Magazine article Science News

Hybrid-Driven Evolution: Genomes Show Complexity of Human-Chimp Split

Magazine article Science News

Hybrid-Driven Evolution: Genomes Show Complexity of Human-Chimp Split

Article excerpt

Not only did the evolutionary parting of human from chimpanzee ancestors occur more recently than had been indicated by previous data, but it also played out over an extended period during which forerunners of people and chimps interbred.

That controversial possibility arises from a new genetic comparison of people, chimps, gorillas, orangutans, and macaque monkeys.

Various parts of the human genome diverged from those of chimps at times that span at least 4 million years, concludes a team led by geneticist David Reich of Harvard Medical School in Boston. A final genetic split, yielding reproductively separate ancestral species of humans and chimps, transpired between 6.3 million and 5.4 million years ago, the scientists report in an upcoming Nature.

Most scientists had held that hominids and ancient chimps branched off from a common ancestor roughly 7 million years ago, with no interbreeding.

Clues to ancient interbreeding lie on the X chromosome, Reich and his coworkers say. People and chimps exhibit far more similarity on that sex-linked DNA strand than on any of the other 22 chromosomes. Genetic detachment of human ancestors, or hominids, from chimps seems to have occurred on the X chromosome about 1.2 million years later than it did on other chromosomes, the scientists report.

A partial genetic cleavage of hominids and chimp ancestors, followed by interbreeding that reshaped the sex chromosomes, then a conclusive split, best explains these findings, in the researchers' view. If they're right, then presumed hominid fossils from more than 6 million years ago (SN: 7/13/02, p. 19) would have preceded the final split and actually come from hybrid creatures.

"Something very unusual happened at the time of [human-chimpanzee] speciation," Reich says.

His team aligned 20 million base pairs from the genomes of five modern primates. The researchers then identified sites containing two alternative versions of the same gene across the species. …

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