>Also, on a slightly different topic, this would mean that life existed at a time when Earth was still getting hit with immense asteroids, of the type that woul

>Also, on a slightly different topic, this would mean that life existed at a time when Earth was still getting hit with immense asteroids, of the type that would leave craters at least 800 kilometers across (as we know was happening to the moon at the same time). Everyone I've read on this topic assumes such a massive hit would sterilize the whole planet. So did the single celled organisms survive such hits, or was life re-created many times? Nobody knows! It's an important question though, especially if you believe in some variant of the 'Great Filter' hypothesis. If the leap from organic molecules to single-celled life is "easy" (as demonstrated by repeated iteration in our past) then the filter lays beyond that. Then the question is whether multicellular life is the big leap?