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Geological time scale. It is estimated that age of our planet earth is about 4.6 x 10⁶ years and first life originated in water (probably seas) about 3.6 x 10⁶ years ago. Geological time scale is the tabulated form showing the sequence and duration of the eras and the periods with their dominant form of life since the beginning of life on the earth.

G. Araduiana developed the first geological time scale. In the geological time scale, the duration of earth's history has been divided as follows :

Era Periods -

Palaeozoic era  has 6 Periods. They are : Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.
Mesozoic era  has 3 Periods. They are Triassic, Jurassic and Cretaceous.
Coenozoic era  has 2 Periods. Tertiary and Quarternary.

Some of the periods are further divided into still smaller time spans called epochs.  Tertiary period 5 epochs namely Palaeocene, Eocene, Oligocene, Miocene and Pliocene.
Quarternary has 2 epochs called  Pleistocene and Recent.