The question of how life came into being in the first place still remained unanswered. To find an answer to this question means looking back billions of years in time and trying to imagine what the conditions on the earth could have been like, when life first appeared. Soviet biochemist, Oparin, and the British biologist, Haldane, tried to do just that. They proposed that "life could have arisen from non-living organic molecules". In other words, to understand the problem of origin of life, one must have a knowledge of the origin of 'organic molecules' on the earth. In the early stages of its development, with the hot gases condensing and molten matter which was solidifying to form what are rocks, today, the Earth acted as the huge factory, producing many kinds of compounds.
The sources of energy available for the formation of numerous type of molecules were cosmic rays, ultraviolet radiations, electrical discharges such as lightning, radioactivity, and heat from volcanoes and hot springs. Molecules of all sorts were being continuously created and destroyed due to their state of agitation. The lighter gases of the atmosphere such as hydrogen, helium, oxygen, nitrogen, etc.. escaped into space unless they could combine with other elements to form liquids or solids. In such cases they remained on the earth.
In particular, oxygen could not remain as free oxygen. It combined with other elements to form compounds. For example, hydrogen and oxygen combined to form water vapour, and remained in the Earth's atmosphere. Similarly, oxygen combined with calcium and carbon to form calcium carbonate, i.e. limestone. Again, nitrogen, hydrogen and oxygen combined together to form ammonium nitrate. Compounds of carbon and hydrogen were also formed sometimes along with nitrogen or oxygen. These compounds are, today, called "organic compounds". The Earth had at the same time started cooling down. As the Earth cooled sufficiently, torrential and prolonged rains were caused due to condensation of steam.
The rains began to accumulate in the depressions on the earth and so the oceans were formed. These hot bodies of water contained abundant and varied organic compounds washed down from the atmosphere. Continued interaction among these compounds in the warm waters resulted in the formation of yet more compounds. The waters of this stage of the Earth's development have been referred to as "hot dilute soup", which amongst other things also contained "amino acids" having a composition of carbon, hydrogen, nitrogen and oxygen. The molecules of amino acids combined together to form large cuinplex molecules, the "proteins" which are the building blocks of life.
Oxygen when acted upon by ultraviolet radiation, forms ozone, a gas through which ultraviolet radiation cannot pass. This happens at a height of about 25 km above the surface of the earth, giving a protective 'ozone layer'. We have, therefore, the happy chain of events-more photosynthesis, more oxygen produced. And in its turn, more ozone produced out of oxygen in the atmosphere. screens the earth from the ultraviolet radiation of the Sun. This allowed living organisms to come to the surface of water and to survive even on land, if they got thrown out of the swirling and splashing water. The oxygenation of Earth's atmosphere was very significant from biological point of view, as organisms of greater complexity and even intel!igence could eventually arise.