. For context, see
|Life on Earth
||The planet Earth forms from the accretion disk revolving
around the young Sun.
||The surface of the Earth cools down enough for the crust to
||Life appears, probably first as self-reproducing RNA
molecules. The atmosphere does not contain any free
||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 employs ATP molecules as short term energy
currency and is used in almost all organisms unchanged to this
Prokaryotes remain the dominant life form on Earth today.
||The split between the bacteria and the archaea
||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 complex 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.
||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 (Cnidaria), Jellyfish (Ctenophora), flat worms
Platyhelminthes and other multicellular animals
appear in the oceans.
||The Cambrian explosion, a rapid set of evolutionary
changes, creates all the major body plans (phyla) of
||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.
||Arthropods, with an exoskeleton that provides
support and prevents water loss, are the first animals
to invade the land. Among the first are Myriapoda
(millipedes and centipedes), later
followed by spiders and scorpions.
||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.
||Plants evolve seeds, structures that protect
plant embryos and enable plants to spread
quickly on land.
||Evolution of the amniotic egg gives rise to the
Amniota, reptiles who can reproduce on land.
Insects evolve flight. Dragonflies (Odonata) still
resemble these early insects. Vast forests of
club moss (lycopods), horsetails, and tree ferns cover the land; when these decay they will
eventually form coal.
||The Permian-Triassic extinction event wipes out about 95%
of all animal species, the most severe mass extinction known. The archosaurs split
from other reptiles. They will later diversify into
crocodiles, dinosaurs, birds
and pterosaurs. Teleosts evolve from
among the Actinopterygii (ray-finned fish), and eventually
become the dominant fish group.
||The climate is very dry, and dry-adapted organism are
favored: the archosaurs and the
gymnosperms. The first mammals appear;
they evolved from synapsid reptiles.
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
||Birds evolve from theropod
dinosaurs. Modern amphibians evolve: the
Lissamphibia; including Anura (frogs), Urodela
(salamanders), and Caecilia
||The supercontinent Pangea begins to break up into several
land masses. The largest is Gondwana, made up of the land
masses which are now Antarctica, Australia, South America, Africa, and India
||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
||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).
||Cetaceans (whales) evolve from
mesonychids, carnivorous ungulates
probably most closely related to the
||Grasses evolve from among the
||The climate begins to dry, savannas and
grasslands take over the earlier forests.
The apes go into decline, and monkeys rise. This
is the heyday of the horses spread throughout the
Northern hemisphere. After 10 MYA they decline in the face of
competition with the artiodactyls.
||The australopithecines (early
hominines) evolve in the savannas of
Africa. North and South America become
joined, allowing migration of animals.
||Homo erectus evolves in Africa and migrates to
other continents, primarily south Asia.
||Neanderthals (Homo Neanderthalensis) evolve
from Homo erectus and live in Europe and the Middle East.
||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.
||Modern humans expand from Asia to Australia and
Europe. Expansion along the coasts happens faster than
||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.
||Neanderthals die out leaving Homo sapiens as the only living species of humans.
||One group of humans in the fertile crescent develop agriculture and, along with it, permanent settlements and cities. These appear first in what is now Iraq. This process of food production caused a massive increase in human population that has continued to the present.
||Humans reach Tierra del Fuego at the tip of South America, the last continental region to be inhabited by humans,
||Recorded history begins.