Scientists have rejected claims that a contaminated vaccine triggered the HIV pandemic in humans.
Scientists believe HIV originated in chimpanzees
According to one theory, HIV jumped to humans after polio vaccine became infected with a similar virus found in chimpanzees in the Congo.
However, a team of international scientists has now carried out genetic tests on chimps in this part of Africa.
Writing in Nature, they said there was no evidence to suggest HIV came from these animals.
The vaccine theory was first mooted in the early 1990s. It hit the headlines again in 1999 after a book set out the case for.
It claimed that in the late 1950s, 400 chimpanzees were captured, killed and used to make one of the first oral polio vaccines in the world.
It suggested scientists used cells from the kidneys of chimpanzees with the SIV virus to develop the vaccine. Some chimpanzees host the SIV virus which is regarded as being similar to HIV.
The polio vaccine was given to at least one million people in what is now the Democratic Republic of Congo (DRC), Rwanda and Burundi.
The site of the 28 vaccination projects correlate closely with the earliest cases of HIV infection.
Dr Michael Worobey from the University of Arizona led a team of international experts
to the DRC to find out if there was any evidence to support the theory.
Their tests showed that SIV is endemic in wild chimpanzees in the country. However, they also found that the virus is very different to all strains of HIV-1, which has infected humans.
The researchers said their findings were "clear-cut" and should lay the vaccine theory to rest.
"HIV-1 did not originate from polio vaccines that were tested in that area in the 1950s," said Dr Worobey.
He said further research is needed to find out how HIV jumped to humans.
"We know HIV-1 comes from chimpanzees but we know little beyond this," he said.
Bush meat link
Paul Sharp, professor of genetics at the University of Nottingham and one of those involved in this latest study, said the findings refuted the vaccine theory.
He said there was now clear evidence to show that while HIV did jump from chimpanzees to humans, it happened well before the 1950s vaccine work.
"We and others have estimated that it happened in the 1930s," he told BBC News Online.
"As to how it happened, we think it may have been as a result of bush meat hunting.
"People may have been butchering chimp carcasses and spilling blood all over the place, which is how they became infected."