The Evolutionary History of Life

The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on the planet first originated until the present day. Earth formed about 4.5 Ga (billion years ago) and life appeared on its surface within 1 billion years. The similarities between all present-day organisms indicate the presence of a common ancestor from which all known species have diverged through the process of evolution.
Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them. The evolution of oxygenic photosynthesis, around 3.5 Ga, eventually led to the oxygenation of the atmosphere, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. Bilateria, animals with a front and a back, appeared by 555 million years ago.
The earliest land plants date back to around 450 Ma (million years ago), although evidence suggests that algal scum formed on the land as early as 1.2 Ga. Land plants were so successful that they are thought to have contributed to the late Devonian extinction event. Invertebrate animals appear during the Ediacaran period, while vertebrates originated about 525 Ma during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252.2 Ma. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates, displacing therapsids in the mid-Triassic; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 Ma killed off the dinosaurs, mammals increased rapidly in size and diversity. Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.
Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous (130 to 90 Ma) 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 6 Ma. Although early members of this lineage had chimpanzee-sized brains, there are signs of a steady increase in brain size after about 3 Ma.

Axis scale: millions of years ago.
Dates prior to 1000 million years ago are speculative.
This timeline of evolution of life represents current scientific theory outlining the major events in the development of life on planet Earth. 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 indicate the presence of a common ancestor from which all known species, living and extinct, have diverged through the process of evolution.
The dates given in this article are estimates based on scientific evidence.

Basic timeline
In its 4.6 billion years circling the sun, the Earth has harbored an increasing diversity of life forms:
for the last 3.6 billion years, simple cells (prokaryotes);
for the last 3.4 billion years, cyanobacteria performing photosynthesis;
for the last 2 billion years, complex cells (eukaryotes);
for the last 1 billion years, multicellular life;
for the last 600 million years, simple animals;
for the last 550 million years, bilaterians, animals with a front and a back;
for the last 500 million years, fish and proto-amphibians;
for the last 475 million years, land plants;
for the last 400 million years, insects and seeds;
for the last 360 million years, amphibians;
for the last 300 million years, reptiles;
for the last 200 million years, mammals;
for the last 150 million years, birds;
for the last 130 million years, flowers;
for the last 60 million years, the primates,
for the last 20 million years, the family Hominidae (great apes);
for the last 2.5 million years, the genus Homo (human predecessors);
for the last 200,000 years, anatomically modern humans.

Periodic extinctions have temporarily reduced diversity, eliminating:
2.4 billion years ago, many obligate anaerobes, in the oxygen catastrophe;
252 million years ago, the trilobites, in the Permian–Triassic extinction event;
66 million years ago, the pterosaurs and nonavian dinosaurs, in the Cretaceous–Paleogene extinction event.
Dates are approximate.
In this timeline, Ma (for megaannum) means “million years ago”, ka (for kiloannum) means “thousand years ago”, and ya means “years ago”.

