Human, Chimp Ancestors May Have Mated, DNA Suggests

Brian Handwerk
for National Geographic News
May 17, 2006

Early human ancestors and chimpanzee ancestors may have mated and produced offspring, according to a new DNA study.

The study suggests that the human and chimp lineages initially split off from a single ape species about ten million years ago. Later, early chimps and early human ancestors may have begun interbreeding, creating hybrids—and complicating and prolonging the evolutionary separation of the two lineages.

The second and final split occurred some four million years after the first one, the report proposes.

"One thing that emerges [from the data] is a reestimate of the date when humans and chimps last exchanged genes," said David Reich, a professor at Harvard Medical School's Department of Genetics in Boston.

"Our data strongly suggest that [the last gene exchange] occurred more recently than 6.3 million years ago and probably more recently than 5.4 million years ago," said Reich, senior author of the study, to be published tomorrow in the journal Nature.

"This paper is very interesting, because it provides a hypothesis that is outside of the currently accepted dogma," said Kateryna Makova, a professor at Pennsylvania State University's Center for Comparative Genomics and Bioinformatics who is unaffiliated with the study.

(Related news: "Chimps, Humans 96 Percent the Same, Gene Study Finds" [2005].)

Did Human, Chimp Ancestors Hybridize?

"The genome analysis revealed big surprises, with major implications for human evolution," biologist Eric Lander said in a release announcing the findings. Lander is director of the Broad Institute, a cooperative institute for genomic medicine in Cambridge, Massachusetts.

A genome is an organism's complete set of DNA. The human genome, for example, contains some 3 billion base pairs, which code for the approximately 30,000 genes that define a person's unique traits. (See our quick overview of human genetics.)

"First, human-chimp speciation occurred more recently than previous estimates. Second, the speciation itself occurred in an unusual manner that left a striking impact across chromosome X," Lander said.

This sex-determining chromosome typically occurs in pairs in cells of females and combined with a Y chromosome in cells of males.

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