Class 12 Biology Evolution Origin of life

Origin of life

  • The universe is almost 20 billion years old.
  • Huge clusters of galaxies comprise the universe.
  • Galaxies contain stars and clouds of gas and dust.
  • A scientific theory called big bang theory explains that, the unimaginable large explosion created the universe.
  • As the universe expanded and cooled, the temperature came down and materials condensed under the influence of gravitation to form present day galaxy.
  • Our galaxy is called Milky Way formed 4.5 billion years back.
  • There was no atmosphere on early earth.
  • Water vapour, methane, carbon dioxide and ammonia released from molten mass covered the surface.
  • The UV rays from the sun broke up water into Hydrogen and Oxygen and the lighter H2
  • Oxygen combined with ammonia and methane to form water, CO2 and others.
  • The ozone layer was formed, as it cooled, the water vapor fell as rain, to fill all the depressions and form oceans.
  • Life appeared 500 million years after the formation of earth, i.e., almost four billion years back.
  • Different scientists had put different views about the origin of life.

Fig. early earth  

Fig. Milky way  

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