The origin of life forms is interlinked with the evolution of the earth, which is again closely associated with the evolution of the universe. The universe is vast, comprises many galaxies and is almost twenty billion years old. The Big Bang Theory attempts to explain the origin of the universe. At the time of its formation, there was no atmosphere on the earth. Its surface was covered by water vapour, methane, carbon dioxide and ammonia released from the molten mass. Gradually, the ultraviolet rays of the sun broke up the water, into hydrogen and oxygen, oxygen combined with ammonia and methane, which led to the formation of water, carbon dioxide and other gases. Some of the water vapour cooled and came down as rain, which filled all the deep depressions on the earth and formed oceans. It is estimated that life appeared on earth five hundred million years after its formation.
There are different theories that try to explain the beginning of life on earth. Some of them are Panspermia, Spontaneous Generation, Biogenesis and Chemical Evolution. Panspermia is the oldest theory founded by Greek thinkers. It explains that units of life called spores were transferred to different planets. The theory of Spontaneous Generation assumed that life came out of non-living, decaying and rotting matter such as stones, straw and mud. Aristotle , John Needham supported the theory. A German biologist, Rudolf Virchow, challenged it with his theory called ‘Omnis cellula e cellula’,. Later on, Louis Pasteur, with his experiments proved the theory of Biogenesis and forever silenced it. Another theory called the theory of Chemical Evolution was expounded by Oparin and Haldane. They proposed that the first form of life originated from pre–existing, non-living organic molecules such as RNA and protein. In 1953, Stanley Lloyd Miller recreated these conditions in a laboratory and observed . He created an electric discharge in a closed flask containing methane, ammonia, hydrogen and water vapour at eight hundred degrees centigrade and observed the formation of amino acids. Later on, while conducting similar experiments, other scientists observed the formation of sugars, nitrogen bases, pigment and fats as well. Thereafter, the theory of Chemical Evolution was most widely accepted. It is probable that all life forms originated in a water environment. This theory of Chemical Evolution, which explains that first life forms arose from non-living molecules through evolutionary forces is now widely accepted.However, the origin of life is a fascinating and curious topic which will forever invite a lot of debate and speculation.