The timeline of life represents the current scientific theory outlining the major events during the development of life on Earth. Dates in this article are consensus estimates based on scientific evidence, mainly fossils.
In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, and individual organisms and molecules, such as DNA and proteins. The similarities between all present day organisms imply a common ancestor from which all known species, living and extinct, have diverged. More than 99 percent of all species that ever lived (over five billion) are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, with about 1.2 million or 14% documented, the rest not yet described. However, a 2016 report estimates an additional 1 trillion microbial species, with only 0.001% described.
There has been controversy between more traditional views of steadily increasing biodiversity, and a newer view of cycles of annihilation and diversification, so that certain past times, such as the Cambrian explosion, experienced maximums of diversity followed by sharp winnowing. Four diagrams of evolutionary models
Extinction
Species go extinct constantly as environments change, as organisms compete for environmental niches, and as genetic mutation leads to the rise of new species from older ones. At long irregular intervals, Earth's biosphere suffers a catastrophic die-off, a mass extinction, often comprising an accumulation of smaller extinction events over a relatively brief period.
The first known mass extinction was the Great Oxidation Event 2.4 billion years ago, which killed most of the planet's obligate anaerobes. Researchers have identified five other major extinction events in Earth's history, with estimated losses below:
End Ordovician: 440 million years ago, 86% of all species lost, including most graptolites Late Devonian: 375 million years ago, 75% of species lost, including most trilobites End Permian, The Great Dying: 251 million years ago, 96% of species lost, including tabulate corals, and most trees and synapsids End Triassic: 200 million years ago, 80% of species lost, including all conodonts End Cretaceous: 66 million years ago, 76% of species lost, including all ammonites, mosasaurs, plesiosaurs, pterosaurs, and non-avian dinosaurs
Smaller extinction events have occurred in the periods between, with some dividing geologic time periods and epochs. The Holocene extinction event is currently under way.
Factors in mass extinctions include continental drift, changes in atmospheric and marine chemistry, volcanism and other aspects of mountain formation, changes in glaciation, changes in sea level, and impact events.
Detailed timeline In this timeline, Ma (for megaannum) means "million years ago," ka (for kiloannum) means "thousand years ago," and ya means "years ago."
Hadean Eon
4540 Ma – 4031 Ma
Date Event 4540 Ma Planet Earth forms from the accretion disc revolving around the young Sun, perhaps preceded by formation of organic compounds necessary for life in the surrounding protoplanetary disk of cosmic dust. 4510 Ma According to the giant-impact hypothesis, the Moon originated when Earth and the hypothesized planet Theia collided, sending into orbit myriad moonlets which eventually coalesced into our single Moon. The Moon's gravitational pull stabilised Earth's fluctuating axis of rotation, setting up regular climatic conditions favoring abiogenesis. 4404 Ma Evidence of the first liquid water on Earth which were found in the oldest known zircon crystals. 4280–3770 Ma Earliest possible appearance of life on Earth.
Archean Eon
4031 Ma – 2500 Ma
Date Event4100 MaEarliest possible preservation of biogenic carbon. 4100–3800 Ma Late Heavy Bombardment (LHB): extended barrage by meteoroids impacting the inner planets. Thermal flux from widespread hydrothermal activity during the LHB may have aided abiogenesis and life's early diversification. Possible remains of biotic life were found in 4.1 billion-year-old rocks in Western Australia. 4000 Ma Formation of a greenstone belt of the Acasta Gneiss of the Slave craton in northwest Canada - the oldest known rock belt. 3900–2500 Ma Cells resembling prokaryotes appear. These first organisms are believed to have been chemoautotrophs, using carbon dioxide as a carbon source and oxidizing inorganic materials to extract energy. 3800 Ma Formation of a greenstone belt of the Isua complex in western Greenland, whose isotope frequencies suggest the presence of life. The earliest evidence for life on Earth includes: 3.8 billion-year-old biogenic hematite in a banded iron formation of the Nuvvuagittuq Greenstone Belt in Canada; graphite in 3.7 billion-year-old metasedimentary rocks in western Greenland; and microbial mat fossils in 3.48 billion-year-old sandstone in Western Australia. 3800–3500 Ma Last universal common ancestor (LUCA): split between bacteria and archaea.
Bacteria develop primitive photosynthesis, which at first did not produce oxygen. These organisms exploit a proton gradient to generate adenosine triphosphate (ATP), a mechanism used by virtually all subsequent organisms. 3000 Ma Photosynthesizing cyanobacteria using water as a reducing agent and producing oxygen as a waste product. Free oxygen initially oxidizes dissolved iron in the oceans, creating iron ore. Oxygen concentration in the atmosphere slowly rises, poisoning many bacteria and eventually triggering the Great Oxygenation Event. 2800 Ma Oldest evidence for microbial life on land in the form of organic matter-rich paleosols, ephemeral ponds and alluvial sequences, some bearing microfossils.
