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Are we sure abiogenesis only happened once? Given how quickly life appeared once the conditions were right, it doesn't seem too implausible that the lightning strike might have happened in two different tidal pools on opposite sides of the planet, and it seems just vaguely plausible that there would be (bacterial) descendants of each that have no interbreeding in their ancestry.



I asked this in a thread on Slashdot once, and got a really good answer. Basically, there are a number of molecules that can have a left or right chirality. These happened by random chance at the beginning instances of abiogenesis, and were replicated faithfully since. So if it happened twice, a number of those molecules (or larger structures) would be backwards. From a macro perspective, things like the heart being on the left side, or the direction of other internal organs (i.e. appendix on the right side). At the molecular level, things like the spiral of DNA. There is some more information in Wikipedia, under abiogenesis and homochirality.


AFAIK all known life shares common ancestry. That doesn't mean life couldn't have arisen twice, but if it did, and if its descendants still exist, we haven't found them.




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