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This timeline of the evolution of life outlines the major events in the development of life on the planet Earth (See Organism). 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.

 

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

  • 3.8 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 the genus 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

4600 Mya

The planet Earth forms from the accretion disc revolving around the young Sun.

4533 Mya

According to one plausible 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.[1]

4100 Mya

The surface of the Earth cools enough for the crust to solidify. The atmosphere and the oceans form.[2]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.[3]

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 Archaea 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.[4]

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.[citation needed]

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.[5]

434 Mya

The first primitive plants move onto land,[6][citation needed] having evolved from green algae living along the edges of lakes.[7] 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,[8] 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

Eoraptor, an early dinosaur.

Gymnosperm forests dominate the land; herbivores grow to huge sizes in order to accommodate the large guts necessary to digest the nutrient-poor plants.[citation needed]

200 Mya

The first accepted evidence for viruses (at least, the group Geminiviridae) exists.[9] 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 the ancestors of modern birds[10]

35 Mya

Grasses evolve from among the angiosperms; grassland dominates many terrestrial ecosystems.

200 kya
(200,000 years ago)

Anatomically modern humans appear in Africa. Around 50,000 years before present they start colonising the other continents, replacing the Neanderthals in Europe and other hominins in Asia. The Holocene epoch starts 10,000 years ago after the Last Glacial Maximum, with continuing impact from human activity.

Present day

With a human population approaching 6.76 billion,[11] the impact of humanity is felt in all corners of the globe. Overfishing, anthropogenic climate change, industrialization, intensive agriculture, clearance of rain forests and other activities contribute to a dramatically rising extinction rate.[12] If current rates continue, humanity will have seen the eradication of one-half of Earth's biodiversity over the next hundred years.[13]

See also

Further reading

  • The Ancestor's Tale by Richard Dawkins, for a list of ancestors common to humans and other living species

References

  1. http://en.wikipedia.org/wiki/Timeline_of_evolution
  2. ^ Planetary Science Institute page on the Giant Impact Hypothesis. Hartmann and Davis belonged to the PSI. This page also contains several paintings of the impact by Hartmann himself.
  3. ^ "However, once the Earth cooled sufficiently, sometime in the first 700 million years of its existence, clouds began to form in the atmosphere, and the Earth entered a new phase of development." How the Oceans Formed (URL accessed on January 9, 2005)
  4. ^ " Between about 3.8 billion and 4.5 billion years ago, no place in the solar system was safe from the huge arsenal of asteroids and comets left over from the formation of the planets. Sleep and Zahnle calculate that Earth was probably hit repeatedly by objects up to 500 kilometers across" Geophysicist Sleep: Martian underground may have harbored early life (URL accessed on January 9, 2005)
  5. ^ "'Experiments with sex have been very hard to conduct,' Goddard said. 'In an experiment, one needs to hold all else constant, apart from the aspect of interest. This means that no higher organisms can be used, since they have to have sex to reproduce and therefore provide no asexual control.'
    Goddard and colleagues instead turned to a single-celled organism, yeast, to test the idea that sex allows populations to adapt to new conditions more rapidly than asexual populations."
    Sex Speeds Up Evolution, Study Finds (URL accessed on January 9, 2005)
  6. ^ "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 or slug like animals such as Protichnites and Climactichnites 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." Oldest fossil footprints on land
  7. ^ "The oldest fossils reveal evolution of non-vascular plants by the middle to late Ordovician Period (~450-440 m.y.a.) on the basis of fossil spores" Transition of plants to land
  8. ^ "The land plants evolved from the algae, more specifically green algae, as suggested by certain common biochemical traits" The first land plants
  9. ^ "The ancestry of sharks dates back more than 200 million years before the earliest known dinosaur. Introduction to shark evolution, geologic time and age determination
  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." Origins of Viruses (URL accessed on January 9, 2005)
  11. ^ . Chiappe, Luis M., & Dyke, Gareth J. (2002). "The Mesozoic Radiation of Birds". Annual Review of Ecology & Systematics 33: 91–124. doi:10.1146/annurev.ecolsys.33.010802.150517. 
  12. ^ An United States Census Bureau estimate of the number of people alive on Earth at any given moment. United States census bureau
  13. ^ The American Museum of Natural History National Survey Reveals Biodiversity Crisis (URL accessed on February 23, 2006)
  14. ^ E. O. Wilson, Harvard University, The Future of Life (2002)

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