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.

When the life appear on the earth after it's formation?

After 500 million years

The correct answer is Option (3) -After 500 million years

The origin of life is considered a unique event in the history of universe. The universe is vast. Relatively speaking the earth itself is almost only a speck. The universe is very old – almost 20 billion years old. Huge clusters of galaxies comprise the universe. Galaxies contain stars and clouds of gas and dust .

In the solar system of the milky way galaxy, earth was supposed to have been formed about 4.5 billion years back. There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface.

The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter H₂ escaped. 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.