Origin
of life – Evolution of life forms
Theory of special
creation states
that life was created by a supernatural power, respectfully referred to
as “God”. According to Hinduism, Lord Brahma created the Earth. Christianity, Islam
and most religions believe that God created the universe, the plants and the
animals.
According to the
theory of spontaneous generation or Abiogenesis, living organisms
originated from non-living materials and occurred through stepwise chemical
and molecular evolution over millions of years. Thomas Huxley coined the term
abiogenesis.
Big bang theory explains the origin of
universe as a singular huge explosion in physical terms. The primitive earth
had no proper atmosphere, but consisted of ammonia, methane, hydrogen and water
vapour. The climate of the earth was extremely high. UV rays from the sun split
up water molecules into hydrogen and oxygen. Gradually the temperature cooled
and the water vapour condensed to form rain. Rain water filled all the depressions
to form water bodies. Ammonia and methane in the atmosphere combined with
oxygen to form carbon-dioxide and other gases.
Coacervates (large colloidal particles that precipitate out in aqueous
medium) are the first pre -cells which gradually transformed into living cells.
According to the theory
of biogenesis life arose from pre-existing life. The term biogenesis also
refers to the biochemical process of production of living organisms This term
was coined by Henry Bastian.
According to the theory
of chemical evolution primitive organisms in the primordial
environment of the earth evolved spontaneously from inorganic substances and
physical forces such, as lightning, UV radiations, volcanic activities, etc.,.,
Oparin (1924) suggested that the organic compounds could have undergone a
series of reactions leading to more complex molecules. He proposed that the
molecules formed colloidal aggregates or ‘coacervates’ in an aqueous
environment. The coacervates were able to absorb and assimilate organic compounds
from the environment. Haldane (1929) proposed that the primordial sea served as
a vast chemical laboratory powered by solar energy. The atmosphere was oxygen
free and the combination of CO2, NH3 and UV radiations
gave rise to organic compounds. The sea became a ‘hot’ dilute soup containing
large populations of organic monomers and polymers. They envisaged that groups
of monomers and polymers acquired lipid membranes and further developed into
the first living cell. Haldane coined the term prebiotic soup and this became
the powerful symbol of the Oparin-Haldane view on the origin of life
(1924-1929).
Oparin and Haldane
independently suggested that if the primitive atmosphere was reducing and if
there was appropriate supply of energy such as lightning or UV light then a
wide range of organic compounds can be synthesized.
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