About the origin of life, several hypotheses have been put forth. Some of them, with their short explanation, are given below.
Theory of special creation
According to this theory, on the earth, life was created by supernatural power, the god. This theory has no scientific evidence; therefore, it is not widely accepted.
Theory of spontaneous generation
According to this theory, life originated on the earth from the non-living things spontaneously. It was believed that the mud of Nile River could give rise to living creatures like frog, toad, snake and crocodile; when warmed in sun.
Modern concept of origin of life
Modern concept was suggested by opera in 1930 AD. According to this theory, the first living cell arose from simple elements. Micro-molecules like carbon, hydrogen, oxygen, nitrogen etc. united to produce macro-molecules like proteins, carbohydrates, enzymes, nucleic acids etc. Then they interact to produce the first living cell by a process called abiogenesis. Abiogenesis means development of living things from non-living matter.
Evolution is defined as the series of changes which occur, each step depending on what has gone before.
Or, Evolution can be defined as the descent with the modification.
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