The timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet 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)[1] are estimated to be extinct.[2][3] Estimates on the number of Earth's current species range from 10 million to 14 million,[4] with about 1.2 million or 14% documented, the rest not yet described.[5] However, a 2016 report estimates an additional 1 trillion microbial species, with only 0.001% described.[6]
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.[7][8]
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,[9] often comprising an accumulation of smaller extinction events over a relatively brief period.[10]
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:[11]
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.[12]
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.[15][16] The Moon's gravitational pull stabilised Earth's fluctuating axis of rotation, setting up regular climatic conditions favoring abiogenesis.[17]
Earliest possible preservation of biogenic carbon.[23][24]
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.[25] Possible remains of biotic life were found in 4.1 billion-year-old rocks in Western Australia.[26][27] Probable origin of life.
Photosynthesizing cyanobacteria using water as a reducing agent and producing oxygen as a waste product.[41] 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.
Formation of the Vredefort impact structure, one of the largest and oldest verified impact structures on Earth. The crater is estimated to have been between 170–300 kilometres (110–190 mi) across when it first formed.[43]
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.[44]Red beds show an oxidising atmosphere, favouring the spread of eukaryotic life.[45][46][47]
First non-marine eukaryotes move onto land. They were photosynthetic and multicellular, indicating that plants evolved much earlier than originally thought.[53]
Possible global glaciation[56][57] which increased the atmospheric oxygen and decreased carbon dioxide, and was either caused by land plant evolution[58] or resulted in it.[59] Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution.[60][61][62]
600 Ma
Accumulation of atmospheric oxygen allows the formation of an ozone layer.[63] Previous land-based life would probably have required other chemicals to attenuate ultraviolet radiation.[42]
580–542 Ma
Ediacaran biota, the first large, complex aquatic multicellular organisms.[64]
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.
Tiktaalik, 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.[81]
365 Ma
Acanthostega is one of the earliest vertebrates capable of walking.[82]
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,[83] 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.[84]
350 Ma
First large sharks, ratfishes, and hagfish; first crown tetrapods (with five digits and no fins and scales).
Synapsids (precursors to mammals) separate from sauropsids (reptiles) in late Carboniferous.[88]
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.
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.[90]
Mesozoic marine revolution begins: increasingly well adapted and diverse predators stress sessile marine groups; the "balance of power" in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
Seed-producing Gymnosperm forests dominate the land; herbivores grow to huge sizes to accommodate the large guts necessary to digest the nutrient-poor plants.[citation needed] First flies and turtles (Odontochelys). First coelophysoid dinosaurs. First mammals from small-sized cynodonts, which transitioned towards a nocturnal, insectivorous, and endothermic lifestyle.
First accepted evidence for viruses infecting eukaryotic cells (the group Geminiviridae).[91] However, viruses are still poorly understood and may have arisen before "life" itself, or may be a more recent phenomenon.
Major extinctions in terrestrial vertebrates and large amphibians. Earliest examples of armoured dinosaurs.
Rise of the angiosperms. Some of these flowering plants bear structures that attract insects and other animals to spread pollen; other angiosperms are pollinated by wind or water. This innovation causes a major burst of animal coevolution. First freshwater pelomedusid turtles. Earliest krill.
Rapid dominance of conifers and ginkgos in high latitudes, along with mammals becoming the dominant species. First psammobiid bivalves. Earliest rodents. Rapid diversification in ants.
63 Ma
Evolution of the creodonts, an important group of meat-eating (carnivorous) mammals.
Grasslands and savannas are established, diversity in insects, especially ants and termites, horses increase in body size and develop high-crowned teeth, major diversification in grassland mammals and snakes.
First tree sloths and hippopotami, diversification of grazing herbivores like zebras and elephants, large carnivorous mammals like lions and the genus Canis, burrowing rodents, kangaroos, birds, and small carnivores, vultures increase in size, decrease in the number of perissodactyl mammals. Extinction of nimravid carnivores. First leopard seals.