Proterozoic Eon
2500 Ma – 539 Ma. Contains the Palaeoproterozoic, Mesoproterozoic and Neoproterozoic eras.
Date Event 2500 MaGreat Oxidation Event led by cyanobacteria's oxygenic photosynthesis. Commencement of plate tectonics with old marine crust dense enough to subduct.2400 Ma Possible land fungi evidence from molecules.2023 MaFormation of the Vredefort impact structure, one of the largest and oldest verified impact structures on Earth. The crater is estimated to have been between across when it first formed. By 1850 Ma Eukaryotic cells, containing membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis. (See Symbiogenesis and Endosymbiont). Bacterial viruses (bacteriophages) emerge before or soon after the divergence of the prokaryotic and eukaryotic lineages. Red beds show an oxidising atmosphere, favouring the spread of eukaryotic life. 1500 MaVolyn biota, a collection of exceptionally well-preserved microfossils with varying morphologies. By 1200 Ma Meiosis and sexual reproduction in single-celled eukaryotes, possibly even in the common ancestor of all eukaryotes or in the RNA world. Sexual reproduction may have increased the rate of evolution. By 1000 Ma Molecular clock type evidence suggests that land fungi appear.Heckman DS, Geiser DM, Eidell BR, Stauffer RL, Kardos NL, Hedges SB. Molecular evidence for the early colonization of land by fungi and plants. Science. 2001 Aug 10;293(5532):1129-33. doi: 10.1126/science.1061457. PMID 11498589. First non-marine eukaryotes move onto land. They were photosynthetic and multicellular, indicating that plants evolved much earlier than originally thought. 750 Ma Beginning of animal evolution. 720–630 Ma Possible global glaciation which increased the atmospheric oxygen and decreased carbon dioxide, and was either caused by land plant evolution or resulted in it. Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution. 600 Ma Accumulation of atmospheric oxygen allows the formation of an ozone layer. Previous land-based life would probably have required other chemicals to attenuate ultraviolet radiation. 580–542 Ma Ediacaran biota, the first large, complex aquatic multicellular organisms. 580–500 Ma Cambrian explosion: most modern animal phyla appear. 550–540 Ma Ctenophora (comb jellies), Porifera (sponges), Anthozoa (corals and sea anemones), Ikaria wariootia (an early Bilaterian).
Phanerozoic Eon
539 Ma – present
The Phanerozoic Eon (Greek: period of well-displayed life) marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, with major mass extinctions at division points.
Palaeozoic Era
538.8 Ma – 251.9 Ma and contains the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods.
Date Event 535 Ma Major diversification of living things in the oceans: arthropods (e.g. trilobites, crustaceans), chordates, echinoderms, molluscs, brachiopods, foraminifers and radiolarians, etc. 530 Ma The first known footprints on land date to 530 Ma. 520 Ma Earliest graptolites. 511 MaEarliest crustaceans. 505 MaJellyfish have existed since at least this time. 485 Ma First vertebrates with true bones (jawless fishes). 450 Ma First complete conodonts and echinoids appear. 440 Ma First agnathan fishes: Heterostraci, Galeaspida, and Pituriaspida. 420 Ma Earliest ray-finned fishes, trigonotarbid arachnids, and land scorpions. 410 Ma First signs of teeth in fish. Earliest Nautilida, lycophytes, and trimerophytes. 488–400 Ma First cephalopods (nautiloids) and chitons. 395 Ma First lichens, stoneworts. Earliest harvestmen, mites, hexapods (springtails) and ammonoids. The earliest known tracks on land named the Zachelmie trackways which are possibly related to icthyostegalians. 375 MaTiktaalik, a lobe-finned fish with some anatomical features similar to early tetrapods. It has been suggested to be a transitional species between fish and tetrapods. 365 MaAcanthostega is one of the earliest vertebrates capable of walking. 363 Ma By the start of the Carboniferous Period, the Earth begins to resemble its present state. Insects roamed the land and would soon take to the skies; sharks swam the oceans as top predators, and vegetation covered the land, with seed-bearing plants and forests soon to flourish. Four-limbed tetrapods gradually gain adaptations which will help them occupy a terrestrial life-habit. 360 Ma First crabs and ferns. Land flora dominated by seed ferns. The Xinhang forest grows around this time. 350 Ma First large sharks, ratfishes, and hagfish; first crown tetrapods (with five digits and no fins and scales). 350 Ma Diversification of amphibians. 325-335 Ma First Reptiliomorpha. 330-320 Ma First amniote vertebrates (Paleothyris). 320 Ma Synapsids (precursors to mammals) separate from sauropsids (reptiles) in late Carboniferous. 305 Ma The Carboniferous rainforest collapse occurs, causing a minor extinction event, as well as paving the way for amniotes to become dominant over amphibians and seed plants over ferns and lycophytes. First diapsid reptiles (e.g. Petrolacosaurus). 280 Ma Earliest beetles, seed plants and conifers diversify while lepidodendrids and sphenopsids decrease. Terrestrial temnospondyl amphibians and pelycosaurs (e.g. Dimetrodon) diversify in species. 275 Ma Therapsid synapsids separate from pelycosaur synapsids. 265 MaGorgonopsians appear in the fossil record. 251.9–251.4 Ma The Permian–Triassic extinction event eliminates over 90-95% of marine species. Terrestrial organisms were not as seriously affected as the marine biota. This "clearing of the slate" may have led to an ensuing diversification, but life on land took 30 million years to completely recover.
Mesozoic Era
From 251.9 Ma to 66 Ma and containing the Triassic, Jurassic and Cretaceous periods.