Flowering plants 10
  • Social insects 11
  • Humans 12
  • Mass extinctions 13
  • See also 14
  • Footnotes 15
  • References 16
  • Further reading 17
  • External links 18


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