Hadean Eon 4000 Ma and earlier.
4600 Ma The planet Earth forms from the accretion disc revolving around the young Sun; complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.
4500 Ma According to the giant impact hypothesis the moon is formed when the planet Earth and the planet Theia collide, sending a very large number of moonlets into orbit around the young Earth which eventually coalesce to form the Moon. The gravitational pull of the new Moon stabilises the Earth’s fluctuating axis of rotation and sets up the conditions in which life formed.
Archean Eon 4000 Ma – 2500 Ma
4000 Ma Formation of Greenstone belt of the Acasta gneisses of the Great Slave Region, in Canada, the oldest rock belt in the world.
4100–3800 Ma Late Heavy Bombardment: extended barrage of impact events upon the inner planets by meteoroids. Thermal flux from widespread hydrothermal activity during the LHB may have been conducive to life’s emergence and early diversification.
3900–2500 Ma Cells resembling prokaryotes appear. These first organisms are chemoautotrophs: they use carbon dioxide as a carbon source and oxidize inorganic materials to extract energy. Later, prokaryotes evolve glycolysis, a set of chemical reactions that free the energy of organic molecules such as glucose and store it in the chemical bonds of ATP. Glycolysis (and ATP) continue to be used in almost all organisms, unchanged, to this day.
3800 Ma Formation of Greenstone belt of the Isua complex of the western Greenland Region, whose rocks show an isotope frequency suggestive of the presence of life.
3500 Ma Lifetime of the last universal ancestor; the split between bacteria and archaea occurs.
Bacteria develop primitive forms of photosynthesis which at first do not produce oxygen.[12] These organisms generate ATP by exploiting a proton gradient, a mechanism still used in virtually all organisms.
3000 Ma Photosynthesizing cyanobacteria evolve; they use water as a reducing agent, thereby producing oxygen as waste product. The oxygen initially oxidizes dissolved iron in the oceans, creating iron ore. The oxygen concentration in the atmosphere slowly rises, acting as a poison for many bacteria. The Moon is still very close to Earth and causes tides 1,000 feet (305 m) high. The Earth is continually wracked by hurricane-force winds. These extreme mixing influences are thought to stimulate evolutionary processes. (See Oxygen catastrophe). Life on land likely developed at this time.
Proterozoic Eon 2500 Ma – 542 Ma
2500 Ma Great Oxidation Event led by Cyanobacteria’s oxygenic photosynthesis. Commencement of plate tectonics with old marine crust dense enough to subduct.
2000 Ma Diversification and expansion of acritarchs.
By 1850 Ma Eukaryotic cells appear. Eukaryotes contain membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis. Bacterial viruses (bacteriophage) emerge before, or soon after, the divergence of the prokaryotic and eukaryotic lineages. The appearance of red beds show that an oxidising atmosphere had been produced. Incentives now favoured the spread of eukaryotic life.
1400 Ma Great increase in stromatolite diversity.
By 1200 Ma Meiosis and sexual reproduction are present in single-celled eukaryotes, and possibly in the common ancestor of all eukaryotes. Sex may even have arisen earlier in the RNA world. Sexual reproduction first appears in the fossil records; it may have increased the rate of evolution.
1200 Ma Simple multicellular organisms evolve, mostly consisting of cell colonies of limited complexity. First multicellular red algae evolve
1100 Ma Earliest dinoflagellates
1000 Ma First vaucherian algae (ex: Palaeovaucheria)
750 Ma First protozoa (ex: Melanocyrillium)
850–630 Ma A global glaciation may have occurred. Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution.
600 Ma The accumulation of atmospheric oxygen allows the formation of an ozone layer. Prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonisation of the land.
580–542 Ma The Ediacaran biota represent the first large, complex multicellular organisms — although their affinities remain a subject of debate.
580–500 Ma Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion.
560 Ma Earliest fungi
550 Ma First fossil evidence for ctenophora (comb jellies), porifera (sponges), and anthozoa (corals & anemones)
Phanerozoic Eon 542 Ma – present
The Phanerozoic Eon, literally the “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, which are divided by major mass extinctions.
Paleozoic Era 542 Ma – 251.0 Ma
535 Ma Major diversification of living things in the oceans: chordates, arthropods (e.g. trilobites, crustaceans), echinoderms, mollusks, brachiopods, foraminifers and radiolarians, etc.
530 Ma The first known footprints on land date to 530 Ma, indicating that early animal explorations may have predated the development of terrestrial plants.
525 Ma Earliest graptolites.
510 Ma First cephalopods (Nautiloids) and chitons.
505 Ma Fossilization of the Burgess Shale.
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.
434 Ma The first primitive plants move onto land, having evolved from green algae living along the edges of lakes. They are accompanied by fungi[citation needed], which may have aided the colonization of land through symbiosis.
420 Ma Earliest ray-finned fishes, trigonotarbid arachnids, and land scorpions.
410 Ma First signs of teeth in fish. Earliest nautiid nautiloids, lycophytes, and trimerophytes.
395 Ma First lichens, stoneworts. Earliest harvestman, mites, hexapods (springtails) and ammonoids. The first known tetrapod tracks on land.
363 Ma By the start of the Carboniferous Period, the Earth begins to be recognisable. 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.
350 Ma First large sharks, ratfishes, and hagfish.
340 Ma Diversification of amphibians.
330 Ma First amniote vertebrates (Paleothyris).
320 Ma Synapsids separate from sauropsids (reptiles) in late Carboniferous.
305 Ma Earliest 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 Therapsids separate from synapsids.
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 30M years to completely recover.
Mesozoic Era From 251.4 Ma
The Mesozoic Marine Revolution begins: increasingly well adapted and diverse predators pressurize sessile marine groups; the “balance of power” in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
245 Ma Earliest ichthyosaurs.
240 Ma Increase in diversity of gomphodont cynodonts and rhynchosaurs.
225 Ma Earliest dinosaurs (prosauropods), first cardiid bivalves, diversity in cycads, bennettitaleans, and conifers. First teleost fishes. First mammals (Adelobasileus).
220 Ma 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
200 Ma The first accepted evidence for viruses that infect eukaryotic cells (at least, the group Geminiviridae) exists. 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 Ankylosaurian dinosaurs
