BIOCHEMICAL ORIGIN OF LIFE -

• It is generally agreed by astronomers, geologists and biologists that the earth is approximately 4500-5000 million years old.
• It is an opinion of a large section of biologists that the original state of the earth bore little resemblance to its present day and had the following probable appearance:
• It was hot about 4000-8000°C and as it cooled, carbon and the less volatile metals condensed and formed the core of the earth.
• The surface was probably barren and rugged as volcanic activity, continuous earth movements and contraction on cooling, folded and fractured the surface.

• The atmosphere is believed to have been totally different in those days.
• The lighter gasses hydrogen (H), helium (He), nitrogen (N), oxygen (O) and argon (Ar) would have escaped because the gravitational field of the partially condensed planet would not contain them..
• But simple compounds containing these elements would have been retained e.g., water (H₂O), ammonia (NH₃), carbon dioxide (CO₂), methane (CH₄), and until the earth cooled below 100°C all water would have existed as vapour. 
• First molecular compound formed were H₂O & NH₃.
• First organic compound formed was CH .
• Free oxygen was absent. Therefore primitive atmosphere was reducing.
• The chemical theory of life was elaborated by two workers independently.
• A. I. Oparin (1923) and J.B.S. Haldane (1929) and was summarized by oparin in his book "The origin of life".
• They held that Spontaneous generation of life under the present environment conditions is not possible.
• Earth's surface and atmosphere during the first billion years of its existence were radically different form today's conditions.
• Earth's initial atmosphere was a reducing one.
• The first life arose from a collection of chemical through a progressive series of chemical reactions in which atoms combined into inorganic molecules, inorganic molecules into simple organic compounds, and simple organic compounds into complex organic compounds, which finally got organized into the living matter.
• The sequence of changes an 8 step process, begins right from the formation of earth.

• Solar radiation, heat radiated by earth and lightning provided energy for evolution of molecules.
• The Oparin-Haldane theory was experimentally supported by Stanley Miller in 1952.