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Timeline of evolution

This timeline of the evolution of life outlines the major events in the development of life on the planet Earth. For a thorough explanatory context, see the history of Earth, and geologic time scale. The dates given in this article are estimates based on scientific evidence.

In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation. Its occurrence over large stretches of time explains the origin of new species and ultimately the vast diversity of the biological world. Contemporary species are related to each other through common descent, products of evolution and speciation over billions of years.

Basic timeline

The basic timeline is a 4.6 billion year old Earth, with (very approximately):

4 billion years of simple cells (prokaryotes),
3 billion years of photosynthesis,
2 billion years of complex cells (eukaryotes),
1 billion years of multicellular life,
600 million years of simple animals,
570 million years of arthropods (ancestors of insects, arachnids and crustaceans)
550 million years of complex animals
500 million years of fish and proto-amphibians,
475 million years of land plants,
400 million years of insects and seeds,
360 million years of amphibians,
300 million years of reptiles,
200 million years of mammals,
150 million years of birds,
130 million years of flowers,
65 million years since the non-avian dinosaurs died out,
2.5 million years since the appearance of Homo,
200,000 years since humans started looking like they do today,
25,000 years since Neanderthals died out.

Detailed timeline

Note that Mya means "million years ago".

Hadean eon

3800 Mya and earlier.

Date	Event
4567.17 Mya	The planet Earth forms from the accretion disc revolving around the young Sun.
4533 Mya	According to one plausable theory, the planet Earth and the planet Theia collide, sending countless moonlets into orbit around the young Earth. These moonlets 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 for the formation of life.
4100 Mya	The surface of the Earth cools enough for the crust to solidify. The atmosphere and the oceans form.PAH infall, and Iron-Sulfide synthesis along deep ocean platelet boundaries, may have led to the RNA world of competing metabolising organic compounds.
Between 4500 and 2500 Mya	The earliest life appears, possibly derived from self-reproducing RNA molecules. The replication of these organisms requires resources like energy, space, and smaller building blocks, which soon become limited, resulting in competition. Natural selection favours those molecules which are more efficient at replication. DNA molecules then take over as the main replicators. They soon develop inside enclosing membranes which provide a stable physical and chemical environment conducive to their replication: proto-cells.
3900 Mya	Late Heavy Bombardment: peak rate of impact events upon the inner planets by meteors. This constant disturbance probably obliterated any life that had already evolved, as the oceans boiled away completely; conversely, life may have been transported to Earth by a meteor.
Somewhere between 3900 - 2500 Mya	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. Glycolysis generates ATP molecules as short-term energy currency, and ATP continue to be used in almost all organisms, unchanged, to this day.

Archean eon

3800 Mya - 2500 Mya

Date	Event
3500 Mya	Lifetime of the last universal ancestor; the split between the bacteria and the neomura occurs. Bacteria develop primitive forms of photosynthesis which at first do not produce oxygen. These organisms generate ATP by exploiting a proton gradient, a mechanism still used in virtually all organisms.
3000 Mya	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 subsequently rises, acting as a poison for many bacteria. The moon is still very close to the earth and causes tides 1000 feet high. The earth is continually wracked by hurricane force winds. These extreme mixing influences are thought to stimulate evolutionary processes. (See Oxygen Catastrophe)

Proterozoic eon

2500 Mya - 542 Mya

Date	Event
By 2100 Mya	Eukaryotic cells appear. Eukaryotes contain membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis.
By 1200 Mya	Sexual reproduction evolves, increasing the rate of evolution.
1200 Mya	Simple multicellular organisms evolve, mostly consisting of cell colonies of limited complexity.
850–630 Mya	A global glaciation may have reduced the diversity of life. Opinion is divided on whether it increased or decreased the rate of evolution.
580-542 Mya	The Ediacaran biota represent the first large, complex multicellular organisms - although their affinities remain a subject of debate.
580–500 Mya	Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion.
Around 540 Mya	The accumulation of atmospheric oxygen allows the formation of an ozone layer. This blocks ultraviolet radiation, permitting the colonisation of the land.

Phanerozoic eon

542 Mya - 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 Mya - 251.0 Mya

Date	Event
530 Mya	The first known footprints on land date to 530 Mya, indicating that early animal explorations may have predated the development of terrestrial plants.
475 Mya	The first primitive plants move onto land, having evolved from green algae living along the edges of lakes. They are accompanied by fungi, which may have aided the colonisation of land through symbiosis.
363 Mya	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.
251.4 Mya	The Permian-Triassic extinction event eliminates over 95% of species. This "clearing of the slate" may have led to an ensuing diversification.

Mesozoic era

Date	Event
From 251.4 Mya	The Mesozoic Marine Revolution begins: increasingly well-adapted and diverse predators pressurise sessile marine groups; the "balance of power" in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
220 Mya	Gymnosperm forests dominate the land; herbivores grow to huge sizes in order to accommodate the large guts necessary to digest the nutrient-poor plants.
200 Mya	The first accepted evidence for viruses (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.
130 Mya	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.

Cenozoic era

65.5 Mya - present

Date	Event
65.5 Mya	The Cretaceous–Tertiary extinction event eradicates about half of all animal species, including all dinosaurs except modern birds
35 Mya	Grasses evolve from among the angiosperms; grassland dominates many terrestrial ecosystems.
14 Mya	The term Anthropocene has been used to describe the period of time during which humans have had a major impact on the planet and its diversity of life. Its beginning is marked by the megafaunal extinction in the Americas which signify the onset of the Holocene extinction event. Fierce debate rages about the influence of man in the initiation of this extinction.
Present day	With a human population approaching 6.67 billion, the impact of humanity is felt in all corners of the globe. Overfishing, anthropogenic climate change, industrialisation, intensive agriculture, clearance of rain forests and other activities contribute to a dramatically rising extinction rate. If current rates continue, humanity will have seen the eradication of one-half of Earth's biodiversity over the next hundred years.