It is clear that the several HIV-1 and HIV-2 strains descend from SIVcpz, SIVgor, and SIVsmm viruses,[2][5][6][7][9][18] and that bushmeat practice provides the most plausible venue for cross-species transfer to humans.[7][9][22] However, some loose ends remain unresolved.
It is not yet explained why only four HIV groups (HIV-1 groups M and O, and HIV-2 groups A and B) spread considerably in human populations, despite bushmeat practices being very widespread in Central and West Africa,[10] and the resulting human SIV infections being common.[22]
It remains also unexplained why all epidemic HIV groups emerged in humans nearly simultaneously, and only in the 20th century, despite very old human exposure to SIV (a recent phylogenetic study demonstrated that SIV is at least tens of thousands of years old).[23]
The discovery of the main HIV / SIV phylogenetic relationships permits explaining broadly HIV biogeography: the early centers of the HIV-1 groups were in Central Africa, where the primate reservoirs of the related SIVcpz and SIVgor viruses (chimpanzees and gorillas) exist; similarly, the HIV-2 groups had their centers in West Africa, where sooty mangabeys, which harbor the related SIVsmm virus, exist. However these relationships do not explain more detailed patterns of biogeography, such as why epidemic HIV-2 groups (A and B) only evolved in the Ivory Coast, which is only one of six countries harboring the sooty mangabey. It is also unclear why the SIVcpz endemic in the chimpanzee subspecies Pan troglodytes schweinfurthii (inhabiting the Democratic Republic of Congo, Central African Republic, Rwanda, Burundi, Uganda, and Tanzania) did not spawn an epidemic HIV-1 strain to humans, while the Democratic Republic of Congo was the main center of HIV-1 group M, a virus descended from SIVcpz strains of a subspecies (Pan troglodytes troglodytes) that does not exist in this country.