HIV-1 is more virulent, more easily transmitted, and the cause of the vast majority of HIV infections globally. Research in this area is conducted using molecular phylogenetics , comparing viral genomic sequences to determine relatedness. 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. 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 and 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 β 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 and with the central estimates varying between and According to the natural transfer theory also called "hunter theory" or "bushmeat theory" , in the "simplest and most plausible explanation for the cross-species transmission" [ 10 ] of SIV or HIV post mutation , the virus was transmitted from an ape or monkey to a human when a hunter was cut or otherwise injured while hunting or butchering an infected animal.
The resulting exposure to blood or other bodily fluids of the animal can result in SIV infection. This over-exposure to bushmeat and malpractice of butchery increased blood-to-blood contact, which then increased the probability of transmission.
