Where did HIV come from? HIV is very similar to SIV (simian immunodeficiency virus). SIV infects several different kinds of higher primates, for example mangabeys, mandrills, green monkeys, and chimps. SIV does not seem to make these primates ill. If HIV evolved from SIV, which SIV was the evolutionary ancestor? Nucleotide sequences of the different strains of HIV and SIV can now be compared by cladistics, which clusters similar genotypes together. The various strains of HIV-1 cluster together with the SIV strains from chimpanzees. HIV-1 and chimp SIV are so mixed together in the analysis that it appears the virus jumped from chimps to humans on at least three occasions. Moreover, HIV-2 clusters together with the version of SIV found in sooty mangabeys. Therefore, HIV had multiple origins from SIV that came from chimps and mangabeys. The original suspects from the 1980s, African green monkeys, are hosts to SIV that are not closely related to any strains of HIV. The method by which the virus entered human populations has not been determined but did not necessarily involve sexual contact. SIV could have infected the first human host by blood contact by a hunter with an infected animal he had killed.

Scientists have even been able to estimate a time of origin for HIV. Evolutionary biologist Bette Korber and colleagues limited their study to group M viruses. They estimated the degree of nucleotide difference between a strain of HIV and the common ancestor of all group M viruses. For each year between about 1983 and 1998, each of the strains became more and more different from the common ancestor. The researchers calculated a statistical line through these data, then extrapolated the line all the way back to a time when there would have been zero difference between the strains and their common ancestor. The line crossed zero for the year 1931. Especially with extrapolation, error ranges become very large, so the estimated age of group M viruses is between 1918 and 1941. It appears that HIV has been present in human populations for only about 70 years, while SIV has been in other primate populations for many millennia. Parasites often have their most severe effects upon first infecting a host species; thereafter, coevolution may result in less severe disease, both because of more resistant hosts and also because of milder parasites. . .

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