What Makes us Different?

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Comparisons of primitive genomes have also led to an astonishing, controversial and somewhat disquieting assertion about the origin of humanity. Along with several colleagues, David Reich of the Broad Institute in Cambridge, Mass., compared DNA from chimpanzees and humans with genetic material from gorillas, orangutans and macaques. Scientists have long used the average difference between genomes as a sort of evolutionary clock because more closely related species have had less time to evolve in different directions. Reich's team measured how the evolutionary clock varied across chromosomes in the different species. To their surprise, they deduced that chimps and humans split from a common ancestor no more than 6.3 million years ago and probably less than 5.4 million years ago. If they're correct, several hominid species now considered to be among our earliest ancestors--Sahelanthropus tchadensis (7 million years old), Orrorin tugenensis (about 6 million years old) and Ardipithecus kadabba (5.2 to 5.7 million years old)--may have to be re-evaluated.

And that's not the most startling finding. Reich's team also found that the entire human X chromosome diverged from the chimp's X chromosome about 1.2 million years later than the other chromosomes. One plausible explanation is that chimps and humans first split but later interbred from time to time before finally going their separate evolutionary ways. That could explain why some of the most ancient fossils now considered human ancestors have such striking mixtures of chimp and human traitssome could actually have been hybrids. Or they might have simply coexisted with, or even predated, the last common ancestor of chimps and humans.

All of that depends in part on the accuracy of fossil dating and the reliability of using genetic variation as a clock. Both methods currently carry big margins of error. But the more primate genomes that geneticists can lay side by side, the more questions they will be able to answer. "We have rough sequences for humans, orangutans, chimps, macaques," says Eric Lander, director of the Broad Institute and a leader of the research team that decoded the chimpanzee genome. "But we don't have the entire gorilla genome yet. Lemurs are coming along, and so are gibbons."

DECODING NEANDERTHALS

Also coming along, thanks to two independent teams of researchers, is the genome of the closest relative of all: the Neanderthal. Ancestors of Neanderthals first appeared some 500,000 years ago, and for a long time it was a toss-up whether that lineage would outlive our own species, at least in Europe and western Asia--or whether, bizarre as it seems today, they would both survive indefinitely. The Neanderthals held out for hundreds of thousands of years. A discovery published online by Nature last month suggests Neanderthals may have made their last stand in Gibraltar, on the southern tip of the Iberian Peninsula, surviving until about 28,000 years ago--and possibly even longer.