- Objections : Living matter cannot survive the extreme cold, dryness and intense radiations of interplanetary space. Moreover,
this theory does not explain how life originated and from where and how spores reached earth.
Modern Theory of Origin of Life
- Also called Oparin-Haldane theory, as it was proposed by Oparin (1923) and supported by Haldane (1928). It stated that
primitive life originated in the water bodies on the primitive earth from non-living organic molecules (e.g., RNA, proteins,
etc.) by chemical evolution through a series of chemical reactions. - Oparin’s theory is also known as primary abiogenesis which means “abiogenesis first and then biogenesis ever since.”
Table : Summary of origin of lifeFree atoms
↓
H, C, N, etc., essential for formation of protoplasm.Simple molecules
↓Free atoms combined to form simple molecules like H 2 , H 2 O, CH 4 , NH 3 , CO 2 , etc.
Primitive atmosphere was reducing atmosphere (without free oxygen) unlike the
present oxidising atmosphere (with free oxygen).Simple organic molecules
↓CH 4 , CO 2 , H 2 O → Sugar, Fatty acids, Glycerol
CH 4 , CO 2 , NH 3 → Amino acids
CH 4 , H 2 O, NH 3 , HCN → Nitrogenous bases (Purines, Pyrimidines)Complex organic molecules
(Macromolecules)
↓Simple organic molecules accumulated in the ancient seas. In primitive
atmosphere, electric discharge, solar energy, lightning, ATP and polyphosphates
might have provided the source of energy for polymerisation reactions of organic
synthesis.
Sugar + Sugar + Sugar → Polysaccharides
Fatty acid + Glycerol → Fats
Amino acid + Amino acid → Proteins
Nitrogen bases + Pentose sugars + Phosphates → Nucleotides
Nucleotides + Nucleotides → Nucleic acidsBiological Evolution (Biogeny)Coacervates or Microspheres
↓Oparin and Sydney Fox proposed that complex organic compounds synthesised
abiogenetically on the primitive earth later tend to accumulate and formed large
colloidal cell-like but non-cellular prebiotic aggregates called protobionts.
These microscopic, spherical, stable and motile aggregates were called coacervates
by Oparin and microspheres by Sydney Fox.
Coacervates are collection of organic macromolecules surrounded by a film of
water molecules. They lack a definite membrane and cannot reproduce.
Microspheres are small spherical cell-like units formed from proteinoids placed
in boiling water and slowly allowed to cool. They have the ability of motility,
growth and binary fission.
Microspheres can be considered as first living cells.Free gene
↓Self replicating, nucleoprotein complex.Earliest cells
↓Lipid-protein membrane-bound units with enzyme controlled metabolism and
nucleic acid regulation but lacked organised nucleus. They were heterotrophs.Prokaryotes
↓Unicellular, cells did not have distinct nucleus and cell organelles. They had
evolved to show chemoautotrophism and photoautotrophism (mostly anaerobic).
They divide by binary fission.Chemical Evolution (Chemogeny)