Six major theories are proposed to explain the origin of life on earth. These theories are as follows:
Theory of Special Creations: It proposes that life on earth is created by a supernatural power, the GOD. It also proposes that all the living organisms, in present form, were created the same day.
Theory of spontaneous generations: It assumes that non-living material in a spontaneous manner gives rise to life. This theory was criticized by Lazzaro Spallanzani, Francisco Redi and Louis Pasteur.
Theory of catastrophism: It assumes that life is originated by the special creation and it is followed by catastrophe due to geographical disturbances.
Theory of cosmozoic: This theory was put forward by Richter and strongly supported by Arrhenius. The theory assumes that life was present in the form of resistant spores and appeared on earth from other planets. Since the condition of earth was supporting the life, these spores grew and evolved into different organisms. This theory was also known as “theory of panspermia or spore theory”.
The modern theory/ Chemical Theory/ Theory of primary abiogenesis: In the modern theory, the hypothesis of abiogenesis was proposed with a condition that the non-living materials can give rise to life in the condition of primitive earth. The condition of the primitive earth is different from the present conditions which do not permit abiogenesis. The idea of chemical theory was put forward by two scientists, A.I. Oparin and J.B.S Haldane.