Origin of life on earth
The origin of life is a fascinating and an unique event in the history of universe. How did life originate on the earth? In order to explain, it several views have been expressed from time to time. Modern biologist agree that the origin of life was a gradual process. Life did not come into existence suddenly; it was the result of a long series of physico-chemical changes. These changes brought about a gradual evolution of inorganic and organic compounds in accordance with the everchanging environmental conditions.
A.I. Oparin (1924), a Russian scientist and J.B.S. Haldane (1929), and English scientist were of the opinion that the Earth originated approximately 5-6 billion years ago. In the beginning, The earth was a burning spinning ball of hot gases and vapours of various elements. Since the original temperature of the earth was very high (5,000-6,000°C), element like hydrogen, oxygen, carbon and nitrogen could not exist in free state.
They combined among themselves or with metals forming simple inorganic molecules. They also proposed that life must have originated from simple molecules (such as methane, ammonia, hydrogen sulphide, etc.). As the earth begin to cool down, its matter started condensing.
They further speculated that the conditions on the earliest Earth were highly different than the present day conditions. The earth's atmosphere was reducing at that time and not oxidizing as today, and the atmosphere was devoid of free oxygen. As the atmosphere cool down to 1,000°C, the more complex organic molecules necessary for life were formed. Synthesis of these complex organic molecules (carbohydrates, fats ,amino acids, etc.) Presumably took place in the sea. Subsequently, the first earliest organisms developed as a result of synthesis of other essential chemicals necessary for life.