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Evolutionary history of life
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The evolutionary history of life on Earth stretches back over , possibly as far as , and there is evidence that evolution continues, even in humans. All present-day organisms use the same large set of complex chemical reactions, which indicates that all modern organisms share a common ancestor. However even the simplest modern organisms are too complex to have emerged directly from non-living materials. Some scientists have proposed that life on Earth was "seeded" from elsewhere, but most research concentrates on various explanations of how life could have arisen independently on Earth.
For about 2,000 million years multi-layered microbial mats were the dominant life on Earth. The evolution of oxygenic photosynthesis led to the oxygenation of the atmosphere, beginning about . While eucaryotic cells may have been present earlier, their evolution accelerated when they acquired the ability to use oxygen as a powerful "fuel" for their metabolisms. The earliest evidence of more complex eucaryotes with organelles, "organs" within a cell, dates from . Multicellular life is composed only of eucaryotic cells, and the earliest evidence for it is from , although specialization of cells for different functions first appears between ( a possible fungus) and (a probable red alga). Sexual reproduction may be a prerequisite for specialization of cells.
The earliest known fossil animals are cnidarians from about , although the earliest animals must have appeared before then. The earliest modern-looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders". There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation. Vertebrates remained an obscure group until the first fish with jaws appeared in the Late Ordovician.
The earliest signs of land plants and land invertebrates date back to about and respectively. The lineage that produced land vertebrates evolved very rapidly between and , and acquired limbs while they were still wholly aquatic. Land plants were so successful that they caused an ecological crisis in the Late Devonian.
During the Permian period synapsids, including the ancestors of mammals, may have dominated land environments, but the Permian–Triassic extinction event came very close to wiping out complex life. During the slow recovery from this catastrophe, archosaurs became the most abundant terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates in the Jurassic and Cretaceous periods, and birds evolved from one group of dinosaurs. During this time mammals' ancestors could survive only as small, mainly nocturnal insectivores, but this may have accelerated the development of mammalian traits. After the Cretaceous–Tertiary extinction event killed off the non-avian dinosaurs – birds are the only surviving dinosaurs – mammals increased rapidly in size and diversity, and some took to the air and the sea.
Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, between and , probably helped by coevolution with pollinating insects. Flowering plants and marine phytoplankton are still the dominant producers of organic matter. Social insects appeared around the same time as flowering plants. Although they occupy only small parts of the insect "family tree", they now form over half the total mass of insects.
Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over . Although early members of this lineage had chimp-sized brains, there are signs of a steady increase in brain size after about .
The mass extinctions that have occurred at least since may have accelerated the evolution of life by making room for new groups of organisms to diversify. The fossil record shows a swift rise in biodiversity from , a slight decline from , and a swift rise from to the present.
[edit] Earliest history of Earth
Millions of years
The oldest meteorite fragments found on Earth are about , and this has convinced scientists that the whole Solar system, including Earth, formed around then.[1] About 40 million years later a planetoid struck the Earth, throwing into orbit the material that formed the Moon.[2]
Until recently the oldest rocks found on Earth were about ,[1] and this led scientists to believe for decades that Earth's surface was molten until then. Hence they named this part of Earth's history the Hadean eon, whose name means "hellish". [3] However analysis of zircons formed indicates that Earth's crust solidified about 100 million years after the planet's formation and that Earth quickly acquired oceans and an atmosphere, which may have been capable of supporting life.[4]
Evidence from the Moon indicates that from it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar system, and Earth, having stronger gravity, should have experienced an even heavier bombardment.[5][6] While there is no direct evidence of conditions on Earth , there is no reason to think that the Earth was not also affected by this late heavy bombardment.[7] 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 contributed at least half.[8]
[edit] Earliest evidence for life on Earth
The earliest organisms were minute and relatively featureless, so their fossils look like small rods, which are very difficult to tell apart from structures which form through physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to .[9] Other finds in rocks dated to about have been interpreted as bacteria,[10] and geochemical evidence seemed to show the presence of life .[11] However these analyses were closely scrutinised, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.[12][13] 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. Currently, the oldest unchallenged evidence for life is geochemical signatures from rocks deposited ,[9][14] although there has been little time for these recent reports (2006) to be examined by critics.
