Philosophers and scientists from time immemorial have attempted to explain how and where life originated on this planet earth. Today, earth is the only planet where life exists. However, when the earth was formed about 4.8 billion years ago, there was absolutely no sign of life on earth. The first form of life is said to have appeared about 3.8 billion years ago.
Several attempts have been made from time to time to explain the origin of life on earth. As a result, there are several theories which offer their own explanation on the possible mechanism of origin of life.
The earth when it was formed about 4.8 billion years ago, was a hot revolving ball of gas consisting of atoms of various elements. Heavy elements such as iron and nickel were found in the center while comparatively lighter ones like those of aluminium and silicon formed the middle layer. The lightest elements like hydrogen, oxygen and carbon were found in the outermost layer. Due to the extremely high temperature, the atoms of these elements could not combine to form molecules.
As the theory of chemical evolution started gaining more support, investigations were initiated by several scientists to establish the conditions under which, the simplest organic compounds were formed in the primitive earth. Two scientists, Stanley Miller and Harold Urey were the first to put to test, the Oparin - Haldane theory, in the year 1953.
According to this theory, the reducing atmosphere of primitive earth helped in the formation of simple inorganic compounds followed by simple organic compounds. Then complex organic compounds and subsequently their interaction leading to the formation of self duplicating nucleic acids.