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>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?

Yes, previously people believed that the heavy bombardment would have sterilized the earth. Apparently (hard to say for sure) that assumption was wrong.

It raises all sorts of exciting questions and completely changes the landscape for what early life looked like.

> Does anyone know? Is the claim that this is definitely bacteria, and not Archaea?

Shouldn't really be either; this should be quite a bit older than Last Universal Common Ancestor and hence older than the Bacteria/Archaea split. But I don't actually understand what precisely was found so can't say.

> did the single celled organisms survive such hits

or perhaps life itself was delivered by these hits ?