Answer :
According to their theory, life evolved in the oceans during a period when the atmosphere was reducing - containing H2, H2O, NH3, CH4, and CO2, but no free O2.
In this theory, Alexander I. Oparin, a Russian biochemist and J.B.S. Haldane, an English biologist put forward the concept that 'the first form of life could have come from pre-existing non-living organic molecules and that formation of life was preceded by chemical evolution i.e., formation of diverse organic molecules from inorganic constituents.