A genetic study of SIV - the Aids-like virus that infects monkeys - suggests that HIV - the virus that causes Aids in humans - came about through the combination of two viruses in chimpanzees.
By Dr David Whitehouse
BBC News Online science editor
Chimps could have been infected by other SIV-type viruses when they preyed on monkeys.
Chimps may harbour more dangerous viruses
The study confirms what has been established about the origin of Aids: it emerged from the forests of western Africa some time in the last century.
Humans caught it from chimpanzees when they ate them as food, or became exposed to their blood in rituals.
We know more about the origin of Aids than most people think. Genetic studies have shown conclusively that HIV is a variant of the Simian Immunodeficiency Virus (SIV) that is found in wild African monkeys and apes.
At some time in the recent past, SIV entered humans and mutated to become HIV. From this incident sprang the epidemic which has killed 20 million people and infected 15 million more.
Where this virus transfer took place is fairly well established: the Guinea-Bissau region of West Africa.
Many scientists believe the transfer occurred more than once because of the multiple strains of Aids that infect humans.
When it happened is more problematic. Significantly, millions of Africans were forcibly removed from their homes as slaves in the 19th Century, and none of them was infected. This suggests the origin of Aids is post-1860.
The first case of Aids reported in the US was in 1981, though it seems an African-American teenager died of it in St Louis in 1969, and that HIV was found in a blood plasma sample from a man living in Congo in 1959.
Studies of the rate of genetic divergence between the two major strains of Aids, HIV-1 and HIV-2, suggest the transfer into humans occurred about 1940, with an error of about 20 years.
Monkey viruses infected chimpanzees
There is also speculation that the increase in international air travel since the 1960s has helped to spread the infection.
The genetic study of SIV, which sheds more light on the trans-species crossover, is by an international team of scientists, and published in the journal Science.
The team suggests two viruses from different monkey species recombined in the chimpanzee to form the SIV strain that causes Aids in humans.
The authors present evidence that this strain, called SIVcpz, arose in the chimpanzee through successive infections by viruses from red-capped mangabeys and greater spot-nosed monkeys.
Chimpanzees prey on both of these species and their ranges overlap in west central Africa.
Elizabeth Bailes of the University of Nottingham, UK, and colleagues say a hybrid origin of SIV in chimpanzees has significant scientific and public health implications.
It provides evidence that humans are not the only ape species to acquire two different SIVs by cross-species transmission under natural conditions, most likely predation.
The researchers raise the possibility that the precursor virus to human Aids, SIVcpz, may itself interact with another virus in chimpanzees to form another Aids-like virus that could also infect humans.
They say: "It will be important to examine whether chimpanzee predation on smaller monkeys has led to additional SIV acquisitions, and possibly co-infection and recombination with SIVcpz, and whether the resulting chimpanzee-adapted SIVs are more likely to infect humans."