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CISCEISC (Science) ISC Class 12
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Biochemical Origin of Life - Seventh step: Origin of Autotrophism

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Estimated time: 9 minutes
CISCE: Class 12

Origin of Autotrophism

  • Early bacteria‑like prokaryotes were anaerobic heterotrophs.
  • They consumed abiotically synthesised organic nutrients present in the “oceanic soup”.
  • Gradual depletion of these nutrients led to intense struggle for existence.
CISCE: Class 12

Emergence of Autotrophism

  • During this struggle, some early prokaryotes evolved enzymes.
  • These enzymes catalysed the synthesis of simple carbohydrates from inorganic substances in ocean water.
  • This marked the beginning of autotrophism (self‑feeding from inorganic raw materials).
  • Energy for this synthesis came from anaerobic breakdown (chemical reactions), not from light.
  • Therefore, the earliest autotrophism is called chemoautotrophism.
  • Natural selection favoured these anaerobic autotrophs under the existing conditions.
  • These early anaerobic autotrophs can be compared to present‑day deep‑sea sulphur bacteria.
CISCE: Class 12

Evolution of Photosynthetic Autotrophs

  • Later, some primitive autotrophic bacteria‑like organisms synthesised chlorophyll‑like green pigments.
  • These pigments were bacteriochlorophylls, formed from magnesium porphyrin present in ocean water.
  • Bacteriochlorophylls could absorb sunlight and provide solar energy for carbohydrate synthesis.
  • Some modern marine planktonic sulphur bacteria show this type of photosynthetic autotrophism.
CISCE: Class 12

Development of Chlorophyll‑Bearing Prokaryotes

  • Subsequently, bacteriochlorophyll molecules underwent structural changes.
  • True chlorophyll evolved and became organised into lamellar units with photosynthetic enzymes.
  • This allowed early prokaryotes to use water instead of sulphides or other inorganic molecules as a hydrogen donor.
  • Chlorophyll‑bearing prokaryotes of the primitive ocean resemble present‑day blue‑green algae (cyanobacteria).
CISCE: Class 12

Key Points: Origin of autotrophism

  • Early prokaryotes were anaerobic heterotrophs that consumed the organic nutrients present in the primitive ocean, leading to competition for survival.
  • Some prokaryotes evolved enzymes to synthesise carbohydrates from inorganic substances, giving rise to chemoautotrophism, which was favoured by natural selection.
  • Later, photosynthetic autotrophs evolved with chlorophyll-like pigments, eventually leading to chlorophyll-bearing prokaryotes similar to present-day blue-green algae (cyanobacteria).

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