When the ancestors of human beings and the ancestors of chimpanzees parted ways 6.3 million years ago, it was probably a very long goodbye. Some of their descendants may even have gone back for a final tryst.
That is the conclusion a group of scientists has reached, using a comparison of the genes of humans and their closest animal relatives to sketch a picture of human origins far more detailed than what fossil bones have revealed.
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.
It also strongly suggests that some of the oldest bones of “proto-humans”—including the 7 million-year-old Toumai skull unearthed in Chad in 2001—may have belonged to a line of non-hybrids that died out, and were not human ancestors at all.
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.