Science: History of Life on Earth: Physical Conditions of Primitive Earth (For CBSE, ICSE, IAS, NET, NRA 2022)

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Physical Conditions of Primitive Earth

  • The physical conditions on primitive earth were not easy for life. The earth was extremely hot-a ball of hot gases.
  • Earth, the planet on which we live, is one of the planets revolving around a medium-sized star , the Sun.

Origin of Life


  • Theory, proposed by the British biologist J. B. S. Haldane and the Russian scientist A. I. Oparin, suggested that life originated in the shallow seas where important organic compounds (such as amino acids) , the building blocks of life, were present in high concentrations (forming a ‘primordial soup’ ) , thus providing the necessary ingredients for emergence of life.
  • Stanley Miller and Harold Urey provided experimental support for hypothesis. Under laboratory conditions they successfully produced amino acids by passing an electric charge (simulating lightning) through a flask containing methane, ammonia and hydrogen in solution.

Diversification of Life

  • Life on earth started in the form of simplest unicellular (prokaryotic) microorganisms. In course of time these organisms evolved to utilize solar energy through chemical process called photosynthesis.
  • Eukaryotes (cells with nucleus) probably appeared about a billion years ago, but life was mostly in the form of unicellular (single celled) organisms.
  • About 600 million years ago, in a geological period called Cambrian, there was a great, almost explosive, diversification of life into multicellular organisms with a variety of body plans and lifestyles, of all those invertebrates and higher plant groups that you are familiar with. Biologists call this period the ‘Cambrian explosion’ .

Darwin՚s Theory of Evolution- Salient Points

Darwin՚s Theory of Evolution
  • All living beings are related through ancestry.
  • The mechanism which causes diversification of species from ancestors is ‘Natural Selection’ .
  • All organisms tend to produce more offspring than can possibly survive. (e. g. only a few frog՚s eggs survive and become frogs) .
  • In fact, population numbers tends to remain fairly constant over long periods of time.
  • Also, organisms in a species show wide variation in characteristics.
  • Some of the variations are inherited, and so passed on to the next generation.
  • Since most offspring do not survive, all organisms must be going through a struggle for survival, being eaten, suffering from disease and competition. The struggle for existence cause large number of individuals to die.
  • The ones who have characteristics that allow them to survive and reproduce better (i.e.. possess most useful adaptations for surviving in the environment) will pass on these characteristics to their offspring. In other words, Nature selects the fittest individuals of the population. Natural Selection is the same as the famous phrase “survival of the fittest” coined by Herbert Spencer. Organisms with slightly less survival value will probably perish first, leaving the fittest to pass on their genes to the next generation.
  • The best adapted individuals were selected by nature to survive and leave offspring for the next generation. Darwin called this mechanism Natural Selection.


Further refinements in the light of advances in population genetics and other areas of biology led to the modern synthetic theory of evolution. It is to Darwin՚s credit that his basic theory of evolution by natural selection finds support even in the most recent developments in molecular biology.

Natural Selection in Action

  • Natural selection is the right mechanism by which species evolve,
  • Since any visible modification in a species evolves slowly over a long time, the effects of natural selection cannot be demonstrated easily.

Levels of Organic Evolution

  • The unit of evolution, in the modern synthetic theory of evolution, is the population. It is the population which evolves and not the individual.
  • Variation occurs at the genetic level through mutation and sexual reproduction in the “gene pool” of the population.
  • Natural Selection causes greater reproduction of the variant genes having adaptive advantage.
  • Evolution at the level of the hereditary material is termed microevolution.
  • Macroevolution or adaptive radiation is the evolution and diversification at the level of species and genera. E. g. Dinosaurs evolved as runners, fliers, swimmers due to macroevolution or adaptative radiation.

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