Life on earth appeared 500 million years after its formation. Different theories were given by different thinkers and scientists.
(i) Theory of Special Creation:
It states that God created life by his divine act off creation, i.e., the earth; light; plants and animals are all being created by the supernatural power.
This theory has following connotations:
(a) All living organisms (species or their types) that we see today were created as such.
(b) Diversity of life form will not change in future.
(ii) Theory of Panspermia or Cosmozoic Theory:
It was given by early Greek thinkers, which states that the spores or panspermia came from outer space and developed into living forms.
(iii) Theory of Spontaneous Generation:
It states that life originated from decaying and rotting matter like straw, mud, etc. Louis Pasteur rejected the theory of spontaneous generation and demonstrated that life came from pre-existing life. In his experiment, he kept killed yeast cells in pre-sterilised flask and another flask open into air. The life did not evolved in the former but new living thing evolved in the later flask.
(iv) Theory of Chemical Evolution:
This theory was given by Oparin and Haldane. According to them, the first form of life came from pre-existing, non-living organic molecules (e.g., RNA, protein, etc.) and chemical evolution was followed by the formation of life, i.e., formation of diverse organic molecules were formed from inorganic constituents. The conditions on earth favouring chemical evolution were high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, etc.