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In principle, it should be possible for whatever happened to create life to have happened in multiple places. Out of the millions of years this could have happened, it seems odd that it didn't happen independently in at least two locations that were each sufficient to host life.

Why then does all life on Earth appear to have a common ancestor? That's weird to me.

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    $\begingroup$ We don't know that all life on Earth had a common ancestor. We know all observed extant life on Earth does. That's quite different. Lineages die out all the time, so if you started with life of different types it would be quite surprising if over hundreds of millions of years you would keep some of both around. $\endgroup$
    – Bryan Krause
    May 8 at 21:30
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    $\begingroup$ Does this answer your question? Why did abiogenesis only happen once? $\endgroup$
    – David
    May 9 at 18:28

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One way to visualize it is to imagine that you have a pile of M&Ms on the table. Let's say that there are 15 red ones, 5 blue ones, and 2 green ones. Every 10 seconds you randomly select one to eat. What is the most likely end result after 100 seconds? Well the green ones will probably get gobbled up first, then the blue ones, until you are left with only red M&Ms. Now imagine that your M&Ms can copy themselves and eat eachother and compete with eachother for resources, well then your whole table would probably be totally covered in red M&Ms after a while and, from the perspective of someone who only had the current state of your table to go on, it would seem that the red ones are all there ever were. We can't say that abiogenesis only happened once, only that every living thing alive today evolved from a single common ancestor. It's sort of like how Mitochondrial Eve wasn't the first human woman. There were plenty of other women running around. It's just that none of their mitochondrial DNA can be found in modern populations. Evolution has something of a "the rich get richer" tendency. Over time, less numerous gene variants tend to die out, while more numerous ones become even more numerous. Run this sort of scenario out long enough and the more numerous variant often ends up with a monopoly while the others get lost to history.

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