Did humans and chimps once interbreed?

http://tinyurl.com/gea6o
17 May 2006
From New Scientist Print Edition

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.

The best explanation for these surprising findings - the relatively
young and variable divergence dates between the human and chimp
lineages, and the evidence for strong selection on the X chromosome -
would be if the two lineages split sometime before the time of the
first proto-human fossils, but later rehybridised (see Diagram) in a
"reverse speciation" event (see "When evolution runs backwards").
Natural selection would favour those hybrid individuals whose X
chromosomes carried fewest of the genes that lower fertility.

So far, Reich admits, this is only a plausible hypothesis, not a proven
fact. For example, he calibrated his molecular clock using the
divergence time between humans and macaques, which is estimated at no
more than 20 million years ago. If this divergence happened earlier,
that would push back the human-chimp split to an earlier date as well -
perhaps far enough that there would be no need to invoke hybridisation.
At the very least, though, Reich's study shows that the separation
between humans and chimps was a long, drawn-out process.
From issue 2552 of New Scientist magazine, 17 May 2006, page 14

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