Chimps outdo people in genetic diversity.

If variety is the spice of life, then chimpanzees come loaded with genetic seasoning. Common chimps, with their three subspecies, exhibit far more diversity along a particular stretch of DNA than people do, a new study finds. Moreover, it reveals a surprisingly close genetic connection between common and pygmy chimps.

The results reflect a tight evolutionary relationship, nurtured by frequent interbreeding, among the different chimp groups, report geneticists Svante Paabo, Henrik Kaessmann, and Victor Wiebe of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Broad genetic consistency among subspecies of these close human relatives supports the notion that chimp groups' unique behaviors in different regions develop through teaching and imitation (SN: 6/19/99, p. 388) rather than through genetic determination, the scientists say.

Until now, genetic studies of chimps have focused on mitochondrial DNA, which is inherited only from the mother. The German team instead studied the X chromosome, which is inherited from both parents. Chemical changes and rearrangements occur rarely in the section examined, thus enhancing efforts to reconstruct ancient evolutionary relationships.

The team determined the chemical arrangement of this DNA segment in 30 common chimps of three subspecies--12 central African chimps, 17 western African chimps, and 1 eastern African chimp. The scientists also tested rive members of the pygmy chimp species.

Overall, the common chimps exhibited about four times as much diversity in this genetic region as a group of 70 people, did, Paabo's team reports in the Nov. 5 SCIENCE. Differences between individuals in a single chimp subspecies often exceeded those between common chimps and pygmy chimps.

The two chimp species may thus have taken different evolutionary directions relatively recently, the scientists contend. They calculate that this split occurred about 930,000 years ago. Prior estimates, based on more changeable sections of DNA, had placed the species' division at around 2.5 million years ago.

Further research will examine whether gorillas and orangutans display the abundant diversity of chimps or the narrower genetic range of people, the team says.

The scientists hold that people's relatively low genetic variation has implications for how they evolved. "The simplest explanation is that at some rather recent point in the past, humans were few in numbers," asserts Paabo. "That point could have been the genetic origin of modern humans."

Paabo and other researchers have similarly argued, using analyses of mitochondrial DNA, that modern humans arose only about 100,000 years ago. This interpretation of the evidence has proven controversial, however (SN: 2/6/99, p. 88).

Since evolutionary processes have yielded chimp subspecies whereas modern humans fall within a unified species, it's not surprising that chimps harbor more genetic diversity than humans do, remarks geneticist Alan R. Templeton of Washington University in St. Louis.

He adds that lesser human diversity doesn't show, as the German team argues, that a decimated human population in the Stone Age depleted human genetic variation.