The origin of the virus HIV-1, the main cause of the worldwide AIDS epidemic, has been a matter of debate ever since its discovery in 1983. Its origin in wild primates-the apes and monkeys-has long been accepted, but the question remained-which ones, and how could it have been transmitted? The first question is answered by a report in Nature: Beatrice H. Hahn of the University of Alabama at Birmingham, Alabama, USA, and colleagues [from the USA, France and the UK] provide the most persuasive evidence yet that HIV-1 entered the human population from the chimpanzee, Pan troglodytes, our nearest living relative.

From their analysis, it is now almost certain that HIV-1 entered the human population from chimpanzees of the subspecies Pan troglodytes troglodytes, which lives in equatorial west Africa, in Cameroon, Gabon and Equatorial Guinea, where the HIV-1 virus is thought, on other evidence, to have emerged in humans.

Chimpanzees, like many other primates, harbour simian immunodeficiency viruses (SIVs). These closely resemble HIV but appear to co-exist far more successfully with their animal hosts, pointing to an ancient association between virus and host. These SIVs are the most likely candidates for the virus ?ancestors? of human HIVs. A source for the less common human immunodeficiency virus, HIV-2, in SIVs fdrom the sooty mangabey monkey (Cercocebus atys) is already well established.

Of all primate SIVs, chimpanzee SIVs most closely resemble HIV-1, and they had always been a possible candidate for its source. But until now, only three chimpanzee SIV strains had ever been identified, in a mere handful of animals, and the question of origin could not be resolved. This was partly because of the very low prevalence of SIV in wild chimpanzees, the wide genetic divergence of one of the chimpanzee SIVs from the others, and the presence of SIV-infected chimpanzees in parts of Africa outside the presumed regions where AIDS originated.

One new clue was provided by Hahn and colleague?s analysis of a completely new SIV strain, SIVcpz-US. DNA corresponding to this strain was isolated from the stored blood and lymph nodes of a wild-caught female chimpanzee who had been brought to a primate-breeding centre in the United States in the early 1960s and tested positive for SIVs in 1985, just before her death. This new strain clustered closely with the other HIV-1-like SIV strains, adding weight to the evidence for a chimpanzee origin of HIV-1.

But Hahn and colleagues took their analysis further, and this is what makes their conclusions so convincing. Molecular biologists had hitherto concentrated on the differences between the SIV viruses themselves - Hahn and colleages thought also to look at differences between their chimpanzee hosts. They decided to determine the distribution of the known SIV strains in the four subspecies of Pan troglodytes, by genetic typing the mitochondrial DNA from infected animals.

And the results proved immensely rewarding. There are four subspecies of chimpanzee in Africa, which have non-overlapping geographical ranges, but all known SIV-infected animals belong to just two of these subspecies. And, most tellingly, the three SIV strains that most closely resemble each other and HIV-1 all come from animals identified as the subspecies Pan troglodytes troglodytes, whose geographical range in equatorial west Africa is consistent with the region where AIDS is thought, on other grounds, to have emerged. The one anomalous SIVcpz, SIVcpzANT, proved to come from a different subspecies, Pan troglodytes schweinfurthii, with a different distribution, further to the east, and has probably played no part in the making of HIV-1.

Three types of HIV-1 are found in humans, the strains M (the most common), O and N (which is known from just two patients in Cameroon). It is likely that each of these represents an independent transmission from chimpanzees to humans.

Commenting on the work in an accompanying article in Nature, virologist Robin Weiss from the Windeyer Institute, University College London, and anthropologist Richard Wrangham from Harvard University, Cambridge, Massachusetts, point out its implications for chimpanzee conservation and welfare. The greatest threat to the wild chimpanzee population in Africa is habitat destruction and hunting for food-bushmeat to feed logging camps and supply city restaurants. It is estimated that thousands of chimpanzees are killed each year, out of a population that can number at most a few hundred thousand. If the killing continues at the present rate, the chimpanzee is certainly doomed to extinction.

Confirmation of the chimpanzee as a source of precursor HIV viruses could provide conservationists with a powerful weapon to stop the killing. Virus transmission from an infected carcass during its butchery and preparation is an entirely plausible route of transfer of the virus into humans. But conservationists are in a dilemma. Would a campaign publicizing the dangers of killing apes for food prove the chimpanzees? salvation, or might it backfire into wholesale slaughter?