[edit] Origins of life on Earth
- For more details on this topic, see Evidence of common descent, Common descent, and Homology (biology).
Biochemists reason that all living organisms on Earth must share a single Last Universal Common Ancestor, because it would be unbelievable that two or more separate lineages could have independently developed the many complex biochemical mechanisms shared by all living organisms.[16][17] However the earliest organisms for which fossil evidence is available are bacteria, which are far too complex to have arisen directly from non-living materials.[18] The lack of fossil or geochemical evidence for earlier types of organism has left plenty of scope for hypotheses, which fall into two main groups: that life arose spontaneously on Earth, and that it was "seeded" from elsewhere in the universe.
[edit] Life "seeded" from elsewhere
The idea that life Earth was "seeded" from elsewhere in the universe dates back at least to the fifth century BC.[19] In the twentieth century it was proposed by the physical chemist Svante Arrhenius,[20] by the astronomers Fred Hoyle and Chandra Wickramasinghe,[21] and by molecular biologist Francis Crick and chemist Leslie Orgel.[22] There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar system via fragments knocked into space by a large meteor impact, in which case the only credible source is Mars;[23] by alien visitors, possibly as a result of accidental contamination by micro-organisms that they brought with them;[22] and from outside the Solar system but by natural means.[23][20] Experiments suggest that some micro-organisms can survive the shock of being catapulted into space and some can survive exposure to radiation for several days, but there is no proof that they can survive in space for much longer periods.[23] Scientists are divided over the likelihood of life arising independently on Mars,[24] or on other planets in our galaxy.[23]
[edit] Independent emergence on Earth
Research on how life might have emerged unaided from non-living chemicals focuses on three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; 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.[25] Research on abiogenesis still has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other.[26][27]
[edit] Replication first: RNA world
Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. This system is far too complex to have emerged directly from non-living materials.[18] The discovery that some RNA molecules can catalyze both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA.[28] These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.[29] RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have.[29][30][31] Ribozymes remain as the main components of ribosomes, modern cells' "protein factories".[32]
Although short self-replicating RNA molecules have been artificially produced in laboratories,[33] doubts have been raised about where natural non-biological synthesis of RNA is possible.[34] 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.[35][36]
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. In this hypothesis lipid membranes would be the last major cell components to appear and until then the proto-cells would be confined to the pores.[37]
[edit] Metabolism first: Iron-sulfur world
A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including 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.[38]
[edit] Membranes first: Lipid world
= water-attracting heads of lipid molecules
= water-repellent tails
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.[39] 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 RNA might then have formed more easily within the liposomes than they would have outside.[40]
[edit] The clay theory
RNA is complex and there are doubts about whether it can be produced non-biologically in the wild.[34] 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.[41] Although this idea has not become the scientific consensus, it still has active supporters.[42]
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.[43]
A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.[44]
[edit] Environmental and evolutionary impact of microbial mats
These multi-layered colonies of bacteria and other organisms are generally only a few millimeters thick, but still contain a wide range of chemical environments.[45] In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there. To some extent each mat forms its own food chain, as the by-products of each group of micro-organisms generally serve as "food" for adjacent groups.[46]
Stromatolites are stubby pillars built as microbes in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.[45] Although earlier reports of fossilized stromatolites from about were criticized on the grounds that the structures in the rocks could have been produced by non-biological processes,[12] in 2006 another find of stromatolites was reported from the same part of Australia, in rocks also dated to .[47]
It is estimated that the appearance of oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The reducing agent used by oxygenic photosynthesis is water, which is much more plentiful than the geologically-produced reducing agents required by the earlier non-oxygenic photosynthesis.[48] From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes.[49] Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms.[50][51]
