The origins of the HIV-1 and HIV-2 are the subject of ongoing research and theoretical speculation. Early in 1999, one group of researchers claimed to have discovered an indisputable link between a primate virus from central West Africa and HIV-1. However, other researchers have argued that a version of HIV may have been prevalent in humans for hundreds or thousands of years before the current epidemic.

Types of retroviruses

HIV-1 and HIV-2 are two members of a family of viruses called retroviruses. Other viruses in this group include human T-cell lymphotrophic virus I (HTLVI), which causes blood and nerve problems, feline immunodeficiency virus (FIV), which causes an AIDS-like illness in some cat species, simian immunodeficiency virus (SIV), which cause an AIDS-like disease in some species of monkey and visna virus, which causes pneumonia in sheep.

The primate theory

In 1999, American, Japanese and French researchers discovered a virus in captive chimpanzees (Pan troglodytes troglodytes) that appears to combine elements of HIV with parts of simian immunodeficiency virus. The chimpanzees are infected with a virus that is between 70 and 90% identical to HIV-1. This virus, which is called SIVcpz, does not appear to cause illness in Pan troglodytes troglodytes^[1].

HIV-2 originated in the sooty mangabey monkeys. As with SIVcpz in chimpanzees, this virus does not cause illness in sooty mangabeys.

Researchers believe that most African primates species have been infected with a sexually transmitted form of SIV for thousands of years and that these viruses no longer harm their hosts. However, when one type of SIV crosses into a different species, it often does cause illness.

This theory suggests that HIV-1 and HIV-2 may have been in the human population for anywhere between 25 and 100 years. It is thought that HIV successfully crossed the species barrier, leading to the current HIV epidemic. Several teams which have looked at the genetic divergence of HIV strains have concluded that HIV-1 strains began to diverge somewhere between 1940 and 1960, judging by the speed of divergence. However, dating the origin of HIV has been difficult due to the unequal rates of evolution among different strains and sub-types. A European research group has developed a technique to date HIV by using particular genetic regions of HIV that evolve at the same rate among various HIV strains. This study found that HIV-1 appeared to separate from its closest monkey-virus relative in about 1800. Furthermore, the HIV-1 group M emerged in about 1930^[2].

Transmission of SIV strains between primate species seems to be a relatively common occurrence through fighting and slaughter. The hunting and consumption of monkeys and apes by humans is thought to have led to HIV infection in people. While it is likely that forms of SIV have been infecting people for a long time, virus mutations may have led to the development of immunodeficiency viruses that were able to cross the species barrier and sustain infections in humans better.

The polio vaccine link?

One variation on the primate theory contends that HIV was spread from chimpanzees to humans through an oral polio vaccine. The theory argues that a polio vaccine developed by Hilary Koprowski from the Wistar Institute in Philadelphia is the source of HIV. This vaccine, administered in central Africa in the late 1950s, was cultured from primates infected with a version of SIV, which subsequently infected humans. The common sense appeal of this theory relies on timing: administration of the vaccine from 1957 to 1960 coincided with the first confirmed case of HIV infection in Africa, detected from blood stored in 1959 in Kinshasa.

An independent analysis of the remaining stock of the vaccine has now been conducted by three research groups in Europe and the United States, finding no evidence to support the theory. There was no evidence of SIV or HIV in the vaccine samples. In addition, chimpanzees, the known source of HIV, were not used in the production of cell cultures. Instead, monkeys were used.