All given times are approximate estimates. We use the abbreviations "MYA" for "million years ago" and "TYA" for "thousand years ago".
Timeline
Life on Earth
Date
Event
4500 MYA
The planet Earth forms from the accretion disk revolving
around the young Sun.
4100 MYA
The surface of the Earth cools down enough for the crust to
solidify.
4000 MYA
Life appears, probably first as self-reproducing RNA
molecules. The atmosphere doesn't contain any free
oxygen.
3900 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 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.
Prokaryotes remain the dominant life form on Earth today.
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
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.
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.
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.
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.
1000 MYA
Multicellular organisms appear: algae and
seaweeds living in the oceans.
The Cambrian explosion, a rapid set of evolutionary
changes, creates all the major body plans (phyla) of
modern animals.
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.
Insects evolve on land and in fresh water from the
myriapods. Some fresh water lobe-finned fish
(Sarcopterygii) develop legs and give rise to the
Tetrapoda. This happens in the water; tetrapods
then use their legs to move out onto land, probably to hunt
insects. Lungs evolve from swim bladders. Amphibians still retain many
characteristics of the early tetrapods.
360 MYA
Plants evolve seeds, structures that protect
plant embryos and enable plants to spread
quickly on land.
The climate is very dry, and dry-adapted organism are
favored: the archosaurs[?] and the
gymnosperms[?]. The first mammals appear;
they evolved from synapsidreptiles.
Initially, they stay small. Gymnosperms (mostly
conifers) are the dominant land plants. Plant eaters
will grow to huge sizes during the dominance of the
gymnosperms to have space for large guts to digest
the poor food offered by gymnosperms
Angiosperm plants evolve flowers,
structures that attract insects and other
animals to spread pollen. The evolution of the
angiosperms cause a major burst of animal evolution.
Half of all known dinosaur species are from the last 30 MY of
the Mesozoic, after the rise of the
angiosperms.
65 MYA
The Cretaceous-Tertiary extinction event wipes out about half of all animal species including all
non-avian dinosaurs, probably because of a
cooling of the climate precipitated by the giant impact of a
meteor. Mammals increase in diversity and size.
Some will later return back to the sea (whales,
sirenians and seals) and others will
evolve flight (bats).
The first anatomically modern humans (Homo sapiens)
appear in Africa some time before this. They also evolved from
Homo erectus. Modern humans enter Asia via the Middle East.
50 TYA
Modern humans expand from Asia to Australia and
Europe. Expansion along the coasts happens faster than
expansion inland.
30 TYA
Modern humans enter North America from Siberia in
numerous waves, some later waves across the Bering land bridge, but
early waves probably by island-hopping across the Aleutians. At least two of the first waves had left few
or no genetic descendants among Americans by the time Europeans
arrived across the Atlantic Ocean.
27 TYA
Neanderthals die out leaving Homo sapiens as the only living species of humans.
15 TYA
Humans develop agriculture and, along with it, permanent
settlements and cities. These appear first in what is now
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