The ancestry of HIV-1 (human immunodeficiency virus–1) has been traced to SIVcpz (simian immunodeficiency virus) infecting chim-panzees (Pan troglodytes)] in west central Africa (1), but the origin of SIVcpz itself remains unknown. Species-specific strains of SIV have been identified in more than 20 species of African primates (2), but all, except SIVcpz, infect monkeys. Here, we present evidence that SIVcpz arose through successive cross-species transmission and recombination events of SIVs infecting monkeys on which chimpanzees prey.