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Due to lack of actual evidence, it is even now difficult to decide whether origin of cells and of later groups of organisms evolved monophyletically or polyphyletically. Without involving in this problem, best evidences at present suggest that the first cell or cells probably arose some two billion years after the origin of earth, that is perhaps three billion years ago.
What point on the earth may be considered as beginning of the life, it not yet clear or known. But it is believed that cell is the major product of the first few billion years and cell is considered as being alive.
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