Re: Have aliens visited earth?
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Actually, the conditions for life to exist are extremely rare. Life as we know it is extremely fragile and it appears, based on probabilistic arguments, that the conditions for life to exist are not that common in the universe. Microbes quite likely exist in abundance but complex life not. This argument was first set forth in the book Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Ward, Donald Brownlee.
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I have not read this book but I am aware of the argument. The problem is we really do only have a sample size of one for a lot of the assumptions being made. Many of the conditions for complex life they consider unlikely may not be so unlikely at all, and many of the conditions for complex life they consider necessary may not be so necessary at all.
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