Modern biologists agree that the origin of life was a continuous steady process. Life did not come into existence suddenly but was a result of a long series of physico-chemical changes. These changes brought about a gradual evolution of first inorganic and then organic compounds in accordance with the ever-changing environmental conditions. The first scientific account of the biochemical origin of life was given by a Russian biochemist, Aleksander I. Oparin, in 1924 and successively by an English biologist, J.B.S. Haldane, in the year 1929.
Later, in the year 1936, Oparin published his views in his book, The Origin of Life. The whole series of events, as envisaged by Oparin and Haldane, can be divided into the following eight steps.
First step of Biochemical origin of Life
Origin of earth and its primitive atmosphere
The earth is supposed to have originated about 5-6 billion years ago both from a part broken off from the Sun or the cosmic dust gradually condensed and presumably formed the entire solar system.
In the beginning, the earth was a fiery spinning ball of hot gases and vapors of various elements . Gradually, through hundreds of millions years, the gases condensed into a molten core and different elements become stratified according to their densities.
Second step of Biochemical origin of Life
Origin of molecules and simple inorganic compounds
Since, the original temperature of the earth was very high (5000°-6000°C), elements like hydrogen, oxygen, carbon, and nitrogen could not exist in Free State. They combined among themselves or with metals forming oxides, carbides and nitrides. All these compounds were present in gaseous state, and water as superheated steam. These compounds formed the atmosphere of the primitive earth.
As the earth began to cool, its matter started condensing and solids, liquids as well as gases co-existed. Superheated steam condensed into water causing rain. The rain drops on approaching the superheated earth immediately evaporated and returned to the atmosphere. This cycle continued for millions of years thus cooling the earth's surface. Large quantities of hydrogen, nitrogen, carbon dioxide, methane, ammonia and water vapours were present on the primitive earth, but the atmosphere was devoid of free oxygen.
Third step of Biochemical origin of Life
Origin of organic compounds
As the atmosphere cooled down to 1000°C, various saturated and unsaturated hydrocarbons were formed. These hydrocarbons later reacted with superheated steam and formed oxy- and hydroxy derivatives—aldehydes, ketones and acids. Due to the condensation and polymerisation of oxy- and hydroxy derivatives, several organic molecules like sugars, glyceral, fatty acids, amino acids and nitrogenous organic bases (purines and pyrimidines) we're formed
The synthesis of carbohydrates, fats and amino acids and other complex organic compounds presumably took place in sea. J. B. S. Haldane described this mixture of sea water as hot dilute soup.
Fourth step of Biochemical origin of Life
Origin of colloids, coacervates and individuality
The organic macromolecules due to intermolecular attraction aggregated into various combinations and then they precipitated out in the aqueous medium in the form of large colloidal particles known as coacervates. Coacervates have more than one types of proteins, nucleoproteins and other organic and inorganic molecules in various combinations. Since protoplasm is also a colloid system, coacervates, as individual structures, have been looked upon as precursors of the first organisms, i.e. prebiotic structures.
Fifth step of Biochemical origin of Life
Nucleoproteins appeared in the primitive ocean as self-duplicating systems. Hence, the nucleoproteins once formed, must have steadily increased in the oceanic soup. By virtue of their self-duplicating property, these are capable of performing hereditary function. The first sign of self-perpetuating life was thus displayed by the nucleic acids. The nucleotide monomers of the nucleic acids presumably first formed only small chains, comparable to the present day genes. A number of such genes might have later aggregated into a large unit called protovirus, comparable to the present day virus.Thus, life was presumably originated in the ocean, about 3.7 billion years ago.
Sixth step of Biochemical origin of Life
Origin of prokaryotes
Following the formation of self-duplicating systems in the form of nucleoproteins, conditions suitable for evolution of cell-like organisms existed in the primitive ocean. According to Opariin, the coacervates with nucleoproteins became the first cellular organisms. These primordial cellular forms were comparable with the bacteria.
Seventh step of Biochemical origin of Life
Origin of autotrophism
Being anaerobic heterotrophs, the early bacteria-like prokaryotes gradually consumed all abiotically synthesized organic nutrients of the oceanic soup, and struggled for existence. During the course of this struggle, some early prokaryotes acquired enzymes which would catalyse synthesis of simple carbohydrate molecules from inorganic substances of the oceanic water.
Later, certain autotrophic bacteria-like primitive organisms synthesized chlorophyll-like green pigment from magnesium porphyrin of the oceanic water. These pigments could absorb sunlight that provided solar energy for synthesis of carbohydrates. Some marine planktonic sulphur bacteria found today show such a photosynthetic autotrophism. Such chlorophyll-bearing prokaryotes of the primitive ocean can be compared with the blue-green algae at the present-day.
Eighth step of Biochemical origin of Life
Origin of eukaryotic cells
Liberation of free oxygen into the primitive atmosphere by blue-green alga-like prokaryotes was a revolutionary change in the early history of the earth. TheTree oxygen oxidised methane and ammonia forming carbon dioxide, nitrogen, and water and the reducing environment thus became oxidising. Hence, the composition of the primitive atmosphere changed to that of the present atmosphere. With the change in the composition of the atmosphere, it is unlikely that the conditions can return in which life can originate. The free oxygen formed a layer of ozone above the sunlight blocking the U-V light and making it possible for the organisms to migrate on the land from the water.
With the liberation of free oxygen in the atmosphere, conditions suitable for aerobic respiration established upon the earth, presumably about 27 billion years ago.
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