The BBC's Sue Nelson reports on the breakthrough

A chimpanzee named Marilyn enabled them to confirm that the Aids virus first passed into people from a particular sub-species of chimp in the Central African rainforest.

Infected chimps do not develop Aids and it may now be possible to learn why. This would greatly help efforts to prevent and treat the disease in humans.

Human infection occurred in the first half of the century as a result of people hunting and eating the chimps, the scientists believe. This practice continues today.

The team said that genetic tests show the main human virus, HIV-1, is closely related to a virus that infects chimps but does not make them sick.

Dr Beatrice Hahn of the University of Alabama and colleagues made the discovery when analysing frozen blood and tissue samples from a lab chimp named Marilyn which died in 1985. The genetic tests were not available at that time.

"She had never been used in Aids research and had not received human blood products after 1969," said Dr Hahn.

Dr. Sarah Nelson-Jones: "This is an exciting piece of research"

Her team, which will publish the results in the journal Nature, found in Marilyn a "grandparent" virus to the human one. Similar viruses had been seen before in four chimps but the latest research confirmed that only Marilyn's sub-species, Pan troglodytes troglodytes, carried the virus which infects humans.

In the early days of Aids research, dozens of chimps were deliberately infected with the HIV virus to see if they developed the disease. To the surprise of scientists, they did not, despite sharing 98% of their genetic material with humans.

HIV found in this species of chimp

Now that the specific sub-species of chimp has been identified, the scientists behind the latest study believe new treatments could be developed by studying why the chimps do not get Aids.

"This is an important finding with significant potential," said Dr Anthony Fauci of the US National Institute of Allergies and Infectious Diseases (NIAID), which helped fund the study.

"This may allow us to study infected chimpanzees in the wild to find out why these animals don't get sick. This information that may help us better protect humans from developing Aids."

It is not yet known whether the chimps do not develop Aids because of their genetic differences with humans or because their immune systems deal better with the virus. The answer will determine whether studying the chimps could lead to better treatments or better vaccines for Aids.

Paul Sharpe: "There's some way to go yet"

Professor Paul Sharp at Nottingham University is one of the team members and told the BBC: "We now need to find out how common this virus is in the chimpanzees in the wild, but that could be difficult as the species is endangered."

The sub-species population is estimated at 80,000 but the annual number killed is thought to be thousands, putting these chimps on the "brink of extinction".

There is evidence that HIV may have transferred to humans throughout history, but only became an epidemic in the 20th century. The reasons for this are increased sexual promiscuity, civil unrest and movement of people to cities, according to Dr Hahn.