Researchers investigating the origins of the AIDS virus have traced it to a similar virus--simian immunodeficiency virus (SIV)--that jumped from chimpanzees to humans. But how chimpanzees first acquired SIV remains unclear. New findings published today in the journal Science indicate that HIV's predecessor arose as a combination of two monkey viruses about a million years ago.

Paul Sharp of the University of Nottingham and his colleagues studied the evolutionary history of a variety of SIV strains. The researchers found that the family trees of the chimpanzee virus (SIVcpz) constructed using different parts of its genome varied significantly, suggesting it arose from the combination of multiple viruses. Many monkey species are naturally infected with different types of SIV, and the team determined that SIVcpz resulted from mixing of the varieties infecting red-capped mangabeys [SIVrcm] and greater spot-nosed monkeys [SIVgsn].

Because chimpanzees hunt smaller monkey species, the authors suggest that the animals became infected with both monkey viruses through hunting. Once inside a host animal the two viruses swapped genetic material, resulting in a new form of SIV that eventually crossed into humans. The scientists conclude that "it will be important to examine whether chimpanzee predation on monkeys has led to other SIV acquistions and whether the resulting chimpanzee-adapted SIVs are more likely to infect humans."