When Kenyan ecologist Wangari Maathai, winner of the 2004 Nobel Peace Prize, claimed in early October that HIV was “created by a scientist for biological warfare,” she weighed into a debate that has been raging since the beginning of the AIDS epidemic.

“Some say that AIDS came from the monkeys, and I doubt that because we have been living with monkeys since time immemorial,” Maathai said.

Over the years, some people have claimed that AIDS was created as part of a conspiracy to wipe out gay men, or was the result of secret biological warfare research that went terribly wrong. Others have produced highly persuasive evidence to argue that AIDS found its way into humans via polio vaccine programs in the 1950s.

The recent alarm over batches of polio vaccine contaminated with a virus called SV40, which might be related to some kinds of cancer, has brought these claims back to the spotlight.

Where exactly did HIV come from? What do we really know, and how true are all these different claims? In this Backgrounder we take a look at the evidence.

Throughout history, humankind has often had to come to terms with new diseases, some of them terrifying in scale. Just in the last quarter of a century, diseases such as Ebola, variant CJD, Legionnaire’s disease, SARS and chronic fatigue syndrome have appeared, seemingly out of thin air. It’s a safe bet to say that other, as-yet-unimagined, diseases will emerge in the future.

The first cases of AIDS were reported in 1980, initially among gay men and IV drug users in the US. Given the widespread animosity towards these groups and the fact that AIDS seemed to come from nowhere to target them, it’s probably not surprising that some people were quick to smell a conspiracy.

But we now know that AIDS was around for a long time before the early 1980s, and that the earliest cases were among people who were neither gay nor drug users.

Scientists have been investigating the origins of AIDS for as long as the disease has been with us: solving the puzzle has the potential to unlock important clues in its medical treatment, perhaps leading to a vaccine or a cure.

Early evidence

In February 1959, Joseph Vandepitte, a Belgian microbiologist, arrived in Leopoldville, the capital of the Belgian Congo, where he had just been made a Professor at a nearby university.

Together with Arno Motulsky, a young American researcher, Vandepitte spent several weeks collecting 672 blood samples from medical staff and hospital patients in Leopoldville and from villagers to the south. They were studying malaria, but their work was to have an important impact on AIDS research.

Twenty-six years later, in 1985, the Belgian Congo had long since gained independence, and Leopoldville was now called Kinshasa. But those 672 blood samples had remained in cold storage, first in Vandepitte’s lab, then later at Emory University in the US.

Researchers in France and the US had recently identified the virus that causes AIDS, and named it HIV. At Emory University, Professor André Nahmias decided to test Vandepitte’s samples for HIV. One of them, sample number L70, came back positive.

From Motulsky’s notes, we know only that the sample came from a Bantu man living in Leopoldville. But that anonymous sample remains the earliest known case of HIV infection and the first concrete evidence that AIDS originated in Africa.

That same year, French researchers discovered another virus, the Simian Immunodeficiency Virus or SIV. Genetically similar to HIV, SIV infects apes and causes an AIDS-like illness in them. Researchers were quick to suggest that the two viruses must be related, and that HIV may be a descendant of SIV.

HIV and SIV belong to a viral family called lentiviruses (‘slow’ viruses). The Feline Immunodeficiency Virus (FIV), which infects cats, and the Bovine Immunodeficiency Virus (BIV), which infects cattle, are distant relations in the same group.

Despite the genetic similarities, humans can’t be infected with SIV. Indeed, there are at least a half-dozen different strains of SIV, each of which infects only one species of ape: one for chimpanzees, one for mandrills, one for sooty mangabeys. But the viruses were so similar that scientists quickly suspected that SIV must have crossed the species barrier from one of these species of apes to humans, becoming HIV.

This phenomenon of crossing from one species to another — zoonosis — is not as uncommon as it might seem. The recent concern over outbreaks of Avian Influenza (‘bird flu’) reflects this. While most viruses are specific to one host, occasionally they change to a form that can infect a different host.

