Origin and Evolution of Life: Introduction to Classification and Probable Stages in the Origin of Life (For CBSE, ICSE, IAS, NET, NRA 2022)

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Introduction to Classification

  • Origin of life: Earth was formed 5 billion years ago, and that time is very hot so the life was not possible at high temperature. Origin of life means that appearing of simple primordial life from non-living matter and evolution of life means that gradual formation of complex organisms from simpler ones.
  • Chemosynthetic Theory of Origin of Life: This theory is widely accepted and proposed by A. I. Oparin and he says that light might have first originated on earth through a series of combinations of chemical substances in the distant past and it all happened in water. Earth is forms 5 billion years ago and that time is made up of hot gases and various chemicals and then is cool down and solid crust formed. In early atmosphere their war absence of free oxygen but ammonia (NH3) , water vapour (H2O) , hydrogen (H2) and methane (CH4) are present. Heavy rain falls on the earth hot surface and after long time water and through chemical reaction occurred in Methane and ammonia from the atmosphere dissolved in the of the seas and gave rise to amino acids, nitrogenous base, sugars and fatty acids etc.
Primitive Condition on Earth

Probable Stages in the Origin of Life

  • First stage; the sources of energy were the ultraviolet rays or electric discharge (lightening) or heat or either alone or a combination of these energy sources caused reactions that produced complex organic compounds (including amino acids) from a mixture of ammonia (NH3) , methane (CH4) , water (H2O) and hydrogen (H2) . The amino acids are the building blocks of proteins which are the main components of protoplasm. In 1953 Stanley Miller and Harold C. Urey set up an experiment with air tight apparatus and in that four gases (NH4, CH4, H2 and H2O) inoculated through electric discharge for 7 days and after that they analyzed the liquid and found organic substances like amino acid, urea, acetic acid, lactic acid, etc.
The Apparatus Used by Stanley Miller and Harold C. …

Apparatus used by Stanley Miller and Harold C. Urey to demonstrate the synthesis of amino acids under conditions that existed on the primitive earth.

  • Second stage; large molecules like peptides (leading to the formation of proteins) , sugars, starch and fat molecules are form when simple organic molecules combined.
  • Third stage; the large molecules of different kinds combined together to form multi-molecular heaps or complexes. Some simple fat molecules arranged themselves around this molecular complex in a sort of membrane. When such complexes reached a certain size they separated from the surrounding in the form of coacervate drops of microscopic size which moving in the liquid with a definite boundary. Coacervate like aggregates were probably the precursors of the first living cells.
  • Fourth stage; some sort of nucleoproteins or nucleic acids may have evolved by random combinations which have provided two more properties to coacervate like bodies; chemical reactions from the nucleic acid and capacity to reproduce through duplication of the nucleic acids. The primitive drop like forms of life were all heterotrophs and unable to manufacture their own food but derived it from environment.

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