Here’s a headline from the AFP in Paris (via Yahoo News):

Fossil find pushes human-ape split back millions of years

This is a heck of a lot better than some of the truly nonsensical proclamations of the press about the discovery some important fossils, namely eight molars and a canine tooth from a (presumed) ancestor of gorillas, e.g. New Fossil Ape May Shatter Human Evolution Theory. But I still have some problems with this story.

I’m no anthropologist, and I do not generally follow the specialized literature of that field closely. But as far as I know, it is now very well established that humans and chimpanzees are more closely related to each other than either is to gorillas. That is what the molecular data indicate, in any case. Here’s a representation of the relationships among our closest living relatives:

This means several things. First, that there is no natural category called “apes” that does not include humans. Humans are apes. Second, that there was no single split between the ancestors of “apes” and those of humans. Different lineages split at different times. The split between the ancestor of orangutans and the ancestor of the other apes (gorillas + chimps + humans) probably occurred first, then the split between the ancestor of gorillas and that of the remaining apes (humans + chimps), and then the split between the ancestor of chimps and humans. (And then splits between the ancestor of chimps and bonobos and various now-extinct relatives thereof, and between the ancestors of various hominins).

(Just in case it doesn’t go without saying, humans are not descended from chimps, they share a common ancestor with chimps. Chimps are cousins, not grandparents, to humans. Further, it is just as accurate to say that “orangutan ancestors diverged from the ‘human’ lineage” as it is to say that “human ancestors diverged from the ‘orangutan’ lineage”. There is no main trunk from which offshoots diverge, there are just branchings within a bush.)

In this particular news story, the reporter notes that “the new fossils, dubbed ‘Chororapithecus abyssinicus‘ by the team of Japanese and Ethiopian paleoanthropologists who found them, place the early ancestors of the modern day gorilla 10 to 10.5 million years in the past, suggesting that the human-ape split occurred before that.”

Moving the split of the ancestors of gorillas from the ancestor of chimps + humans back does not affect the “human-ape split”. Why? Because there was no such split. Move the gorilla lineage divergence back from 6-8 to 10-10.5 million years (My), that’s fine, but it does not automatically tell you when the human and chimpanzee ancestors’ lineages split, because that occurred later (nonetheless, the authors of the original paper suggest a new divergence for chimp/human ancestors at 9 My). In contrast, if you found an older split for the ancestors of humans and chimpanzees, you might have to move some splits that happened before it back also. As a result, there are arguments that pushing the gorilla lineage split back to 10 My from the current 6-8 My would require moving the orangutan lineage split way back (the authors of the original article say to 20 My).

Notably, the author of the news story also reports that “Conventional scientific wisdom, based on genetic “distances” measured by molecular geneticists, had placed the divergence between chimps and humans some five to six million years ago. Orangutans are thought to have parted company with our ancestors 13 to 14 million years ago.” As a very general proposition, and not including any nuanced discussion, gorillas would be expected to have ancestors of an age somewhere in between 5-6 and 13-14 million years, and 10-10.5 My is right in between these.

Maybe I am missing something, but as interesting and important as this discovery is for understanding some specifics of gorilla evolution, it does not shake things up that much as far as the other apes, including a certain hairless one, are concerned. That is, if these really are teeth from a direct ancestor of gorillas and not an example of convergence according to diet in a non-ancestral species, as some people have already suggested it might be. The question of why the molecular data and the fossils don’t quite agree is interesting, but that’s a pretty standard issue.

Finally, let me say once again that this sort of case is a question about the path of evolution, the specific branchings and timings thereof in the history of particular lineages, and has no bearing on the fact of evolution, namely whether or not species are related through common ancestry and descent with modification. Find me a gorilla fossil older than 500 My and we’ll talk about substantial challenges to the fact of evolutionary relationships.

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The original report and comments article in Nature are:

Suwa, G., Kono, R.T., Katoh, S., Asfaw, B., and Beyene, Y. 2007. A new species of great ape from the late Miocene epoch in Ethiopia. Nature 448: 921-924.

Dalton, R. 2007. Oldest gorilla ages our joint ancestor. Nature 448: 844-845.