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Although evidence of early life is scarce and often difficult to interpret, it appears that life appeared on Earth relatively soon (on the geologic time scale) after the planet had cooled enough for liquid water to be present. The dominant theory within the scientific community is that the first living organisms originated on Earth, evolving from non-living self-replicating molecules, and gained other characteristics of living organisms (see abiogenesis). An alternative theory suggests that the first simple organisms came from outer space, perhaps on a meteorite (see panspermia).
Overview
For the majority of its history, life has been restricted to the sea, and has been dominated by microbial mats. These layered colonies consisted of bacteria and other microbes with a diverse range of behaviours, from decomposition to photosynthesis. The evolution of oxygenic photosynthesis produced free oxygen, which initially reacted with the Earth's crust. When this weathering process had stabilised, free oxygen began to accumulate. This allowed organisms to grow larger and more complex, which in turn paved the way for multicellularity and sex.
Sexual reproduction allows for more rapid evolution, but there were no organisms large enough to handle until shortly before the Cambrian period, . The Cambrian period saw an explosion of animal diversity, and most recognisable groups of organisms—as well as some bizarre extinct types—appeared in this period. Although the seas flourished with life, the land was still barren—a number of biological challenges had to be overcome before land could inhabited. Plants, animals and fungi all seem to have become established on the land around the same time, around the Silurian period (); they diversified and expanded over the coming millions of years, and the appearance of forests of land plants changed Earth's environment sufficiently to trigger one of the five largest extinction events. Other such events dramatically changed the dominant organisms on both land and sea; the largest—the Permian-Triassic extinction event —wiped out the vast majority of animal taxa .
Following the P–T extinction event, vertebrates such as the dinosaurs rose to become the dominant large land animals. The flora was revolutionised with the advent of the flowering plants in the Cretaceous period (); co-evolution with insects may have hastened their diversification and given rise to social insects, which, while a small part of the insect "family tree", account for 50% of the total mass of all insects. The extinction of the dinosaurs , and a subsequent warm spell, allowed mammals to radiate.
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]
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.
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.
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 the biochemists Francis Crick and 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]
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]
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]
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 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]
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]
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]
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]
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]
The presence of steranes in Australian shales indicates that eukaryotes were present .[54] Fossils of the alga Grypania have been reported in rocks (originally dated to but later revised[64]), and indicates that eucaryotes with organelles had already evolved.[65] A diverse collection of fossil algae were found in rocks dated between and .[66] The earliest known fossils of fungi date from .[67]
Multicellular organisms and sexual reproduction
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.[69] Multicellularity evolved independently in organisms as diverse as sponges and other animals, fungi, plants, brown algae, cyanobacteria, slime moulds and myxobacteria.[70][64] 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".[71]
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;[72] and even the opportunity for a group of cells to behave "intelligently" by sharing information.[68] These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.[72]
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.[72]
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.[72]
How sex evolved
The following hypotheses attempt to explain how and why sex evolved:
- It may have enabled organisms to repair genetic damage.[73] The most primitive form of sex may have been one organism repairing damaged DNA by replicating an undamaged strand from a similar organism.[74]
- Sexual reproduction may have originated from selfish parasitic genetic elements propagating themselves by transfer to new hosts.[75]
- It may have evolved from cannibalism, where some of the victim's DNA was incorporated into the cannibal organism.[74]
- Sexual reproduction may evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids.[76]
- 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.[77]
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.[78] 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.[79] 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.[74]
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 , is the earliest known organism which has differentiated, specialized cells, and is also the oldest known sexually-reproducing organism.[72] The fossils interpreted as fungi appear to have been multicellular with differentiated cells.[67] The "string of beads" organism Horodyskia, found in rocks dated from to , may have been an early metazoan;[64] however it has also been interpreted as a colonial foraminiferan.[80]
Emergence of animals
Animals are multicellular eucaryotes,[note 2] and are distinguished from plants, algae, and fungi by lacking cell walls.[82] All animals are motile,[83] 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.
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.[84]
The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,[85] were the first animals more than a very few centimeters long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate kingdom, Vendozoa.[86] Others, however, been interpreted as early molluscs (Kimberella),[87][88] echinoderms (Arkarua);[89] and arthropods (Spriggina,[90] Parvancorina[91]). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans.[92] However there seems little doubt that Kimberella was at least a triploblastic bilaterian animal, in other words significantly more complex than cnidarians.[92]
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.[93]
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.[93] 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 at 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.[94] 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[95] – for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have have been closely related to the modern tardigrades.[96] 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.