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Evolution

Origin of Life and Theories of Evolution

Origin of Earth

  • According to the Big Bang theory proposed by Abbe Lemaitre in 1931, the universe arose as an eruption of hot, energy-rich sub-atomic particles about 15 billion years ago.
  • Formation of hydrogen and helium after the big bang and their condensation, formed the galaxies.
  • It is estimated that the Earth originated along with the Sun and other planets about 4.6 billion years ago from a huge cloud of cosmic dust in rotating motion called solar nebula.

  • The atmospheric conditions on the primitive earth were not conducive for free molecular water to exist. Water only accumulated as the earth cooled.

  • According to scientists, the primitive atmosphere was composed of CO2, N2, water vapour, H2, H2S, NH3 and CH4.
  • The primitive atmosphere was considered to be reducing in nature due to the sufficient availability of hydrogen atoms and the absence of free oxygen.

  • Hence, the self-replicating entities that first formed on primitive earth were both heat tolerant as well as capable of surviving in an anaerobic environment.

Theories on Origin of Life

  • Several theories have been put forth to explain the origin of life on earth.

Some of the important ones are as follows:

  • Theory of Panspermia: Panspermia literally means 'seeds everywhere'. According to this theory, spores of life are present everywhere in the universe and from somewhere in the universe, some spores of life entered Earth and thus life originated on Earth.
  • Theory of Special Creation: According to this theory, life on earth was created by a supernatural power (God).

  • Theory of Spontaneous Generation or Abiogenesis: According to this theory, life originated spontaneously from non-living materials.

  • Theory of Biogenesis: According to this theory, life originated from pre-existing life. It was supported by many scientists such as Francisco Redi, Lazzaro Spallanzani and Louis Pasteur, who disproved the earlier theory of spontaneous generation.

    • This theory was experimentally proved by Louis Pasteur by his swan-neck flask experiment.

  • Theory of Physicochemical Evolution: It was proposed by Alexander I. Oparin and John BS. Haldane. According to them, life originated after a long period of inorganic molecular evolution, where physicochemical changes preceded organic evolution.

    • This theory was experimentally proved by Stanley Miller and Harold Urey in 1953.

Urey - Miller Experiment

  • In 1953, Stanley Miller and Harold Urey conducted a series of experiments to prove the theory of physicochemical evolution put forth by Oparin and Haldane.
  • Primitive atmosphere had high temperature, volcanic storms, and reducing atmosphere, containing CH4, NH3, H2, etc.

  • Urey and Miller took the same compounds in a closed flask along with water vapour at 800C and created an electric discharge.

  • Formation of biomolecules such as amino acids, simple sugars, fats, etc. was observed in the flask.

Modern Theory of Origin of Life

  • The inorganic molecules present in the primitive atmosphere combined to form simple organic compounds such as sugars, fatty acids, glycerol, amino acids, nitrogen bases etc.

  • With further cooling of the earth, the simple organic compounds combined to form complex compounds.

  • These complex organic compounds accumulated in the sea water making it a sort of “primordial soup”.

  • In this sea water, large molecules aggregated and formed an envelope of water around them, these aggregates are called coacervates.

  • The envelope of water around these coacervates increased the chances of chemical reactions.

  • Gradually, they started breaking down, building up and even multiplying by taking up new materials from the ocean and releasing the degraded materials.

  • Therefore, coacervates exhibited the basic properties of life such as metabolism, growth and reproduction.

  • Gradually, nucleic acids took the precise control of coacervates and transferred them into protobionts (the first living systems).

  • Enzymes developed from some of the proteins in the protobio…

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