This theory is the most accepted theory and was given by a Russian biochemist A.L. Oparin(1923) & an English biologist J.B.S. Haldane(1928). According to this theory, spontaneous generation of life under present day conditions is not possible. When the earth was formed 4.6 billion years ago, that time its atmosphere was very much different from that of today. That time it was reducing atmosphere & consisted of H2, water vapours(H2O), NH3 and methane. It is called Primary Atmosphere. Temp of the earth was very high & a lot of energy was available from solar radiations, UV rays(ozone shield absent), electric discharges, volcanic eruptions and cosmic rays were frequent. Such an Atmosphere where 1 right type of chemicals were present 2 Oxygen was absent 3 Lots of energy sources were available was required for the formation of earth. It is believed that before the origin of life first there was a long chain of evolution of chemicals. then these chemicals gave rise to organisms. This formation of living organisms from the non- living organisms is known as abiogenesis. The evolution of chemicals & the abiogenesis has taken place in the following ways:
a) Formation of early Molecules: As the earth cooled, free atoms of different elements came together at random & formed inorganic molecules. The earliest molecules formed by reactions between these elements were gases likeH2, NH3, H2O(water vapours), CH4, CO, CO2.
b) Formation of simple organic molecules: The early molecules reacted with one another to form simple organic molecules e.g sugars, fatty acids, glycerols, amino acids, purines, and pyrimidines. All the organic molecules got collected in the sea water and continued to stay there.