ses (SIVs) are rare, and only three cases had previously been identified. The recent discovery of a fourth case by Dr. Hahn=s team, together with a detailed investigation of where each of the apes came from, cleared up some of the details. The genetic material of the four chimpanzee SIVs suggests that three of them, including the new one, are closely related. The fourth belongs to a different branch of the SIV family tree.
By analyzing genes from the chimpanzees, Dr. Hahn and her colleagues have shown that three related SIVs come from a sub-species of chimpanzee that lives in Gabon, Cameroon, and Equatorial Guinea. The fourth virus was found in a second sub-species that lives farther to the east. This species lives in the same region where AIDS was first found in humans. In this region, these animals have long been hunted for food, so blood from the carcasses could easily have entered the hunters= bodies through superficial wounds.
Hahn=s group showed how, after jumping species on at least three occasions, chimpanzee SIV evolved into the three families of HIV-1 strains recognized today. The part of Africa where the related SIVs are found is the only place where all three forms of the basic HIV-1 virus, known as M, N, and O, are found. Only type M has spread from its homeland. The other two are confined locally.
Analysis of the part of the SIV family tree containing the three related SIVs and the three basic types of HIV-1 shows that all six are closely related, and that the three types of HIV-1 seem to have had different origins within the western chimpanzee SIV group. This suggests that the M, N, and O strains each made an independent leap from chimpanzee to man.
The chimpanzees also carry their version of the virus but it does not make them sick, a fact which may reveal important clues about controlling AIDS in humans, since humans share 98.5 percent of their genes with chimpanzees. A problem which may ham...