All given times are approximate estimates. We use the abbreviations "MYA" for "million years ago" and "TYA" for "thousand years ago".
| 4500 MYA |
The planet Earth forms from the accredition disk revolving around the young Sun.
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| 4100 MYA |
The surface of the Earth cools down enough for the crust to solidify.
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| 4000 MYA |
Life appears, probably first as self-reproducing RNA molecules.
The atmosphere does not contain any free oxygen.
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| 3900 MYA |
Cells resembling prokaryotes appear. These first organisms are chemoautotrophs?: they use carbon dioxide as carbon source and oxidize inorganic materials to extract energy. Prokaryotes are still the dominant life form on Earth. Later prokaryotes invent glycolysis, a set of chemical reactions that free the energy of organic molecules such as glucose. Glycolysis employs ATP molecules as short term energy currency and is used in almost all organisms unchanged to this day.
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| 3900 MYA |
The split between the bacteria and the archaea occurs.
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| 3500 MYA |
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.
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| 3000 MYA |
Photosynthesizing cyanobacteria? evolve; they use water as reductant, thereby producing oxygen as waste product. The oxygen initially oxydizes dissolved iron in the oceans, creating iron ore. Then the oxygen concentration in the atmosphere rises, acting as a poison for many bacteria.
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| 2500 MYA |
Some bacteria evolve the ability to utilize oxygen to more efficiently use the energy from organic molecules such as glucose. Virtually all organisms using oxygen employ the same set of reactions, the citric acid cycle and [oxidative phosphorylation]?.
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| 2100 MYA |
More complicated cells appear: the eukaryotes, which contain various organelles. The closest relatives of these are probably the Archaea. Most have organelles which are probably derived from symbiotic bacteria: mitochondria, which use oxygen to extract energy from organic molecules and appear similar to today's Rickettsia?, and often chloroplasts, which derive energy from light and synthesize organic molecules and originated from cyanobacteria and similar forms.
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| 1200 MYA | Sexual reproduction evolves and leads to an explosion in the rate of evolution.
While most life occurs in oceans and lakes, some cyanobacteria may already have lived in moist soil by this time.
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| 1000 MYA |
Multicellular organisms appear: algae and seaweeds living in the oceans.
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| 600 MYA |
Sponges, worms and other multicellular animals appear in the oceans.
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| 565-525 MYA |
The [Cambrian explosion]?, a rapid set of evolutionary changes, creates all the major body plans of modern animals.
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| 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, and very likely plants and fungi work symbiotically together.
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| 450 MYA |
Arthropods, with an exoskeleton that provides support and prevents water loss, are the first animals to invade the land. Among the first are spiders and scorpions, later followed by insects.
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| 365 MYA |
Fresh water fish moving onto land evolve lungs (from swim bladders) and legs (from fins); they turn into amphibians?.
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| 360 MYA |
Plants evolve seeds, structures that protect plant embryos and enable plants to spread quickly on land.
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| 300 MYA |
Evolution of the [amniotic egg]? turnes amphibians into reptiles? who can reproduce on land.
Insects evolve flight. Vast forests cover the land; decaying trees will eventually form coal.
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| 200 MYA |
Dinosaurs and pterosaurs (flying reptiles) appear as branches off the reptile tree.
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| 220 MYA |
The first mammals appear; they evolved from reptiles. Initially, they stay small.
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| 200 MYA |
Birds split off from the reptiles. Birds are the closest surviving relatives of the dinosaurs.
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| 130 MYA |
Plants evolve flowers, structures that attract insects and other animals to spread pollen.
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| 65 MYA |
Dinosaurs and many other species become exctinct in the course of about 1 million years, probably because of a cooling of the climate precipitated by the giant impact of a meteor. (See [cretaceous extinctions]?.)
Mammals increase in diversity and size. Some will later return back to the sea (whales and seals) and others will evolve flight (bats). Australia is separated from other land masses; the marsupial mammals evolve exclusively here.
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| 45 MYA |
Mammals most likely related to today's hippopotamuses return to the sea and evolve into whales.
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| 3 MYA |
North and South America become joined, allowing migration of animals.
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| 1800 TYA |
[Homo erectus]? evolves in the savannas of Africa and migrates to other continents.
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| 130 TYA |
Neanderthals? evolve from Homo erectus and live in Europe and the Middle East.
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| 100 TYA |
The first anatomically modern humans (Homo sapiens) appear in Africa. They also evolved from Homo erectus.
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| 35 TYA |
Neanderthals die out.
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| 15 TYA |
Humans develop agriculture and, along with it, permanent settlements and cities.
These appear first in what is now Iraq.
Humans cross for the first time from Siberia to Alaska on a land bridge created by an ice age. They migrate south throughout the Americas, reaching the tip of South America about 10 TYA.
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| 4 TYA |
Recorded history begins.
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