The Common Ancestor?

Presumably you mean the most recent common ancestor of humans and all modern apes (since the most recent common ancestor would itself be classified as an ape, and since the most recent common ancestor of humans with chimpanzees is more recent than the most recent common ancestor of humans - and chimps - with orangutans).The answer is that we probably have not found it but there's no way to tell for sure. Most species - especially land vertebrates - have left no fossils whatsoever. And it is impossible to tell for certain whether a fossil actually is in the direct line of ancestry rather than simply an offshoot except in very special cases.I imagine that you actually want the last common ancestor of humans and chimpanzees. Although there's no fossil I can point to and say that it is that, there is Orrorin tugenensis which likely belongs on the human side of the divide, and dates to shortly after the lineages diverged.So, we don't have any expectation of finding "the common ancestor". What we expect to find is a sampling of species that includes - maybe - some common ancestors and a lot more near relatives, like Orrorin. And that's what we do find.

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While such an individual may have existed, I don't see why it is at all necessary or even reasonable to assume she did. What defines one as human vs. chimp is not locked away in a single gene. T

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In fact Dr. A did give reasons why it IS necessary. Provided the divisions between the gorilla and human/bonobo/chimp lineages and the human and bonobo/chimp lineages are clean it must be the case.To repeat the explanation in my words. Start with a common ancestor of living humans and bonobos/chimps from the ancestral population after the split with the gorilla lineage is complete. This individual (by definition) must have both human and bonobo/chimp descendants.Either this individual has a child which also has both human and bonobo/chimp descendants or it does not. If it has such a child we move on to that child and repeat, at each stage choosing a child with both human and bonobo/chimp descendants.Since we know that the lineages have diverged this process cannot go on forever. Therefore at some point we must find an individual who has human descendants and bonobo/chimp descendants, but no child with both human and bonobo/chimp descendants.It follows that this individual must have at least one child with human descendants and no bonobo/chimp descendants and at least one child with bonobo/chimp descendants but no human descendants.Therefore Doctor Adequate is correct