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Human Speciation

May 18, 2006

Of all the Latinized names given to hominid fossils, the one I had the most trouble remembering was Sahelanthropus tchadensis. A skull found in Chad in 2001 may not actually be an ancestor to humans and chimps, after all.While it was a pain to spell, the fossil was considered an important link—potentially a common ancestor to both humans and chimpanzees, dating to a million years or so before the two species split. At least, that’s what they thought, until now.

New research suggests that the chimp/human divergence happened not once, but at least twice, both before and after S. tchadensis was wandering around Africa. In other words, a million years or so after the two species split, they got back together to mate, swapped a little DNA, and produced hybrids. Modern chimps and humans are descended from those hybrids. This means S. tchadensis was probably part of a line of non-hybrids which died out—just a stubby branch on our family tree.

The Washington Post gave the details of this new theory, this morning:

According to the new theory, chimps and humans shared a common apelike ancestor much more recently than was thought. Furthermore, when the two emerging species split from each other, it was not a clean break. Some members of the two groups seem to have interbred about 1.2 million years after they first diverged — before going their separate ways for good.

If this theory proves correct, it will mean modern people are descended from something akin to chimp-human hybrids. That is a new idea, and it challenges the prevailing view that hybrids tend to die out.

They suggest the new theory is important in understanding how new species arise. Personally, I’d like to see this quote in giant flashing neon signs (the emphasis is mine):
“This is contributing to the idea that species are kind of fuzzy. They become real over time, but it takes millions of years,” said James Mallet, a geneticist at University College London who was not involved in the new research. “We probably had a bit of a messy origin.”

Here are the not-so-fuzzy details:
When Nick Patterson of MIT and his colleagues at the Broad Institute compared the genes of humans and chimps, they found that one of the chromosomes — the female sex chromosome X — was 1.2 million years younger than the others. It appeared the two species shared a common ancestor who gave them both their X chromosomes, and did so more recently than the ancestors who gave them all the other chromosomes.

The best explanation, the scientists think, is that ancient humans and chimps broke away from each other not once, but twice. The first time was more than 6.3 million years ago. The second time was at least a million years later.

What probably happened was that some of the evolving human ancestors bred with the evolving chimps. This was perhaps not as strange as it seems, for although there were some physical differences between the two groups, “the early humans must have looked pretty much like chimpanzees,” said Mallet, the London geneticist.

Males have only one X chromosome, which is necessary for reproduction. As is often the case with hybrids, the male offspring from these unions would probably have been infertile.

But the females, which have two X chromosomes, would have been fertile. If some of those hybrid females then bred with proto-chimp males, some of their male offspring would have received a working X from the chimp side of the family. They would have been fertile — and with them the hybrid line would have been off and reproducing on its own.

And what does that mean for the skull from Chad? He apparently lacked the essential X, suggesting he was descended from the first divergence, not the second. Our line lived on to mix once again with the chimps, while S. tchadensis’ line died out. Sorry, bro.

Image of skull via the Washington Post.