Map of the African continent, highlighting habitats of the four different chimpanzee subspecies. Populations that show evidence of contemporary natural infections with SIVcpz strains are Pan troglodytes troglodytes (light orange) and P.t. schweinfurthii (dark orange). Superimposed is a diagram illustrating the cross-species transmission events that led to the emergence of HIV-1 group M, the initiator of the human pandemic . The lower panel in the figure illustrates the speciation events in the homo-pan lineage. The arrow indicates the putative time span of the MHC repertoire reduction in the pan lineage.