The disturbing and fiercely disputed theory that the Aids virus was unleashed accidentally by western medical researchers in Africa in the 1950s gained new impetus yesterday, with the emergence of fresh witness testimony about the use of chimpanzees in an African mass vaccination trial.
Ed Hooper, a writer who has spent a decade seeking the source of the epidemic, told an international conference on the origins of Aids in London that the new evidence was the "smoking gun" in support of his claim that Aids crossed from chimps to humans when doctors used their kidneys to prepare an experimental oral polio vaccine (OPV).
But Mr Hooper's opponents were also triumphant yesterday, pointing to results from tests on an archived sample of the vaccine which showed that it contained no trace of DNA from either a chimp or an HIV-type virus.
Surviving members of the US-Belgian research team involved in the trials deny that they used chimps to make the vaccine, although they have given contradictory accounts of what happened in the past.
One of them, Stanley Plotkin, said: "There is no gun, there is no bullet, there is no shooter. There's only smoke created by Mr Hooper."
The now elderly researchers and other opponents of Mr Hooper are attending the two-day conference, in what amounts to a showdown with enormous repercussions.
If the international scientific community ever accepted the OPV theory of the origins of Aids, there would be powerful legal, moral and political consequences for relations between Africa and the west.
There are 34m HIV-infected people in the world today. Almost 20m people have already died, 15,000 in Britain. However, most of the dead, are from sub-Saharan Africa.
It is already accepted that the HIV-1 virus, the dominant cause of Aids, crossed into humans from a virus, SIV-cpz, carried by chimps.
But most researchers maintain that the transfer happened as the result of an accidental blood-to-blood infection, when a native hunter with an open wound was catching or slaughtering a chimp for food - the "natural transfer" theory.
In his book The River, published last year, Mr Hooper argued that the sites of most of the earliest known cases of Aids, in the former Belgian colonies of the Congo, Rwanda and Burundi, coincided with the sites of mass vaccinations with an experimental polio vaccine called Chat between 1957 and 1960. The vaccine was produced by a team led by US virologist Hillary Koprowski.
Mr Hooper argues that there is powerful circumstantial evidence that the makers of Chat had used local chimps, instead of the more commonly used rhesus monkeys, to supply the kidneys used to make early polio vaccines.
The surviving researchers, led by Dr Koprowski, say that they used captured chimps to test, not make, the vaccine. Yet there is a baffling lack of documentation on what was going on in the African facilities.
Yesterday Mr Hooper produced testimony from witnesses which strengthened his claim that Dr Koprowski's collaborators were regularly removing kidneys from chimps and sending them abroad.
In a signed statement, Louis Bugyaki, who worked as a vet in Stanleyville - now Kisangani - where the Koprowski team's lab and chimp facility was located, said he had been told by two of the Belgian doctors working there that chimp organs, mostly kidneys, were being sent to the US.
"It is possible that the main purpose of sending the kidneys there was to provide cells in which the Koprowski polio vaccines could be grown," said Mr Bugyaki, who now lives in Brussels.
"I was told by the [doctors] that the sending of chimp kidneys abroad was to be kept a secret. I think there is a strong possibility that this was a commercial secret, and that the vaccine-makers did not want the role that the chimps had played to be known."
Another witness, Juma Jamnadas, who worked as a microscopist at a lab in Bujumbura in Burundi in the 1950s, said that he saw single kidneys regularly being removed from chimps and sent to a lab in the Congo where a collaborator of Dr Koprowski's, Tadeusz Wiktor, worked and produced a range of vaccines.
Dr Plotkin described Mr Bugyaki's testimony as "hearsay", and said that the vet had given contradictory information to him.
Yesterday Claudio Basilico, professor of microbiology at the New York University school of medicine, revealed that there were no signs of contamination in tests by independent labs on a tiny set of samples of vaccine held by the Wistar Institute in Philadelphia, where much of the Chat vaccine used in Africa was made.
However, this does not disprove the OPV-Aids theory, since there were several different batches of Chat vaccine and it is unlikely that all would have been contaminated with chimp SIV.