In 1999 an American researcher, Beatrice Hahn, found another missing piece of the puzzle. Although SIV infection in chimpanzees is extremely rare — only a handful of cases have ever been identified — Hahn’s team found striking similarities between some chimp SIV and HIV-1. The viruses were so similar that Hahn argued they could not have arisen separately.

So HIV found its way into humans from chimpanzees. But how?

The most widely accepted explanation for the way this happened is the ‘hunter hypothesis’ which suggests that, at some point in the past, a mutated SIV found its way from a chimpanzee into a human via the African ‘bushmeat’ industry, in which apes are hunted for food.

A mutated virus, perhaps the viral offspring of two strains of SIV which had infected a single animal, somehow found its way into the bloodstream of a human, perhaps during the butchering process or, less likely, via eating uncooked chimp meat.

The species barrier was crossed and HIV was born.

When did this happen? Clearly it had already happened by 1959, but we now believe it was many years earlier. At the Los Alamos National Laboratory in the US, Bette Korber has developed a complex computer model that maps the genetic changes to HIV from the mid-1980s to 1999. Using a supercomputer called ‘Nirvana’, the model says that the earliest human ancestor of HIV must have existed around 1931. Korber says the virus must have jumped to humans around that date.

Humans have been hunting apes in Africa since time immemorial, so if SIV is a naturally occurring virus, why didn’t it cross the species barrier long before this? Beatrice Hahn believes it probably did, but changes in human society in post-World War 2 Africa enabled it to gain a firm foothold.

“Increasing urbanisation, breakdown of traditional lifestyles, population movements, civil unrest, and sexual promiscuity are all known to increase the rates of sexually transmitted diseases and thus likely triggered the AIDS pandemic,” points out Hahn.

The polio vaccine hypothesis

Perhaps the most commonly held alternative explanation for the hunter hypothesis is the idea that HIV found its way into humans via polio vaccines. Poliomyelitis (polio) is an infectious disease that attacks the central nervous system and causes paralysis. Until relatively recently, polio outbreaks were a common occurrence.

The first polio vaccine, developed by Jonas Salk, was hailed as a medical miracle in 1955. But contamination problems, leading to the deaths of 11 children, meant that a better vaccine was needed.

Working separately, Albert Sabin and Hilary Koprowski developed two alternative polio vaccines, both derived from polioviruses grown in primate tissue cultures. Sabin tested his vaccine in the USSR while Koprowski conducted tests on his vaccine, called CHAT, in the Belgian Congo between 1956 and 1960. Eventually the Sabin vaccine proved superior and was adopted.

The polio vaccine hypothesis, which first emerged in a 1992 Rolling Stone article, suggests that the Koprowski vaccine became contaminated with SIV during the tests in the Belgian Congo.

The hypothesis gained momentum when British journalist Edward Hooper published a book called The River in 1999. Hooper travelled to Africa and carefully charted the Koprowski vaccine program, concluding that HIV had its origins in infected chimp kidneys used to produce the vaccine, a claim refuted by Koprowski who insisted that macaques, not chimpanzees, were used to manufacture his vaccine. An examination of stored samples of the CHAT vaccine found no evidence of HIV or SIV.

In April 2004, in a letter published in the scientific journal Nature, Michael Worobey and others tested the polio vaccine hypothesis. They examined the SIV found in chimps in the area where Koprowski did his research. Using highly sensitive molecular analysis, they compared the SIV with HIV, finding significant differences.

HIV could not have come from chimps in the region where Koprowski worked, Worobey said. Together with Bette Korber’s computer model showing that HIV originated many years before the polio vaccine trials were conducted, and the absence of virus in the stored CHAT vaccine, Worobey argued that the polio vaccine hypothesis should finally be laid to rest.

While the polio vaccine theory may seem compelling, the scientific evidence against it is strong and mounting. When science is confronted with multiple explanations for a single phenomenon, a principle called Occam’s Razor says that the simplest explanation should usually be preferred.