Life appeared 500 million years after the formation of earth, i.e.. almost four billion years back. Early Greek thinkers thought units of life called spores were transformed to different planets including earth. 'Panspermia' is still a favourite idea of some astronomers. Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life. However, this did not answer how the first life form came on Earth.

Oparin of Russia and Haldane of England proposed that the first form of life would have come from pre-existing non-living organic molecules (RNA. proteins etc.) and formation of life was preceded by chemical evolution. i.e.. formation of diverse organic molecules from inorganic constituents. The conditions on earth were - high temperature, volcanic storms, reducing atmosphere containing CH₄. NH₃ etc.

In 1953, S.L. Miller, an American scientist, created similar conditions in a laboratory scale. He created electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800° C. He observed formation of amino acids. In similar experiments others observed, formation of sugars, nitrogen bases, pigment and fats. With this limited evidence, the first part of the conjectured story, i.e.. chemical evolution was more or less accepted.

Louis Pasteur by careful experimentation demonstrated that life comes from :-

pre-existing life

The correct answer is Option (4) -pre-existing life

Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life. He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’. Spontaneous generation theory was dismissed once and for all. However, this did not answer how the first life form came on earth.