The origin of the virus that causes Aids has been traced to chimpanzees living in the forests of southern Cameroon.

The closest known cousin of the Human Immunodeficiency Virus (HIV) has been identified among chimps living south of the West African country’s Sanaga River, allowing scientists to pinpoint where the germ jumped the species barrier to people.

The findings by an international research team fill a "missing link" in the evolution of HIV, the cause of the world’s most deadly infectious disease with an annual death toll of more than three million.

It bolsters the standard theory that Aids began when an ape version of HIV crossed into humans, probably first infecting a bushmeat hunter, and demolishes more sensational alternative explanations.

Some conspiracy theorists have suggested that HIV could have escaped from a bioweapons laboratory, while another controversial hypothesis advanced by the journalist Edward Hooper holds that the epidemic began with a batch of contaminated polio vaccine in what is now the Democratic Republic of Congo.

Neither possibility fits with the new evidence, which is published tomorrow in the journal Science.

Paul Sharp, Professor of Genetics at the University of Nottingham, a member of the research group, said the work indicates that the HIV-1 virus that causes Aids almost certainly arose in south-east Cameroon, in the early part of the 20th century.

"Particularly when you consider that HIV-1 probably originated more than 75 years ago, it is most unlikely that there are any viruses out there that will prove to be more closely related to the human virus," he said. "Thus, the initial jump of a virus from a chimpanzee to a human probably occurred in that region."

The ape version of HIV, known as Simian Immunodeficiency Virus (SIV), has long been considered the source of the human virus, and chimps are by far the most likely species to have conveyed it to people.

Only a few captive animals, however, have been identified as carrying chimp SIV (SIVcpz), leaving the wild reservoir of the virus that first infected humans unknown. The possibility remained that another type of ape might have passed it on, and the location of the crossover was still uncertain.

In the new research, trackers combed the forest floor at 10 sites in the jungles of Cameroon for chimp faeces, samples of which were then sent for genetic analysis. This detected SIVcpz in up to 35 per cent of chimpanzees in some populations.

An evolutionary analysis of the genetic sequence of these chimp viruses has now shown that it is very closely related to HIV-1. Different chimp communities had slightly different genetic variants of SIVcpz, with those that are closest to the human virus found in south east Cameroon.

As well as solving the mystery about the origin of the virus, the new findings pave the way for future work exploring the natural history and behaviour of the simian form of HIV in its natural host.

SIVcpz does not cause an Aids-like illness among chimpanzees, despite its similarity to the human virus and the very close genetic relationship between chimps and humans. The new findings should assist research into why this is so, which could ultimately help scientists to understand the workings of HIV.

"We’re currently working to understand which genetic differences between SIVcpz and HIV-1 evolved as a response to the species jump," Professor Sharp said.

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