1) the extraterrestrial theory states that alien beings from outer space colonized the Earth.
2) the chemical evolution theory states that the conditions of early Earth were sufficient for the generation of organic compounds and eventually for cells to spontaneously form, without outside input.
3) one theory hypothesizes that meteors bombarding the Earth may have carried in organic molecules or even cells, as well.
4) Darwin and Wallace's theory of evolution by natural selection offers a thorough explanation of how life originated on Earth.
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