"We think it's likely that the cross-species transmission took place locally" in the Cameroon region, Hahn said.
Hunters in the region who caught and ate chimps were probably the first to contract HIV, she adds. (See a photo of a Cameroonian family preparing a meal of bush meat.)
"Based on what we know about the biology of these viruses, you need to be exposed to infectious blood or body fluids," she said. "You don't get it by petting a chimp."
From southeastern Cameroon the virus appears then to have spread south via the Sangha and Congo Rivers.
"Eventually the virus ended up in a major metropolitan area, which would either be Kinshasa [Democratic Republic of the Congo] or Brazzaville [Republic of the Congo]," Hahn added. "That's where we believe the AIDS pandemic really started."
Chimps in turn are thought to have picked up SIV by eating infected monkeys.
A study published in 2003 in Science suggested a chimp strain of SIV arose through repeated transmissions and recombination of similar viruses found in red-capped mangabeys and greater spot-nosed monkeys. (Read a story about this study.)
This previous research was led by Paul Sharp of the Institute of Genetics at the University of Nottingham in England.
"We now know there are more than 30 species of monkeys across Africa which have their own forms of SIVs," said Sharp, who was also involved with the new study.
Chimps, however, are the only apes known to be infected.
"The chimp virus is a mosaic of viruses that infect prey monkeys," the University of Alabama's Hahn said. "Chimps had to eat these monkeys first but they have certainly had their infection a lot longer than humans."
She says the diversity of new SIV strains detected in Cameroon chimps raises the possibility that the animals could pass on new types of HIV infection to humans.
This, says Hahn, could hinder the global fight against AIDS.
"Cross-species transmissions could have already occurred and may have gone unrecognized," she added.
Hahn says it's unlikely that further, recently acquired chimp infections have led to AIDS epidemics in humans. But such transmissions could undermine the effectiveness of AIDS vaccines currently under development.
"If we ever had a vaccine that worked and you then bring in a new virus that's 30 or 40 percent different, that's probably not a good thing," she said.
"It might be much more difficult to detect one of these other chimp viruses if it jumped into humans," the University of Nottingham's Sharp added.
On the other hand, further studies of wild chimp populations could help in finding new treatments for AIDS sufferers, Sharp says.
"As far as we know, the chimps don't get any type of AIDS-like disease," he said. "That's part of the reason for being very interested in the infection of chimps, because chimps and humans are very similar genetically."
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