195 Ma First pterosaurs with specialized feeding (Dorygnathus). First sauropod dinosaurs. Diversification in small, ornithischian dinosaurs: heterodontosaurids, fabrosaurids, and scelidosaurids.
190 Ma Pliosaurs appear in the fossil record. First lepidopteran insects (Archaeolepis), hermit crabs, modern starfish, irregular echinoids, corbulid bivalves, and tubulipore bryozoans. Extensive development of sponge reefs.
176 Ma First members of the Stegosauria group of dinosaurs
170 Ma Earliest salamanders, newts, cryptoclidid & elasmosaurid plesiosaurs, and cladotherian mammals. Sauropod dinosaurs diversify.
165 Ma First rays and glycymeridid bivalves.
161 Ma Ceratopsian dinosaurs appear in the fossil record (Yinlong)
155 Ma First blood-sucking insects (ceratopogonids), rudist bivalves, and cheilostome bryozoans. Archaeopteryx, a possible ancestor to the birds, appears in the fossil record, along with triconodontid and symmetrodont mammals. Diversity in stegosaurian and theropod dinosaurs.
130 Ma The rise of the Angiosperms: These flowering plants boast structures that attract insects and other animals to spread pollen. This innovation causes a major burst of animal evolution through co-evolution. First freshwater pelomedusid turtles.
120 Ma Oldest fossils of heterokonts, including both marine diatoms and silicoflagellates.
115 Ma First monotreme mammals.
110 Ma First hesperornithes, toothed diving birds. Earliest limopsid, verticordiid, and thyasirid bivalves.
106 Ma Spinosaurus, the largest theropod dinosaur, appears in the fossil record.
100 Ma Earliest bees.
90 Ma Extinction of ichthyosaurs. Earliest snakes and nuculanid bivalves. Large diversification in angiosperms: magnoliids, rosids, hamamelidids, monocots, and ginger. Earliest examples of ticks.
80 Ma First ants.
70 Ma Multituberculate mammals increase in diversity. First yoldiid bivalves.
68 Ma Tyrannosaurus, the largest terrestrial predator of North America appears in the fossil record. First species of Triceratops.
Cenozoic Era 66 Ma – present
66 Ma The Cretaceous–Paleogene extinction event eradicates about half of all animal species, including mosasaurs, pterosaurs, plesiosaurs, ammonites, belemnites, rudist and inoceramid bivalves, most planktic foraminifers, and all of the dinosaurs excluding their descendants, the birds.
From 66 Ma Rapid dominance of conifers and ginkgos in high latitudes, along with mammals becoming the dominant species. First psammobiid bivalves. Rapid diversification in ants.
63 Ma Evolution of the creodonts, an important group of carnivorous mammals.
60 Ma Diversification of large, flightless birds. Earliest true primates, along with the first semelid bivalves, edentates, carnivorous and lipotyphlan mammals, and owls. The ancestors of the carnivorous mammals (miacids) were alive.
56 Ma Gastornis, a large, flightless bird appears in the fossil record, becoming an apex predator at the time.
55 Ma Modern bird groups diversify (first song birds, parrots, loons, swifts, woodpeckers), first whale (Himalayacetus), earliest rodents, lagomorphs, armadillos, appearance of sirenians, proboscideans, perissodactyl and artiodactyl mammals in the fossil record. Angiosperms diversify. The ancestor (according to theory) of the species in Carcharodon, the early mako shark Isurus hastalis, is alive.
52 Ma First bats appear (Onychonycteris).
50 Ma Peak diversity of dinoflagellates and nanofossils, increase in diversity of anomalodesmatan and heteroconch bivalves, brontotheres, tapirs, rhinoceroses, and camels appear in the fossil record, diversification of primates.
40 Ma Modern-type butterflies and moths appear. Extinction of Gastornis. Basilosaurus, one of the first of the giant whales, appeared in the fossil record.
37 Ma First Nimravid carnivores (“False Saber-toothed Cats”) — these species are unrelated to modern-type felines
35 Ma Grasses evolve from among the angiosperms; grasslands begin to expand. Slight increase in diversity of cold-tolerant ostracods and foraminifers, along with major extinctions of gastropods, reptiles, and amphibians. Many modern mammal groups begin to appear: first glyptodonts, ground sloths, dogs, peccaries, and the first eagles and hawks. Diversity in toothed and baleen whales.
33 Ma Evolution of the thylacinid marsupials (Badjcinus).
30 Ma First balanids and eucalypts, extinction of embrithopod and brontothere mammals, earliest pigs and cats.
28 Ma Paraceratherium appears in the fossil record, the largest terrestrial mammal that ever lived.
25 Ma First deer.
20 Ma First giraffes, hyenas, bears and giant anteaters, increase in bird diversity.
15 Ma Mammut appears in the fossil record, first bovids and kangaroos, diversity in Australian megafauna.
10 Ma 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.
6.5 Ma First hominin (Sahelanthropus).
6 Ma Australopithecines diversify (Orrorin, Ardipithecus)
5 Ma First tree sloths and hippopotami, diversification of grazing herbivores like zebras and elephants, large carnivorous mammals like lions and dogs, burrowing rodents, kangaroos, birds, and small carnivores, vultures increase in size, decrease in the number of perissodactyl mammals. Extinction of Nimravid carnivores
4.8 Ma Mammoths appear in the fossil record.
4 Ma Evolution of Australopithecus, Stupendemys appears in the fossil record as the largest freshwater turtle, first modern elephants, giraffes, zebras, lions, rhinos and gazelles appear in the fossil record.
3 Ma The Great American Interchange, where various land and freshwater faunas migrated between North and South America. Armadillos, opossums, hummingbirds, and vampire bats traveled to North America while horses, tapirs, saber-toothed cats, and deer entered South America. The first short-faced bears (Arctodus) appear.
2.7 Ma Evolution of Paranthropus
2.5 Ma The earliest species of Smilodon evolve
2 Ma First members of the genus Homo appear in the fossil record. Diversification of conifers in high latitudes. The eventual ancestor of cattle, Bos primigenius evolves in India
1.7 Ma Extinction of australopithecines.
1.2 Ma Evolution of Homo antecessor. The last members of Paranthropus die out.
600 ka Evolution of Homo heidelbergensis
350 ka Evolution of Neanderthals
300 ka Gigantopithecus, a giant relative of the orangutan dies out from Asia
200 ka Anatomically modern humans appear in Africa.[40][41][42] Around 50,000 years before present they start colonising the other continents, replacing the Neanderthals in Europe and other hominins in Asia.
40 ka The last of the giant monitor lizards (Megalania) die out
30 ka Extinction of Neanderthals, first domestic dogs.
15 ka The last Woolly rhinoceros (Coelodonta) are believed to have gone extinct
11 ka The giant short-faced bears (Arctodus) vanish from North America, with the last Giant Ground Sloths dying out. All Equidae become extinct in North America
10 ka The Holocene Epoch starts 10,000[43] years ago after the Late Glacial Maximum. The last mainland species of Woolly mammoth (Mammuthus primigenius) die out, as does the last Smilodon species

