Biochemical theory of the origin of life Part -I
Many theories have been provided to explain the mystery of the origin of life on earth but most of them are discarded due to lack of evidence. The most widely accepted theory which explains the origin of life is biochemical theory proposed by Oparin and Haldane. This article will explain the biochemical theory of the origin of life.
Introduction The different theories were proposed to explain the mystery of the origin of life on the earth but all the theories were discarded. The most scientific and widely accepted theory of the origin of life is the biochemical theory which was proposed by Haldane and Oparin. H. Urey and Stanley Miller also proved this theory by performing an experiment.
Russian biochemist A.I. Oparin has nicely presented the biochemical explanation of the origin of life in his book 'The origin of life on Earth' published in 1936. According to him the origin and evolution of life occurred along with the origin and evolution of earth and its atmosphere. The primitive atmosphere of earth had water, methane and ammonia form which as a result of a series of changes, a colloidal body called coacervate and a mixture of biologically important macromolecules like proteins, lipids and acids, etc. were formed. However, this theory can be explained in the different phases under two separate headings:
Chemical Origin Earth, other planets, their environment and chemical substance are formed in this stage due to various chemical changes. Chemical origin involves the following steps:
Origin of earth and its primitive atmosphere The age of the universe has been estimated to be 20 billion years. Scientists are of different views about the origin of the solar system, but the most prevailing view is that the solar system originated 4.5 to 5 billion years ago in the form of a cosmic cloud of dust and gases called as a nebula. Condensation of these clouds and gases caused an increase in pressure and temperature resulting in a thermonuclear reaction. During this process, the whole mass rotated separating the dense central part as 'sun'. Surrounding less dense cloud and gases developed into planets including earth by cooling. Thus the earth was originated from the sun about 5 billion years ago. In the beginning, the earth was in the form of hot gases and vapours of various elements. This type of environment was definitely unfavourable for any form of life. Gradually the gases and vapour cooled down and various elements started to solidify. Some heavy elements like iron, nickel, formed the core of the earth. The outer crust was formed by silicon, aluminium, magnesium, etc. While the lightest elements like hydrogen, nitrogen, helium, carbon, etc. formed the gaseous atmosphere. The original temperature of the earth was estimated to be about 5K-6K degree Celsius. At this high temperature, these gases could not exist in the free state. They combined with each other to form water, carbides, methane, ammonia, nitrates, etc. Oparin suggested that from these simple compounds, complex organic compounds were formed gradually under the influence of electric charges, ultraviolet rays or corpuscular radiations.
Origin of simple compounds Hydrogen was the more active gas in the primitive atmosphere, hence it combined with carbon and hydrogen in the presence of UV rays and resulting in the formation of methane gas. Similarly, ammonia gas originated due to the combination of hydrogen and nitrogen gases. Ethane, propane, hexane and butanes like compounds are then formed from methane gas. These compounds help in the formation of other organic compounds like fatty acids, glycerol, sugars, amino acids and nitrogenous bases.
Origin of oceans, rivers, lakes and mountains As the earth cooled down gradually some gases liquified and some of the liquids changed into solids. Steam condensed and resulted in rain. The rain droplets on reaching the superheated earth surface immediately evaporated and returned to the atmosphere. This cycle of water continued for millions of years and resulted in the cooling of the earth surface. As the earth surface became cool enough to hold water the large water bodies like oceans and sea came into existence. The oceanic water contains atmospheric ammonia and methane which got dissolved and washed down with the water dropped from the atmosphere in the form of rain. Mineral rocks also dissolved resulting in the accumulation of fo salts and minerals in the oceanic water Therefore, primitive oceans were alkaline. The primitive atmosphere of the earth was without free oxygen.
Formation of complex compounds Simple organic compounds gradually formed complex organic compounds under the influence of electrical charges, ultraviolet rays and corpuscular radiations of radioactive substances. Hydrocarbon, derivatives of hydrocarbon, fatty acids, and amino acids were formed during this process. The synthesis of carbohydrates, fats, amino acids and other complex organic substances probably occurred in the sea, which had been described by Haldane as " the hot dilute soup" or broth containing molecules of these organic substances.
Formation of nucleic acids and nucleoproteins Purines and pyrimidines are thought to be originated from hydrogen cyanide in primitive earth due to condensation reaction. These nitrogenous bases reacted with sugar and phosphate to form nucleotides, which in turn formed nucleic acid by polymerization.
Conclusion The formation of nucleic acids and nucleoproteins was the basic materials for the origin of life. From these nucleic acids and nucleoproteins, life was originated in the sea. The further process is explained in the second phase of the biochemical theory of the origin of life called the biochemical evolution of life.