The same researchers who argued that humans probably first got HIV from butchering chimpanzees for food have traced the origin of the virus back one step further: to the monkeys that the chimpanzees ate. They believe the simian precursor to HIV was created in chimps that ate two kinds of monkeys with different but related viruses, red-capped mangabeys and spot-nosed guenons.

Scientists made the deduction by sequencing the genes of the simian immunodeficiency viruses in chimpanzees and 30 monkey species, and then compiling "family trees" to see which were most closely related. They believe two monkey viruses were involved because the virus from spot-nosed guenons was closest in the part of the genome that contains the code for the virus's protein envelope, while the virus from the mangabey was closest in a different segment.

There is no way to know when the two viruses merged inside one chimpanzee. Chimp virus has been found in two subspecies from central Africa, but not yet in the westernmost subspecies, nor in a related species found south of the Congo River. The virus's failure to spread to all chimpanzees before they diversified into subspecies suggests that it is relatively new, researchers said. Subspecies have been separated for eons by large rivers like the Congo and the Ubangi, since chimps cannot cross water.

The new conclusion is important, said Dr. Beatrice Hahn, a University of Alabama-Birmingham virologist and one of the authors, "because it shows that chimpanzees acquired their virus exactly the same way humans did, by hunting bushmeat." Neither chimps nor monkeys get sick from it.

Unlike the other great apes, chimpanzees are formidable hunters, often working together. Hunting males tear their catches limb from limb and eat them on the spot, share their carcasses or trade them to females for sex, so blood-to-blood contact from "open cuts or chomping on bones" is easy to imagine, one researcher said.

It is still unclear exactly how chimpanzees infect each other and why the disease is not more rampant among them, since they have many sexual partners and fight frequently, often biting each other, which in rare human cases has passed the virus. Nursing is surely one route, Hahn said, because some chimps captured in infancy are infected. The full study, "Hybrid Origin of SIV in Chimpanzees," appears in the Friday issue of Science (2003;300(5626):1713).

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