Write an explanatory note on the origin of life.

(N/A) The origin of life on Earth occurred millions of years ago,and the study of the history of life forms on Earth is called evolutionary biology.
The history of life comprises two events: firstly,the origin of life,and secondly,the mechanism involved in the changes of living organisms through time,or the evolution of life.
The origin of life is considered a unique event in the history of the universe.
Origin of the universe: The universe is about $15-20$ billion years old.
It comprises huge clusters of galaxies,which contain stars and clouds of gas and dust.
According to the Big Bang theory proposed by Abbe Lemaitre $(1931)$,our universe arose as a cataclysmic eruption.
The temperature lowered,and gases like hydrogen and helium were formed sometime later.
By the force of gravitation,the gas condensed,and galaxies were formed.
Origin of Earth: Earth is supposed to have been formed about $4.5$ billion years ago in the solar system of our Milky Way galaxy.
Its surface was initially covered with water vapor,methane,carbon dioxide,and ammonia released from molten mass.
Later,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 the Earth cooled,water vapor fell as rain and filled all depressions to form oceans and other water bodies.