The ancestry of the virus that caused the AIDS epidemic has been traced to two strains of virus found in monkeys in Africa.
The viruses probably passed into chimpanzees when the apes ate infected monkey meat, researchers say.
Earlier studies have shown that HIV1, the virus that causes the most common form of human AIDS, originated from a simian immunodeficiency virus, or SIV, that is found in chimpanzees. But how chimps came to have SIV has been a mystery.
American and English researchers analysed the genetic pattern of a number of SIV strains in African monkeys and concluded that at least two strains found in the red-capped mangabeys and in the greater spot-nosed monkeys in south-central Africa combined to form the type of SIV now found in African chimps.
It was this form of SIV that spread into the human population to start the HIV1 epidemic that has killed millions of people, researchers report today in the journal Science.
"The recombination of these monkey viruses happened in chimpanzees and the chimp transmitted it to humans on at least three occasions," said Frederic Bibollet-Ruche, a virologist at the University of Alabama, Birmingham, and a co-author of the study. "The transfer between chimps and humans probably happened before 1930."
Bibollet-Ruche said that three types of HIV1, called M, N and O, probably were transmitted from chimps to humans decades ago. A second type of AIDs, called HIV2, is known to have been transmitted from the sooty mangabey in West Africa to humans directly, with out going through the chimp.