The origin of life on the earth took place millions of year ago and the study of history of life forms on earth is called evolutionary biology.

The history of life actually comprises of two events i.e. firstly origin of life and secondly the mechanism involved in the changes of living organisms through time or evolution of life.

The origin of life is considered a unique event in the history of universe.

Origin of universe : The universe is about $15-20$ billion years old.

It comprises of huge clusters of galaxies, which contains stars and clouds of gas and dust.

According to the Big Bang theory given by Abbe Lemaitre ($1931$), our universe arose as cataclysmic eruption.

The temperature lowered and the gases like hydrogen and helium were formed sometimes later.

By the force of gravitation, the gas condensed and galaxies were formed.

Origin of earth : Earth was supposed to have been formed about $4.5$ billion years back in the solar system of our milky way galaxy.

Its surface was covered with water vapour, methane, carbon dioxide and ammonia released from molten mass.

Then, later on the water molecules were broken into hydrogen and oxygen by the $UV$ rays of the sun.

The lighter hydrogen gas escaped from the surface while oxygen combined with other compounds.

Subsequently, the ozone layer was formed.

As it cooled, the water vapour fell as rain and filled all depressions to form oceans and other water bodies.