There are a few different ideas that have been put forward to explain how HIV came about. The two thrown around most, by far, are: “The government did it,” or “It was definitely people having sex with monkeys!”
There are lots of conspiracy theories surrounding HIV; some people do not believe it causes AIDS at all, and others believe that HIV is a man-made virus.
HIV-1 from chimpanzees and gorillas to humans
Scientists generally accept that the known strains (or groups) of HIV-1 are most closely related to the simian immunodeficiency viruses (SIVs) endemic in wild ape populations of West Central African forests. Particularly, each of the known HIV-1 strains is either closely related to the SIV that infects the chimpanzee subspecies Pan troglodytes troglodytes (SIVcpz) or closely related to the SIV that infects western lowland gorillas (Gorilla gorilla gorilla), called SIVgor. The pandemic HIV-1 strain (group M or Main) and a very rare strain only found in a few Cameroonian people (group N) are clearly derived from SIVcpz strains endemic in Pan troglodytes troglodytes chimpanzee populations living in Cameroon. Another very rare HIV-1 strain (group P) is clearly derived from SIVgor strains of Cameroon. Finally, the primate ancestor of HIV-1 group O, a strain infecting 100,000 people mostly from Cameroon but also from neighboring countries, has been recently confirmed to be SIVgor. The pandemic HIV-1 group M is most closely related to the SIVcpz collected from the southeastern rain forests of Cameroon (modern East Province) near the Sangha River. Thus, this region is presumably where the virus was first transmitted from chimpanzees to humans. However, reviews of the epidemiological evidence of early HIV-1 infection in stored blood samples, and of old cases of AIDS in Central Africa have led many scientists to believe that HIV-1 group M early human center was probably not in Cameroon, but rather farther south in the Democratic Republic of the Congo, more probably in its capital city, Kinshasa (formerly Léopoldville).
Using HIV-1 sequences preserved in human biological samples along with estimates of viral mutation rates, scientists calculate that the jump from chimpanzee to human probably happened during the late 19th or early 20th century, a time of rapid urbanisation and colonisation in equatorial Africa. Exactly when the zoonosis occurred is not known. Some molecular dating studies suggest that HIV-1 group M had its most recent common ancestor (MRCA) (that is, started to spread in the human population) in the early 20th century, probably between 1915 and 1941. A study published in 2008, analyzing viral sequences recovered from a recently discovered biopsy made in Kinshasa, in 1960, along with previously known sequences, suggested a common ancestor between 1873 and 1933 (with central estimates varying between 1902 and 1921). Genetic recombination had earlier been thought to "seriously confound" such phylogenetic analysis, but later "work has suggested that recombination is not likely to systematically bias", although recombination is "expected to increase variance".The results of a 2008 phylogenetics study support the later work and indicate that HIV evolves "fairly reliably". Further research was hindered due to the primates being critically endangered. Sample analyses resulted in little data due to the rarity of experimental material. The researchers, however, were able to hypothesize a phylogeny from the gathered data. They were also able to use the molecular clock of a specific strain of HIV to determine the initial date of transmission, which is estimated to be around 1915-1931.
HIV-2 from sooty mangabeys to humans
Similar research has been undertaken with SIV strains collected from several wild sooty mangabey (Cercocebus atys atys) (SIVsmm) populations of the West African nations of Sierra Leone, Liberia, and Ivory Coast. The resulting phylogenetic analyses show that the viruses most closely related to the two strains of HIV-2 that spread considerably in humans (HIV-2 groups A and B) are the SIVsmm found in the sooty mangabeys of the Tai forest, in western Ivory Coast.
There are six additional known HIV-2 groups, each having been found in just one person. They all seem to derive from independent transmissions from sooty mangabeys to humans. Groups C and D have been found in two people from Liberia, groups E and F have been discovered in two people from Sierra Leone, and groups G and H have been detected in two people from the Ivory Coast. These HIV-2 strains are probably dead-end infections, and each of them is most closely related to SIVsmm strains from sooty mangabeys living in the same country where the human infection was found.
Molecular dating studies suggest that both the epidemic groups (A and B) started to spread among humans between 1905 and 1961 (with the central estimates varying between 1932 and 1945).