According to the findings, which are to be unveiled this week, it has only now created a global killer epidemic because of the huge social and political upheaval in Africa in the 20th century, particularly after the Second World War.
Scientists believe that the discovery, which comes after a 20-year hunt, raises the prospect of studying wild chimpanzees to ascertain why they appear to be immune to the virus. Such research could lead to a vaccine against the human disease.
A chimpanzee named Marilyn has helped to confirm how the virus passed into people. Frozen blood samples from the animal have been analysed, proving that wild chimpanzees living in Africa are the natural "reservoir" of the human disease, which now affects 30 million people worldwide.
In a study to be published this week in Nature, the scientists argue that the transmission of chimp viruses into humans occurred relatively recently - perhaps not more than 70 years ago.
Blood samples were taken from Marilyn just before she died in 1985 after spending much of her 26 years in captivity. They reveal that she was infected with a virus that is a genetic ancestor to HIV-1, the principal Aids virus.
Feng Gao, professor of medicine at Alabama University, and a leading member of the international team of scientists who made the discovery, said: "We have long suspected a virus from African primates to be the cause of human Aids, but exactly which animal species was unknown."
Marilyn, who belonged to a subspecies of chimpanzee called Pan troglodytes troglodytes, was caught in Gabon, west-central Africa, before being used in medical research at a US Air Force base in New Mexico. Scientists believe she became infected in Africa as she had not been injected with human blood products since 1969, before the HIV epidemic reached the West.
Although Aids-like viruses have been found in three other chimps, it was a genetic analysis of Marilyn's virus that finally provided the clue the scientists needed to conclude that chimps were the natural hosts of HIV.
Paul Sharp, professor of genetics at Nottingham University and a member of the research team, said that although he has harboured many doubts over what could have caused Aids, the latest study on Marilyn has clarified the issue. "I'm really at the stage where I'm no longer hedging my bets. I'm pretty convinced by what we have found," he said.
Beatrice Hahn, a leading Aids scientist at Alabama, said close contact between people in Africa and wild chimps almost certainly led to HIV crossing the "species barrier" from monkey to man. The prime culprit for transmission to man is the bushmeat trade.
"Chimpanzees are frequently hunted for food, especially in west-central Africa, and we believe that HIV-1 was introduced into the human population through exposure to blood during hunting," Professor Hahn said.
Although African monkeys have long been suspected of being the natural reservoir of the human Aids virus, scientists had not previously been able to find the evidence they needed to enable them to make a positive identification of the "guilty" species.
A comparison of Marilyn's virus with other monkey viruses shows that chimps have been infected for many tens of thousands of years. Although it is likely that the species barrier has been broken many times, only in this century did it result in a widespread human epidemic.
"Increasing urbanisation, breakdown of traditional lifestyles, population movements, civil unrest and sexual promiscuity are all known to increase the rates of sexually transmitted diseases, and thus likely triggered the Aids pandemic," Professor Hahn said.
Simon Wain-Hobson, a world authority on Aids viruses at the Pasteur Institute, in Paris, who analysed the first chimpanzee SIV [simian immune-deficiency virus] to be discovered, said the latest research answers many questions.
"This is the most complete description of HIV's origin to date and adds another piece to the unfolding jigsaw, but a number of pieces to the puzzle still remain to be solved," he said.Reuse content