Historical extinctions
6000 ya Small populations of American Mastodon die off in places like Utah and Michigan
4500 ya The last members of a dwarf race of Woolly Mammoths vanish from Wrangel Island near Alaska
613 ya (1400) The moa and its predator, Haast’s Eagle, die out in New Zealand
386 ya (1627) The last recorded wild Aurochs die out
325 ya (1688) The dodo goes extinct
245 ya (1768) The Steller’s sea cow goes extinct
130 ya (1883) The quagga, a subspecies of zebra, goes extinct
99 ya (1914) Martha, last known Passenger Pigeon, dies
77 ya (1936) The Thylacine goes extinct in a Tasmanian zoo, the last member of the family Thylacinidae
61 ya (1952) The Caribbean monk seal goes extinct[44]
5 ya (2008) The Baiji, the Yangtze river dolphin, becomes functionally extinct

Did Dark Matter Kill the Dinosaurs? It is well accepted that a meteorite collided with the earth 65 million years ago causing the dinosaur die-off. But a new reason for the meteorite may be dark matter – that not fully-explained stuff that makes up 85% of all matter and is believed to surround galaxies in a sort of sphere, holding them together. A new model of a different type dark matter interacts electronically and exists in a thin layer in the middle of the Milky Way, sandwiched between its top and bottom halves.
Most of the time that would have no consequence for Earth. But every 35 million years, as our sun orbits the center of the galaxy, it would cross that dark-matter equator, creating a disturbance that could jostle the comets that hover at the fringes of our solar system, sending one plunging toward Earth. Geological records do suggest heavy cratering on Earth at about those intervals, and fossil records suggest corresponding die-offs.

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I would like to think of myself as a full time traveler. I have been retired since 2006 and in that time have traveled every winter for four to seven months. The months that I am "home", are often also spent on the road, hiking or kayaking. I hope to present a website that describes my travel along with my hiking and sea kayaking experiences.
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