i) Ancient theories of origin of life: Various theories have been put forward to explain the phenomenon of origin of life. A few of them were only speculations while others were based on scientific grounds. These theories are –
(a) Theory of special creation.
(b) Theory of spontaneous generation or Abiogenesis.
(c) Biogenesis
(d) Cosmozoic theory
(e) Theory of sudden creation from inorganic material.
(f) Naturalistic theory
(ii) Oparin’s Modern Theory:
(a) Oparin (1924) proposed that “life could have originated from non-living organic molecules.”
(b) He believed in Biochemical origin of life. Haldane (1929) also stated similar views. Oparin greatly expanded his ideas and presented them as a book “The origin of life” in 1936.
(c) According to this theory, the Earth originated about 4,500 million years ago. When the earth was cooling down, it had a reduced atmosphere. In this primitive atmosphere nitrogen, hydrogen, ammonia, methane, carbon mono-oxide and water were present. Energy was available in the form of electric discharges by lightening and ultraviolet rays. As soon as the earth crust was formed, it was very much folded. Torrential rains poured over the earth for centuries and were deposited in deep places.
(d) Miller’s Experiment: An American scientist (Biologist) Stanley Miller (1953) performed an experiment under support Oparin’s theory of origin of life. He believed that basic compounds which are essential for life can be synthesised in the laboratory by creation in the laboratory, on a small scale, the conditions which must have existed at the time of origin of life on earth.
(e) Miller took a flask and filled it with methane, ammonia and hydrogen in proportion of 2:1:2 respectively at 0°C. This proportion of gases probably existed in the environment at time of origin of life. This flask was connected with a smaller flask, that was filled with water, with the help of glass tubes. In the bigger flask, two electrodes of tungsten were fitted. Then a current of 60,000 volts was passes, through gases containing bigger flask for seven days. At the end of seven days, when the vapours condensed, a red substance was found in the U-tube. When this red substance was analyzed, it was found to contain amino acids, Glycine and nitrogenous bases which are found in the nucleus of a cell.
(f) The entire process of the origin of life, as proposed by Oparin, can be summarised as under –
(i) The Chemical Evolution:
(1) Step 1: Formation of simple molecules
(2) Step 2: Formation of Simple organic compounds
(3) Step 3: Formation of complex organic compounds
(4) Step 4: Formation of nucleic acids and nucleoproteins
(ii) Organic Evolution:
(1) Step 5: Formation of Coacervates
(2) Step 6: Formation of Primitive cell
(3) Step 7: Origin of autotrophism
(4) Step 8: Origin of Eukaryotic cells
Evidences of Organic Evolution
The following are the evidences in favour of Organic Evolution:
(i) Evidences from Classification
(ii) Evidences from Comparative Anatomy
(a) Analogy and Homology
(b) Vestigeal organs
(iii) Evidences from Physiology
(iv) Evidences from Serology
(v) Evidences from Embryology
(vi) Evidences from Palaceontology
(vii) Evidences from geographic distribution
(viii) Evidences from Genetics
