FIG. 10.

Possible scenarios for the origin of HIV-1 group O. (A) Chimpanzees harboring SIVgor-like viruses infected gorillas and humans independently. (B) Chimpanzees harboring SIVgor-like viruses infected gorillas, which then passed the virus on to humans. (C) Chimpanzees harboring SIVgor-like viruses infected humans, who then passed the virus to gorillas. Support for each scenario would come from the discovery of one of the hypothetical viruses shown in green. In all three trees, nodes marked by circles indicate ancestral viruses that are assumed to have infected chimpanzees.