Scientists have long known that captive chimps carry their own version of the AIDS virus, called SIV, or simian immunodeficiency virus. But it was extraordinarily hard to find in wild chimpanzees, complicating efforts to pin down just how the virus could have made the jump from animal to man.
Fitting that final piece of the puzzle required seven years of research just to develop tests to genetically trace the virus in living wild chimps without hurting the endangered species. Then trackers had to plunge through the dense forests of West Africa and scrape up fresh ape feces, more than 1,300 samples in all.
Until now, "no one was able to look. No one had the tools," said Dr. Beatrice Hahn of the University of Alabama at Birmingham. She led the team of international researchers that reported the success in Friday's edition of the journal Science.
"We're 25 years into this pandemic," Hahn said. "We don't have a cure. We don't have a vaccine. But we know where it came from. At least we can make a check mark on one of those."
Hahn's team tested chimp feces for SIV antibodies, finding them in a subspecies called Pan troglodytes troglodytes in southern Cameroon.
Chimps tend to form geographically distinct communities. By genetically analyzing the feces, researchers could trace individual infected chimps. The team found some chimp communities with infection rates as high as 35 percent, while others had no infection at all.
Every infected chimp had a common base genetic pattern that indicated a common ancestor, Hahn said.
There are three types of HIV-1, and the "M" strain is responsible for most of the worldwide epidemic. Genetic analysis let Hahn identify chimp communities south of Cameroon's Sanaga River, especially in the country's southeast corner, whose viral strains are most closely related to that most common HIV-1 subtype.
"The genetic similarity was striking," Hahn said.
So how did the jump happen?
The first human known to be infected with HIV was a man from Kinshasa in the nearby country of Congo who had his blood stored in 1959 as part of a medical study, decades before scientists knew the AIDS virus existed.
Researchers believe someone in rural Cameroon was bitten by a SIV infected chimp - or was cut while butchering one - became infected with the ape virus, and then passed it on to someone else.
Waterways were the highways for transporting hardwood, ivory and other items to urban areas. Infected people probably traveled via a second Cameroon river, the Sangha, south to the Congo River and on to Kinshasa.
"How many different transmission events occurred between that initial hunter and this virus making it to Kinshasa, I don't know. It could have been one, it could have been 10, it could have been 100," Hahn said. "Eventually, it ended up in an urban area, and that's where it really got going."
The chimps originally acquired SIV, by the way, from similar viruses infecting monkeys in west-central Africa.
Somewhere in all that spread, the virus became more deadly to people than it is to chimps, who seldom are bothered much by SIV.
The research seems to settle any question of HIV's origin, said Dr. Anthony Fauci, the National Institutes of Health's AIDS chief.
When tracing a virus' evolution, "it's important to get as close to the source as you can," he said. "It's of historic interest."