Simian immunodeficiency viruses (SIVs) are primate lentiviruses that infect no fewer than 36 different nonhuman primate species in sub-Saharan Africa (4, 9, 62). Two of these viruses, SIVcpz from chimpanzees (Pan troglodytes) and SIVsmm from sooty mangabeys (Cercocebus atys), have crossed species barriers on multiple occasions and have generated human immunodeficiency virus (HIV) types 1 and 2 (20, 24, 25, 94). AIDS, one of the most devastating infectious diseases to have emerged in recent history, is thus the result of cross-species infections of humans by lentiviruses of primate origin (33).

Identification of the primate source that spawned the HIV-1 pandemic is of scientific and public health importance. To this end, it is now well established that the immediate precursor of HIV-1 is a lentivirus that infects chimpanzees of the subspecies Pan troglodytes troglodytes in west central Africa (17, 24). What is also known but perhaps less widely appreciated is that SIVcpz strains have been transmitted to humans on at least three independent occasions and that the current HIV-1 group M pandemic, which has affficted more than 60 million people and caused more than 20 million deaths, resulted from just one of these transmission events in the first half of the twentieth century (42, 77, 97). The reasons for HIV-1’s sudden emergence, the adaptive changes that followed, and the mechanisms underlying its unique pathogenicity in humans have not yet been determined.