Posted by JOY on Wednesday, November 30, 2011 at 4:52am.
You have essentially two possibilities, life started on Earth abiogenesis (http://en.wikipedia.org/wiki/Abiogenesis)
or it arrived here from elsewhere (Panspermia see http://en.wikipedia.org/wiki/Panspermia)
If you google abiogenesis or panspermia you will get more information. The above two wiki pages will get you started.
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