New research suggests that chimpanzees and human beings each acquired their versions of the AIDS virus the same way -- by killing and butchering other primates infected with similar microbes.

The prevailing theory of how the first human got AIDS is that about 75 years ago, someone in central Africa cut himself while butchering a chimpanzee for food. The animal was infected with simian immunodeficiency virus (SIV), which after a few adaptive mutations evolved into human immunodeficiency virus (HIV) in its new host. This is known in AIDS research circles as the "cut-hunter hypothesis."

A team of European and American scientists offered in today's issue of Science what might be called the "cut-chimpanzee hypothesis" of how that species acquired SIV, the genetic "father" of HIV. Sometime deep in prehistory, they say, a chimpanzee killed and consumed two monkeys -- a red-capped mangabey and a greater spot-nosed monkey -- each infected with its own particular strain of SIV. Both viruses got into the chimp's bloodstream, probably through an open wound.

This scenario is based on the researchers' discovery that the virus chimpanzees now carry is a combination of the mangabey and spot-nosed monkey strains of SIV. For the two strains to mix and fuse, a chimp would have to be simultaneously infected by both -- and that is likely to happen only through the bloody activity of predation.

The research and its conclusions "emphasize the parallels with the origin of HIV infection in humans," said Paul M. Sharp, an evolutionary geneticist at the University of Nottingham, in Britain.

About 30 different African primates harbor SIV, often with no ill effects, and each species's strain is different.

Genetic analysis showed that chimpanzee SIV -- the closest relative to the human virus -- had some genes that are similar to the red-capped mangabey's SIV and others that are like the greater spot-nosed monkey's SIV.

Sharp and his collaborators in France and the United States attempted to nail down the relationship between the strains. They looked at four genes from eight different species of primate, and constructed dozens of hypothetical family trees for the virus strains. The ones that made the most sense had the chimpanzee SIV as the product of the fusion of virus strains from red-capped mangabeys and greater spot-nosed monkeys.

This made geographical sense. One of the three subspecies of chimpanzees that carry SIV occupy the same region of west central Africa near Cameroon and Gabon as the two other animals. And although they were once thought to be gentle vegetarians, it is now known that chimps hunt the monkeys for food.

The hybrid scenario requires that at some point an individual chimp was infected with SIV from both species, and that the viruses underwent a "recombination event," which mixed their genes inside a cell.

The new research adds to the AIDS virus's biography by filling in the story of its ancestors. Sharp says there is also a lesson of more immediate importance.

It is remotely possible that chimpanzee populations -- which are rare, scattered and hard to study -- harbor other SIVs capable of infecting human beings. He and other researchers are looking for them. And they still might acquire other similar viruses in the future.

"It could happen tomorrow," Sharp said.

Human beings appear to have gotten the AIDS virus from African primates twice. Human immunodeficiency virus 1 (HIV-1) -- which accounts for virtually all the cases of AIDS around the world -- came from chimpanzees. HIV-2, a slightly different strain that is the cause of a very small number of AIDS cases in west Africa, came from sooty mangabeys.