Chemosynthetic Theory of Origin of Life
The earth was formed about five billion years ago. At that time it was extremely hot. The existence of life in any form at that high temperature was not possible.
Origin of life means the appearance of simplest primordial life from non-living matter. Evolution of life means the gradual formation of complex organisms from simpler ones.
Several theories have been put forth to explain the origin of life. The widely accepted theory is the Chemosynthetic theory of origin of life, proposed by A.I. Oparin. It states that life might have originated at first on earth through a series of combinations of chemical substances in the distant past and it all happened in water.
The earth originated about 5 billion years ago. It was initially made up of hot gases and vapours of various chemicals. Gradually it cooled down and a solid crust was formed.
The early atmosphere contained ammonia (NH3), water vapour (H2O), hydrogen (H2), methane (CH4). At that time, there was no free oxygen. This sort of atmosphere (with methane, ammonia and hydrogen) is still found on Jupiter and Saturn.
Heavy rains fell on the hot surface of earth, and over a very very long period the water bodies appeared that still contained hot water. Methane and ammonia from the atmosphere dissolved in the water of the seas.
In this water, chemical reactions occurred and gave rise to amino acids, nitrogenous bases, sugars and fatty acids which further reacted and combined to give rise to biomolecules of life such as proteins and nucleic acids.
Early atmosphere of earth had no free oxygen. Chlorophyll-bearing organisms later released free oxygen which gave greater possibilities for life to evolve.
Miller Urey Experiment
In 1953, Stanley Miller and Harold C. Urey set up an experiment with an air-tight apparatus in which four gases (NH4, CH4, H2 and H2O) were subjected to an electric discharge for one week. On analyzing the liquid, they found a variety of organic substances in it, such as amino acids, urea, acetic acid, and lactic acid.