Australopithecus evolves. Stupendemys appears in the fossil record as the largest freshwater turtle, first modern elephants, giraffes, zebras, lions, rhinoceros and gazelles appear in the fossil record
First members of genus Homo, Homo Habilis, appear in the fossil record. Diversification of conifers in high latitudes. The eventual ancestor of cattle, aurochs (Bos primigenus), evolves in India.
1.7 Ma
Australopithecines go extinct.
1.2 Ma
Evolution of Homo antecessor. The last members of Paranthropus die out.
Anatomically modern humans appear in Africa.[103][104][105] Around 50 ka they start colonising the other continents, replacing Neanderthals in Europe and other hominins in Asia.
Last woolly rhinoceros (Coelodonta antiquitatis) are believed to have gone extinct.
11 ka
Short-faced bears vanish from North America, with the last giant ground sloths dying out. All Equidae become extinct in North America. Domestication of various ungulates.
^Astrobio (September 24, 2001). "Making the Moon". Astrobiology Magazine (Based on a Southwest Research Institute press release). ISSN2152-1239. Archived from the original on 2015-09-08. Retrieved 2015-03-04. Because the Moon helps stabilize the tilt of the Earth's rotation, it prevents the Earth from wobbling between climatic extremes. Without the Moon, seasonal shifts would likely outpace even the most adaptable forms of life.{{cite journal}}: CS1 maint: unfit URL (link)
^Hahn, Jürgen; Haug, Pat (May 1986). "Traces of Archaebacteria in ancient sediments". Systematic and Applied Microbiology. 7 (2–3): 178–183. doi:10.1016/S0723-2020(86)80002-9. ISSN0723-2020.
^Clarke, Tom (April 30, 2002). "Oldest fossil footprints on land". Nature. doi:10.1038/news020429-2. ISSN1744-7933. Retrieved 2015-03-09. The oldest fossils of footprints ever found on land hint that animals may have beaten plants out of the primordial seas. Lobster-sized, centipede-like animals made the prints wading out of the ocean and scuttling over sand dunes about 530 million years ago. Previous fossils indicated that animals didn't take this step until 40 million years later.
^"Graptolites". British Geological Survey. Retrieved 2022-09-24.
^Martin, R. Aidan. "Evolution of a Super Predator". Biology of Sharks and Rays. North Vancouver, BC, Canada: ReefQuest Centre for Shark Research. Retrieved 2015-03-10. The ancestry of sharks dates back more than 200 million years before the earliest known dinosaur.
^Rybicki, Ed (April 2008). "Origins of Viruses". Introduction of Molecular Virology (Lecture). Cape Town, Western Cape, South Africa: University of Cape Town. Archived from the original on 2009-05-09. Retrieved 2015-03-10. Viruses of nearly all the major classes of organisms - animals, plants, fungi and bacteria / archaea - probably evolved with their hosts in the seas, given that most of the evolution of life on this planet has occurred there. This means that viruses also probably emerged from the waters with their different hosts, during the successive waves of colonisation of the terrestrial environment.
^Greshko, Michael (2020-02-11). "Oldest evidence of modern bees found in Argentina". National Geographic. Archived from the original on February 23, 2021. Retrieved 2022-06-22. The model shows that modern bees started diversifying at a breakneck pace about 114 million years ago, right around the time that eudicots—the plant group that comprises 75 percent of flowering plants—started branching out. The results, which confirm some earlier genetic studies, strengthen the case that flowering plants and pollinating bees have coevolved from the very beginning.
McKinney, Michael L. (1997). "How do rare species avoid extinction? A paleontological view". In Kunin, William E.; Gaston, Kevin J. (eds.). The Biology of Rarity: Causes and consequences of rare—common differences (1st ed.). London; New York: Chapman & Hall. ISBN978-0-412-63380-5. LCCN96071014. OCLC36442106.
Kyrk, John. "Evolution". Cell Biology Animation. Archived from the original(SWF) on 2012-10-22. Retrieved 2015-03-18. Interactive timeline from Big Bang to present