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Geologists divide Earth's history into time units based on significant geologic, climate, and
biological events.
Eons
are the longest geologic time divisions.
Eons are divided into
eras
based on the
organisms that characterized each era where fossil evidence exists. Periods are subdivided into eras, which are
further subdivided into
epochs
.
From approximately 4.6 billion years ago to 2.5 billion years ago, little is known
about Earth. The
Archaean eon
, which lasted from 4.0 billion years ago when the Earth's crust formed to 2.5
billion years ago, is when life first appeared on Earth.
The Proterozoic eon
spans the period from 2500 to 541 million years ago. Because the rocks have
been less altered by heat and pressure, this vast period of time is easier to study than the preceding Archaean
eon. Prokaryotes like cyanobacteria dominated life at the start of the Proterozoic eon. Stromatolites were
plentiful.
The Paleozoic era
lasted about 289 million years and started about 541 million years ago. The six
periods are the
Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
The Mesozoic era
lasted about 252 million years and the Cenozoic era lasted about 186 million years.
It is split into three sections: Triassic, Jurassic, and Cretaceous.
The Cenozoic era
is also known as the Age of Mammals, the Age of Birds, the Age of Insects, and
the Age of Flowering Plants. The appearance of all of these forms in a diverse range of species marks this
epoch. The Cenozoic era began 66 million years ago and continues to the present day. It is divided into three
periods: the Paleogene (43 million years), the Neogene (20.4 million years), and the Quaternary (20.4
million
years) (2.6 million years).
