HIV-1 transmission from chimpanzees and gorillas to humans

HIV-1 transmission 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 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 to the SIV that infects Western lowland gorillas (Gorilla gorilla gorilla), called SIVgor.^{[3][4][5][6][7][8]} 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.^[3] Another very rare HIV-1 strain (group P) is clearly derived from SIVgor strains of the same country.^[6] Finally, the primate ancestor of HIV-1 group O, a strain infecting tens of thousands of people mostly from Cameroon but also from neighboring countries, is still uncertain, but there is evidence that it is either SIVcpz or SIVgor.^[5] 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.^[3] 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 epicenter was probably not in Cameroon, but rather farther south in the Democratic Republic of the Congo, more probably in its capital city, Kinshasa.^{[3][9][10][11]}

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 19^th or early 20^th century, a time of rapid urbanisation and colonisation in equatorial Africa. Exactly when the zoonosis occurred is not known. Some molecular datation 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 20^th century, probably between 1915 and 1941.^[12][13][14] 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).^[15][16]

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 [results]", although recombination is "expected to increase variance".^[15] The results of phlyogenetics study supported the later work and indicated that HIV evolves "fairly reliabl