It is thought that humans acquired SIVcpz from chimpanzees in Cameroon. But where did chimpanzees get the virus? Perhaps is is endemic to this species of primates; however, a number of strains of SIV that are species-specific are found in west Africa. All infect monkeys except SIVcpz. In 2003, Bailes and colleagues* suggested that the chimpanzee form of SIV arose as a result of recombination of monkey strains of SIV acquired by chimpanzees who prey on those monkeys. Since not all subspecies of chimpanzee harbor SIVcpz, it is likely that chimpanzees acquired SIV after the subspecies split apart, that is relatively recently. There is evidence that SIV has been subject to recombination events among different virus types and SIVcpz is most closely related to SIV from red-capped mangabeys (Cercocebus torquatus) and from greater spot-nosed monkeys (Cercopithecus nictitans). It appears that that SIVcpz is a hybrid of these monkey virus types.

It appears that the two monkey viruses were acquired by chimpanzees by inter-species transmission and when present simultaneously in the same chimpanzee, they recombined to a form that was not only infectious for chimpanzees but was capable of spreading to humans. Probably the recombined virus was originally pathogenic for chimpanzees as HIV is pathogenic for humans and the chimpanzees that are present today are those that were selected for resistance to the recombinant virus.

* Hybrid origin of SIV in chimpanzees. Bailes E, Gao F, Bibollet-Ruche F, Courgnaud V, Peeters M, Marx PA, Hahn BH, Sharp PM. Science. 2003 Jun 13;300(5626):1713.