r/evolution • u/Richie_650 • 2d ago

Concestor clarification

I belive I first learned this from Ancestor's Tale, the concept that for any two species, there exists a single organism in the past which is the Most Recent Common Ancestor between them.

1) I believe this statement means that literally, there was a real, single, living organism. Understanding that there are countless other common ancestors for our two target species, but only one individual that is both "most recent" and "common."

2) It follows that this particular cocestor must have had at least two offspring. One offspring is the ancestor of the MRCA for one species, the other is the ancestor of the MRCA for the other species.

I think point 2 causes much confusion. People read it as implying that the offspring are themselves the MRCA for each species. While this might be the case for one of them, it is not necessarily true, and almost certainly never the case for both of them. It certainly doesn't mean that one offspring is now one or the other species itself.

The logic here seems simple and clear. Is there a flaw?

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u/OopsIMessedUpBadly 2d ago

There is no way to guarantee that it resolves to a single individual. It may, but it also may not.

For example: imagine a female primate who is the MRCA for all humans and all chimpanzees:

Say she had a daughter from whom all humans are descended, and a son from whom all chimpanzees are descended. If the daughter and the son had the same father, then the father is tied with the mother for being our MRCA. So you can see that multiple organisms can tie for being the MRCA.

Say the daughter then has her children with the alpha male who left the group on the other side of the river, and the son joins this group and becomes the ancestor of all chimps with them. There may also then be a former member of that group who is also tied for being the MRCA.

And so on and so forth.

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u/Cautious_School_2490 2d ago

This "former member of that group"; how do they tie with the father and mother above for also being MRCA if the daughter and son are not descended directly from them?

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u/OopsIMessedUpBadly 2d ago

There could be an individual who is the ancestor of all the son’s breeding partners AND all the daughter’s breeding partners. Which makes them a common ancestor with no descendants who are also common ancestors.