Oxygen became a significant component of Earth's atmosphere about .[52] Although eucaryotes may have been present much earlier,[53][54] the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eucaryotic cells, from which all multicellular organisms are built.[55] The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created selection pressure for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via endosymbiosis, where one organism lives inside another and both of them benefit from their association.[56]
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.[56]
[edit] Diversification of eucaryotes
| Eucaryotes |
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Eucaryotes may have been present long before the oxygenation of the atmosphere,[53] but most modern eucaryotes require oxygen, which their mitochondria use to fuel the production of ATP, the internal energy supply of all known cells.[55] In the 1970s it was proposed and, after much debate, widely accepted that eucaryotes emerged as a result of a sequence of endosymbioses between "procaryotes". For example: a predatory micro-organism invaded a large procaryote, probably an 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.[59][60][61] Another hypothesis proposes that mitochondria were originally sulfur- or hydrogen-metabolising endosymbionts, and became oxygen-consumers later.[62] On the other hand mitochondria might have been part of eucaryotes' original equipment.[63]
There is a debate about when eukaryotes first appeared: the presence of steranes in Australian shales may indicate that eukaryotes were present ;[54] however an analysis in 2008 concluded that these chemicals infiltrated the rocks less than and prove nothing about the origins of eukaryotes.[64] Fossils of the alga Grypania have been reported in rocks (originally dated to but later revised[65]), and indicates that eucaryotes with organelles had already evolved.[66] A diverse collection of fossil algae were found in rocks dated between and .[67] The earliest known fossils of fungi date from .[68]
[edit] Multicellular organisms and sexual reproduction
[edit] Multicellularity
The simplest definitions of "multicellular", for example "having multiple cells", could include colonial cyanobacteria like Nostoc. Even a professional biologist's definition such as "having the same genome but different types of cell" would still include some genera of the green alga Volvox, which have cells that specialize in reproduction.[70] Multicellularity evolved independently in organisms as diverse as sponges and other animals, fungi, plants, brown algae, cyanobacteria, slime moulds and myxobacteria.[71][65] For the sake of brevity this article focusses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the evolution of complexity could be regarded as "rather anthropocentric".[72]
The initial advantages of multicellularity may have included: 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;[73] and even the opportunity for a group of cells to behave "intelligently" by sharing information.[69] These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.[73]
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.[73]
The available evidence indicates that eucaryotes evolved much earlier but remained inconspicuous until a rapid diversification around . The only respect in which eucaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eucaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function.[73]
[edit] How sex evolved
The following hypotheses attempt to explain how and why sex evolved:
- It may have enabled organisms to repair genetic damage.[74] The most primitive form of sex may have been one organism repairing damaged DNA by replicating an undamaged strand from a similar organism.[75]
- Sexual reproduction may have originated from selfish parasitic genetic elements propagating themselves by transfer to new hosts.[76]
- It may have evolved from cannibalism, where some of the victim's DNA was incorporated into the cannibal organism.[75]
- Sexual reproduction may evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids.[77]
- Or it may have evolved as a form of vaccination in which infected hosts exchanged weakened symbiotic copies of parasitic DNA as protection against more virulent versions. The meiosis stage of sexual reproduction may then have evolved as a way of removing the symbiotes.[78]
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.[79] However conjugation is not a means of reproduction and is not limited to members of the same species, and there are cases where bacteria transfer DNA to plants and animals.[80] Nevertheless it may be an example of the "selfish genetic element" hypothesis, as it transfers DNA by means of such a "selfish gene", the F-plasmid.[75]
[edit] Fossil evidence for multicellularity and sexual reproduction
The earliest known fossil organism that is clearly multicellular, Qingshania,[note 1], dated to , appears to consist of virtually identical cells. A red alga called Bangiomorpha, dated at