Modern Theory of Origin of Life
According to this theory life originated on early earth through physicochemical pro cesses of atoms combining to form molecules, molecules in turn reacting to produce inor ganic and organic compounds. Organic compounds interacting to produce all types of macromolecules which organised to form the first living system or cells.
Thus according to this theory ' life ' originated upon our earth spontaneously from non living matter . First inorganic compounds and then organic compounds were formed in accordance with everchanging environmental conditions . This is called chemical evolution which cannot occur under present environmental conditions upon earth. Conditions suitable for origin of life existed only upon primitive earth.
Oparin - Haldane theory is also called chemical theory naturalistic theory.
A.I. Oparin ( 1894–1980 ) was a Russian Scientist. He published his book " The origin of Life " in 1936 and an English edition in 1938. J.B.S. Haldane (1892–1964) was born in England but migrated to India in July 1957 and settled in Bhubaneshwar, Orissa. He was biologist, biochemist and geneticist. Both Oparin ( 1938 ) and Haldane ( 1929 ) gave similar views regarding the origin of life.
Modern views regarding the origin of life include chemical evolution and biological evolution :
Chemical Evolution ( Chemogeny )
The Atomic Phase. Early earth had innumerable atoms of all those elements (e.g. , hydrogen, oxygen, carbon, nitrogen, sulphur, phosphorus, etc.) which are essential for the formation of protoplasm.
Formation of Simple Molecules. Free atoms combined to form simple molecules such as H2 (Hydrogen), N2 ( Nitrogen ), H2O (Water vapour), CH4 (Methane ), NH3, (Ammo nia), CO2 (Carbon dioxide). Hydrogen atoms were most numerous and most reactive in primitive atmosphere. First hydrogen atoms combined with all oxygen atoms to form water and leaving no free oxygen. Thus primitive atmosphere was reducing atmosphere (without free oxygen) unlike the present oxidising atmosphere (with free oxygen)
Hydrogen atoms also combined with nitrogen, forming ammonia (NH3). So water and ammonia were probably the first molecules of primitive earth.
Formation of Simple Organic Molecules ( Monomers ). The early simple molecules interacted and produced simple organic molecules such as simple sugars ( e.g . ribose , deoxyribose , glucose , etc.), nitrogenous bases ( e.g. , purines , pyrimidines ), amino acids, glycerol, fatty acids, etc.
Torrential rains must have fallen. As the water rushed down, it must have dissolved away and carried with it salts and minerals, and ultimately accumulated in the form of oceans. Thus ancient oceanic water contained large amounts of dissolved NH3 , CH4 , HCN nitrides, carbides, various gases and elements
Some external sources must have been acting on the mixture for reactions. These external sources might be ( i ) solar radiations such as ultraviolet light , X - rays , etc. , ( ii ) energy from electrical discharges like lightning, ( iii ) high energy radiations are other sources of energies (probably unstable isotopes on the primitive earth). There was no ozone layer in the atmosphere. A soup - like broth of chemicals formed in oceans of the early earth from which living cells are believed to have appeared, was termed by J.B. Haldane ( 1920 ) as ' prebiotic soup ' (also called ' hot dilute soup '). Thus the stage was set for combination of various chemical elements. Once formed, the organic mol ecules accumulated in water because their degra dation was extremely slow in the absence of any life or enzyme catalysts.
Miller's Experiment
Stanley Miller in 1953, who was then a graduate student of Harold Urey (1893–1981) at the University of Chicago, dem onstrated it clearly that ultra violet radiation or electrical discharges or heat or a combina tion of these can produce com plex organic compounds from a mixture of methane (CH4), ammonia (NH3), hydrogen (H) and water vapour. The ratio of meth ane , ammonia and hydrogen in Miller's experiment was 2:1:2
Miller circulated four gases- methane, ammonia, hydrogen and water vapour in an air tight apparatus and passed electrical discharges from electrodes at 800 ° C. He passed themixture through a condenser. He circulated the gases continu ously in this way for one week and then analysed the chemical composition of the liquid inside the apparatus. He found a large number of simple organic compounds including some amino acids such as alanine , glycine and aspartic acid.Miller conducted the experiment to test the idea that organic molecules could be synthesized in a reducing environmen.
Other substances, such as urea, hydrogen cyanide, lactic acid and acetic acid were also present. In another experiment Miller circulated the mixture of the gases in the same way but he did not pass the electric discharge. He could not get the significant yield of the organic compounds.
Formation of Complex Organic Molecules ( Macromolecules )
A variety of amino acids, fatty acids, hydrocarbons, purines and pyrimidine bases, simple sugars and other simple organic molecules accumulated in the ancient seas. In the primaeval atmosphere electrical discharge, lightning, solar energy, ATP and polyphosphates might have provided the source of energy for polymerisation reactions of organic synthesis. S.W. Fox of the university of Miami has demonstrated that if a nearly dry mixture of amino acids is heated, polypeptide molecules are synthesized. Similarly simple sugars could form polysaccharides and fatty acids could combine to produce fats. Amino acids could form proteins, when other factors were involved. Thus the small simple organic molecules combined to form large complex organic molecules, e.g., amino acid units joined to form polypeptides and proteins, simple sugar units combined to form polysaccharides, fatty acids and glycerol united to form fats, sugars, nitrogenous bases, and phosphates combined into nucleotides which polymerized into nucleic acids in the ancient oceans.
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