AIDS is caused by HIV (human immunodeficiency virus), which arose in non-human primates in West and Central Africa. While different sub-groups of the virus acquired human infectivity at other times, the global pandemic had its roots in the evolution of one specific strain – HIV-1 subgroup M – in Léopoldville in the Belgian Congo (now Kinshasa in the DRC (the Democratic Republic of the Congo)) in the early 1920s.
HIV-1 from chimpanzees and gorillas to humans
Scientists usually accept that the known groups (or strains) of HIV-1 are most closely linked to the SIVs (simian immunodeficiency viruses endemic) in ape populations of West Central African forests. In particular, each of the HIV-1 strains is related to the SIV that infects the SIVcpz (chimpanzee subspecies Pan troglodytes) or closely related to the SIV that affects western lowland gorillas, called SIVgor.
The epidemic HIV-1 strain (group Main or M) and a unique strain found only in a few Cameroonian people (group N) are derived from SIVcpz strains endemic in Pan troglodytes chimpanzee residents living in Cameroon.
Another very unique HIV-1 strain (group P) is derived from the SIVgor strains of Cameroon. Finally, the primate ancestor of HIV-1 group O, a strain sadly infecting 100,000 people, mostly from Cameroon and neighboring countries, was confirmed in 2006 to be SIVgor.
The epidemic HIV-1 group M is most closely linked to the SIVcpz received from Cameroon’s southeastern rain forests (East Province) near the Sangha River.
Thus, this area is presumably where the virus was initially transmitted from chimpanzees to humans. However, epidemiological evidence of early HIV-1 infection in stored blood samples and of old cases of AIDS in Central Africa has led many scientists to believe that HIV-1 group M early human center was probably not in Cameroon, but somewhat further south in the Belgian Congo (the modern-day the Democratic Republic of the Congo) mostly in its capital city, Léopoldville (current day Kinshasa).
Using HIV-1 sequences stored in human biological samples and estimates of viral mutation rates, experts calculate that the constant jump from chimpanzee to human probably occurred during the late 19th or early 20th century, a time of accelerated urbanization of equatorial Africa. Exactly when the zoonosis really occurred is unknown. Some molecular records investigations suggest that HIV-1 group M had its MRCA (most recent common ancestor) (that is, started to rapidly spread in the human population) in the early 20th century, presumably between 1915 and 1941. A study published in 2008, investigating viral sequences obtained from a biopsy made in Kinshasa, in 1960, along with previously recognized sequences, proposed a common ancestor between 1873 and 1933 (with central estimations varying between 1902 and 1921).
Genetic recombination had earlier been thought to “severely confound” such phylogenetic study, but later “work has proposed that recombination is not likely to systematically results [bias].” However, recombination is “suspected to increase variance.” The latest results of a 2008 phylogenetics research support the later work and show that HIV evolves “fairly reliably.”
Further research was sadly hindered due to the primates’ turning critically endangered. Sample reports resulted in little data due to the rarity of the experimental material. The scientists, however, were able to hypothesize a phylogeny from the gathered data carefully. They were also able to utilise the molecular clock of a particular strain of HIV to fix the initial transmission date, which is calculated to be around 1915–1931.
According to the natural transfer theory (also called “bushmeat theory” or “hunter theory”), in the most plausible and most straightforward explanation for the cross-species transmission” SIV or HIV (after the mutation), the virus was transferred from a monkey or an ape to a human when a bushmeat vendor/handler or hunter was cut or bitten while butchering or hunting the animal. The following exposure to blood or other bodily fluids of the ape or the monkey can result in SIV infection.
Before World War II, some Sub-Saharan Africans were forced out of the village areas because of the greedy European demand for resources. Since rural African tribal people were not keen to try new agricultural practices in the forest, they turned to non-domesticated animals as their primary meat source. This malpractice of butchery and over-exposure to bushmeat increased blood-to-blood contact, increasing the transmission probability.