Gorilla SIV distribution

Nature has a little article this week by Fran Van Heuverswyn et al. announcing that SIV (the primate relative of HIV) has been found in wild populations of Western lowland gorillas.

The finding of distinct but related SIVgor strains in gorillas living nearly 400 km apart suggests that, as in chimpanzees, SIV infection is endemic in gorillas. An alternative explanation could be that gorillas acquire SIV sporadically from chimpanzees, but this seems unlikely as no chimpanzee community surveyed so far, including several from habitats that overlap with those of the SIVgor-positive gorillas, harbour group O-like viruses (see supplementary information). The phylogenetic relationships shown in Fig. 1b argue that chimpanzees were the original reservoir of SIVs now found in chimpanzees, gorillas and humans; that distinct chimpanzee communities in southern Cameroon transmitted divergent SIVcpz to humans, giving rise to HIV-1 groups M and N1; and that chimpanzees transmitted HIV-1 group O-like viruses either to gorillas and humans independently, or to gorillas that then transmitted the virus to humans (Van Heurverswyn et al. 2006:164).

OK, here's what I want to know: If the virus originated in chimpanzees, I can understand that it might have moved to humans by bushmeat consumption. But how did it move to gorillas?

The article suggests it may have crossed from humans to gorillas through hunting, and the gorilla lineages do cluster phylogenetically with some human HIV lineages, so it's not impossible. But it's hard to figure just how this is supposed to work in practice. Well, it's a stumper!

References:

Van Heuverswyn F and 15 others. 2006. Human immunodeficiency viruses: SIV infection in wild gorillas. Nature 444:164. DOI link