Dutch Researchers Theorize Chimpanzees May Have Survived AIDS Epidemic Two Million Years Ago

Dutch researchers theorize that an AIDS-like epidemic wiped out huge numbers of chimpanzees 2 million years ago, leaving modern chimps with resistance to HIV and its variants. If true, the hypothesis would explain why chimps, which share more than 98 percent of their DNA with humans, do not develop AIDS. The findings will be published in the coming issue of the Proceedings of the National Academy of the Sciences and are already online ("Evidence for an Ancient Selective Sweep in the MHC Class I Gene Repertoire of Chimpanzees," 10.1073/pnas.182420799).

The theory stems from a study of 35 chimps conducted by the Biomedical Primate Research Center in the Netherlands. The study chimps shared an unusually uniform cluster of genes in the area that controls immune system defenses against disease. "Chimps show more genetic variation than humans in all areas -- with this one exception, which is seriously condensed," said Dr. Ronald Bontrop, leader of the Dutch team, which worked with University of California statisticians.

The chimps' lack of diversity in genes related to the immune system suggests that a lethal sickness attacked chimps in the distant past. This unknown disease would have wiped out all or almost all the chimps lacking the immune system genes to fend it off, leaving survivors with a uniform set. This, combined with the knowledge that modern chimps are largely immune to HIV and its simian variants, pointed toward an AIDS-like disease as the culprit. Scientists believe that HIV originated in apes or monkeys and was transferred or mutated its way into the human population about 50 years ago.

The Dutch researchers' theory notes that chimps split into four subspecies around 1.5 million years ago. Since all subspecies represented in the study share the same genetic reduction, the researchers estimated the epidemic happened before the split. Other studies have suggested that bonobo apes also have a similar genetic reduction, pushing the time of the epidemic back to 2 million years ago, when bonobos and chimps shared a common ancestor.

Bontrop said that chimpanzee immune systems appear to defeat HIV by targeting part of the virus' proteins that do not mutate. A similar defense mechanism may be at work in humans who are repeatedly exposed to HIV but do not get sick, suggesting an area for further study.

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