All given times are approximate estimates. We use the abbreviations "MYA" for "million years ago" and "TYA" for "thousand years ago".
The surface of the Earth cools down enough for the crust to solidify.
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.
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.
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.
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.
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.
|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.
Multicellular organisms appear: algae and seaweeds living in the oceans.
Sponges, worms and other multicellular animals appear in the oceans.
The Cambrian explosion, a rapid set of evolutionary changes, creates all the major body plans of modern animals.
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.
Fresh water fish moving onto land evolve lungs (from swim bladders) and legs (from fins); they turn into amphibians.
Plants evolve seeds, structures that protect plant embryos and enable plants to spread quickly on land.
Insects evolve flight.
Vast forests cover the land; decaying trees will eventually form coal.
The first mammals appear; they evolved from reptiles. Initially, they stay small.
Birds split off from the reptiles. Birds are the closest surviving relatives of the dinosaurs.
Plants evolve flowers, structures that attract insects and other animals to spread pollen.
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.
Mammals most likely related to today's hippopotamuses return to the sea and evolve into whales.
North and South America become joined, allowing migration of animals.
Homo erectus evolves in the savannas of Africa and migrates to other continents.
Neanderthals evolve from Homo erectus and live in Europe and the Middle East.
The first anatomically modern humans (Homo sapiens) appear in Africa. They also evolved from Homo erectus.
Neanderthals die out.
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.
Recorded history begins.