First of all, the study revealed that the Xq13.3 sequence is almost four times as variable and three times as old in chimpanzees as it is in humans. Although this conflicts with the tale told by isolated chunks of DNA analyzed in earlier research, the findings support many other studies that characterize humans as less diverse. The researchers believe that a bottleneck or crash in the human population might have dramatically reduced its genetic diversity. "The results show a strikingly lower amount of variation in humans than in chimps," said Pbo. "The simplest explanation for this is that at some rather recent point in the past, humans have been few in numbers. From a genetic point of view, this point could be the origin of modern humans."

Most of the variation in the chimpanzee sample comes from the central African chimpanzees, with the least amount of diversity found among the western chimpanzees. The central chimpanzee sequences also appear to be the oldest of the chimpanzee lineages. According to Pbo, the variation is similar to what geneticists see with the human African 'Eve'. "The central chimps are the most genetically diverse and other groups seem to be derived from them. Thus, if you like, the chimp 'Eve' of this sequence lived in central Africa," he said. But Pbo and his colleagues acknowledged that mitochondrial DNA studies, which show the most diversity in the western population, clash with this version of events.

The research team encountered another curious contradiction between the mitochondrial and nuclear genomes when they drew up an evolutionary tree for their sample. Mitochondrial studies divide the western, central, and eastern chimps into discrete groups. By contrast, the Xq13.3 sequence tree showed that the geographical subspecies were in fact highly intermixed
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Contact: Heather Singmaster
hsingmas@aaas.org
202-326-6414
American Association for the Advancement of Science
3-Nov-1999