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Ancient human and chimps mixed

Findings of interbreeding could help broaden conventional thinking regarding origins of species

By | May 18, 2006

The ancestors of humans and chimpanzees may have interbred and exchanged significant numbers of genes after the initial split between the species, scientists report in the May 17 online edition of Nature. The findings could help rethink mainstream thought during the origin of species "by suggesting there can be a quite of bit of exchange as species emerge and diverge," James Mallet at University College London, who did not participate in this study, told The Scientist. The researchers compared the genomes of humans, chimps, gorillas, and more distantly related primates such as orangutans and macaques, yielding roughly 20 million base pairs of aligned sequence. They focused not on the average level of genetic divergence between humans and their relatives, which can reveal approximately when each species emerged, but on genetic divergence across the human genome, to see when sequences diverged. The researchers unexpectedly found the youngest divergent regions between humans and chimps appear to be no more than 6.3 million years old and probably no more than 5.4 million years old. This suggests the emergence of humans was more recent than previously suggested by the roughly 6.5- to 7.4-million-year-old Toumaï fossil, humanity's oldest known ancestor. Moreover, the age of divergent regions varied from less than 84% to more than 147% of the average, a surprising range of more than 4 million years. Reich said he would expect a range of about 1 million years given what has been found in the typical ape. The researchers suggest an explanation for their findings could be that human and chimp speciation took place over a long time with episodes of hybridization between the emerging species after their initial split before they separated permanently. "Their explanation seems plausible," Mallet said. "It's not surprising per se on the basis of what we know about other animals, but if it happens with humans, we really start believing it." The conventional model of speciation focuses on allopatry, in which two populations diverge after a barrier isolates them and prevents gene flow, and any hybrids that result are less fit and do not contribute genetically in a meaningful way. "Hybridization could in fact be common during speciation, and could mix traits to create adaptations that help emerging species in new environments," coauthor David Reich at Harvard Medical School in Boston told The Scientist. The called it striking that human and chimp X chromosomes appear to have diverged far less than autosomes did, experiencing only 83% of the average divergence. Reich and his colleagues say interbreeding could account for this unusual reduction in divergence, as interbreeding would place strong selection pressure to eliminate traits that contribute to hybrid sterility and inviability, which are often X-linked. These findings could also help settle a controversy suggesting a higher mutation rate amongst males in primates than in other species, Reich said. Prior comparisons of sequence divergence rates in human and rat autosomes and X chromosomes suggested males had a mutation rate about twice that of females, but analyses of humans and chimps suggested a male mutation rate six to seven times higher. The low divergence Reich and his colleagues found between the human and chimp X chromosomes suggest they split very recently. When the investigators factored in the recentness of this split, they said the formerly anomalous human-chimp data matched human-rat findings. Completion of the gorilla and other primate genomes would enable future tests of the interbreeding hypothesis by distinguishing whether the evolution of certain genomic regions at different rates is due to hybridization or to other mechanisms, such as conserved functionally important regions or the presence of mutation hotspots, Derek Wildman at Wayne State University School of Medicine, who did not participate in the research, told The Scientist. Charles Q. Choi cchoi@the-scientist.com Links within this article I. Ganguli. "Chimp papers by the barrel." The Scientist, September 1, 2005. http://www.the-scientist.com/article/display/22758/ N. Patterson et al. "Genetic evidence for complex speciation of humans and chimpanzees," Nature, published online May 17, 2006. http://www.nature.com James Mallet http://www.the-scientist.com/article/display/14251/ M. Brunet et al. "New material of the earliest hominid from the Upper Miocene of Chad." Nature, April 7, 2005. PM_ID: 12110880 David Reich http://www.hms.harvard.edu/dms/bbs/fac/reichda.html C. Holding, "Chimps are not like humans," The Scientist, May 27, 2004. http://www.the-scientist.com/article/display/22203/
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