Why isn’t life on earth more diverse than it in fact is?

Via Kottke, a New Scientist piece on the hypothesis that the earliest common ancestor of all life on earth was a mega-organism of planetary scale.  That’s one way of solving a problem that I sometimes wonder about.  If, as we now hear from exobiology types, life can be expected to originate wherever in the universe conditions will sustain it, how many times did it arise on this planet?  If the answer is “many times” and life has arisen independently many times in the history of the earth, why do all of the organisms we see look so similar that they might have a common ancestor?  Of course several solutions have been proposed to this problem, but the idea of a mega-organism that assimilated some previous life-forms and drove the rest to extinction would seem to be another.

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