Answer: Oparin — Haldane theory of chemical origin of life.
The Russian scientist Alexander Ivanovich Oparin (1924) and British scientist J.B.S. Haldane (1929) proposed the ‘Theory of chemical evolution of life’. According to this theory, life originated from non-living matter, some three billion years ago, in a primitive atmosphere, through a process of chemical evolution. The entire process of chemical evolution can be divided into the following steps:
i. Origin of earth and its primitive atmosphere:
a. The origin of life on earth is closely related to the origin of earth itself.
b. Evidences suggest that earth originated about 4.6 billion years ago.
c. When it was broken from the sun, it was a glowing fire and a rotating cloud of hot gases, vapours of various elements and pieces of rocks and metals called Nebulous.
d. This mass of gas exploded with a Big Bang into several small pieces. The earth, as a piece was broken from the sun.
e. As the earth was moving away from the sun, it was getting cooled. This led to the condensation of gases.
f. The heavy elements like iron, nickel, etc. sank to the centre and formed the solid core of earth.
g. The lightest elements like helium, hydrogen, oxygen, nitrogen, carbon, etc. occupied the atmosphere of the earth.
h. The geological conditions on the primitive earth about 3600 million years ago were much different from those found today. In fact, the atmosphere of primitive earth was reducing type and there was no free oxygen.
ii. Formation of ammonia, water and methane:
a. The prebiotic environment had very less resemblance with the present day environment.
b. The earth was very hot initially and so the atoms could not combine with each other very easily.
c. The primitive earth contained a large amount of hydrogen, nitrogen, carbon and oxygen.
d. Of these, hydrogen was very active.
e. It combined with nitrogen to form ammonia (NH3), with oxygen to form water (H2O); and with carbon to form methane (CH4).
f. As the temperature was high, ammonia and methane remained as gases and water as steam.
g. As years passed, the temperature of the earth decreased.
h. Steam condensed into water which resulted in rain and the earth became cold.
i. Water gradually accumulated and this led to the formation of rivers, streams, lakes, seas and oceans.
j. Compounds like ammonia, methane, etc. were dissolved in rain water and accumulated in the sea. Thus, the first chemicals formed on the earth were water, ammonia, methane, etc.
iii. Synthesis of simple organic compounds:
a. The next step in molecular evolution was the formation of micromolecules.
b. As the earth surface considerably cooled, the highly reactive free radicals –CH and – CH2 condensed to form a variety of both saturated and unsaturated hydrocarbons.
c. The simple inorganic molecules of the oceanic water interacted with one another to form simple organic molecules such as sugar, fatty acids, glycerol, amino acids, purines and pyrimidines.
d. The energy for these reactions was provided by the electrical discharges (lightning), ultraviolet rays (solar radiations), volcanic activities and decay of radioactive elements.
CH4 + NH3 + H2O → amino acids
CH4 + HCN + NH3 + H2O → purines + pyrimidines
CH4 + H2O → sugar + glycerol + fatty acids.
Haldane described the sea containing molecules of these organic substances in abundance as ‘the hot dilute soup’ or ‘primitive broth’.
iv. Formation of complex organic compounds:
I. The hot dilute soup was sterile and oxygen free.
II. Simple organic substances came together in increasing numbers.
III. They colloided, reacted and aggregated forming new complex molecules such as . polysaccharides, fats, proteins, nucleosides and nucleotides.
IV. The protein molecules made their existence by polymerization of amino acids.
V. These proteins showed enzymatic reaction and were called protoproteins.
VI. Due to their enzymatic nature, these proteins accelerated the rate of other chemical reactions.
VII. The formation of protein molecules is considered as a landmark in the origin of life.
v. Formation of nucleic acid:
I. The next step in chemical evolution was formation of nucleic acid.
II. It was formed by the aggregation of phosphoric acid, sugar, purines and pyrimidines.
III. They were linked in various combinations to form different types of nucleotides.
IV. Thousands of nucleotides joined together to form nucleic acid.
V. It acquired self replication ability, which is a fundamental property of living forms.
vi. Formation of Protobiont or precells:
I. The nucleic acid along with inorganic and organic molecules formed the first form of life and were called protobionts or precells.
II. The proteins formed colloidal hydrophilic complexes surrounded by water molecules.
III. Oparin and Sydney Fox demonstrated the formation of this aqueous suspensions of polymers.
IV. Oparin called these aggregates as Coacervates, while Sydney Fox called them Protenoids or Microspheres.
vii. Formation of First Cell:
I. The nucleic acids in pre-cells had the capacity to multiply but gradually it started directing a series of chemical reactions like protein synthesis.
II. That was the significant step in the transformation of pre-cell into a cell.
III. This chemical evolution gave rise to the biological evolution.
viii. Biological evolution:
I. The first cells or primitive cells were marine and heterotrophic in nature.
II. They obtained their food from the surrounding areas.
III. Growth and multiplication of these cells caused depletion in the food and increase in CO2 due to fermentation.
IV. These were the favourable circumstances for mutation, which resulted in the development of chromophores.
V. The chromophores had the ability to trap light energy and convert it into chemical energy.
VI. This event helped in the transformation of heterotrophs into autotrophs.
VII. Due to release of oxygen as a byproduct of photosynthesis, the primitive reducing atmosphere was
VIII. slowly and gradually converted into an oxidizing atmosphere.
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