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As discussed previously, the earth was earlier assumed as a hot gaseous mass that led to the origin of life when cooled.
Many theories explain the origin of life, and some of them are as follows:
Theory of special creation:
The theory of special creation proposed that a supernatural power created life. Hence it is called life on earth as a divine creation.
It also stated that the diversity among living organisms was always the same from the creation time and will continue to be the same.
Earth-centred view of the universe
This theory has the following connotations:
  • All living organisms were created similarly. Hence, there are no differences in their appearances.
  • They were created in the way they exist, and there is no evolution.
  • Their bodies and organs are fully developed to meet the demands of life. Hence, there is no adaptation.
Objections against the theory of special creation:
  • It was a theory proposed purely on religious belief.
  • They are no scientific evidence to support the theory.
  • The age of various fossils demonstrates that biological organisms appeared on earth at various times.
As this theory lacked scientific evidence, it was not accepted.
Spontaneous generation:
The theory of spontaneous generation, often known as abiogenesis, states that life is formed spontaneously from abiotic, dead substances.
The various observations supporting this theory are as follows:
  • Fishes originated from mud.
  • Fly larvae developed from rotten meat.
  • Frogs developed from moist soil.
  • Insects developed from decaying matter.
This theory was criticized by Lazzaro Spallanzani, Francisco Redi and Louis Pasteur by their experiments which concluded that life originated from pre-existing life and named the theory biogenesis.
 Redi’s experiment:
In \(1668 ,\)an Italian physician Francesco Redi proved that life spontaneously originated from pre-existing life by the following experiment.
  • Redi took cooked meat so that no living organisms are alive in it.
  • He placed the meat in three different jars.
  • He left one of the jars uncovered, one sealed with a cork and the other with gauze.
A picture illustrating Redi's experiment
  • The decaying meat in the jar attracted flies.
  • In the uncovered jar, flies entered and layed eggs which eventually gave birth to new larvae.
  • In the jar covered with cork and gauze, flies were unable to enter the jar. Hence, no larvae were found.
  • However, larvae were found on the gauze as the flies laid eggs on them.
Hence, through this experiment, Redi proved that life spontaneously originated from pre-existing life (flies) rather than non-living material (cooked meat).
A video explaining Redi's experiment
Pasteur's experiment:
  • In this experiment, Louis Pasteur took broths (killed yeast) in a long-necked flask, and then he bent the neck of the flask (swan neck flask).
A picture illustrating Pastuer's experiment
  • He boiled the broth in the flask so that the microorganisms in this flask were killed.
  • The curved neck of the flasks acted as a filter.
  • When these flasks were kept months together, no life appeared in them as the neck of the flask trapped the dust particles.
  • When this neck was broken off, the broths developed colonies of microbes such as bacteria and moulds.
  • Hence, through this experiment, he concluded that the flasks kept airtight with dead yeast did not produce any life forms. However, another flask that was kept open to the air produced living organisms from the deceased yeast.