Did Humans And Chimps Once Interbreed?

I think they still do in iran and San Francisco!!

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Did Humans And Chimps Once Interbreed?

New ScientistDid humans and chimps once interbreed?

17 May 2006
From New Scientist
Bob Holmes


IT GOES to the heart of who we are and where we came from. Our human ancestors were still interbreeding with their chimp cousins long after first splitting from the chimpanzee lineage, a genetic study suggests. Early humans and chimps may even have hybridised completely before diverging a second time. If so, some of the earliest fossils of proto-humans might represent an abortive first attempt to diverge from chimps, rather than being our direct ancestors.

We can observe the traces of this complex history in the human genome today, says David Reich, a population geneticist at the Broad Institute and Harvard University in Cambridge, Massachusetts. Reich and his colleagues compared the genomes of humans, chimps and gorillas using a "molecular clock" to estimate how long ago the three groups diverged. The further back two species diverged, the more differences will have accumulated between their genome sequences.

The team estimated that humans and chimps diverged no more than 6.3 million years ago, and probably less than 5.4 million years ago, although some parts of the genome showed divergence times up to 4 million years older. Even if we split from our ape relatives 6.3 million years ago, that is still later than some of the earliest fossils showing human-like traits such as altered tooth structure and bipedalism (Nature, DOI: 10.1038/nature04789). "That makes the fossil record even more interesting," says Reich. "What were those fossils?"

The answer might lie in a second striking observation. Reich's team found that the X chromosome diverged later than any of the other chromosomes. One way this can arise is if natural selection has been acting unusually strongly on genes on the X chromosome. That is significant because in every animal species studied, genes that make hybrids less fertile than their parental species tend to be found on the X chromosome or its equivalent, so hybridisation can create strong selection pressures on this chromosome.

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