New research suggests that the simian immunodeficiency virus (SIV) lineage was already circulating in monkeys and apes at least 32,000 years ago. During the millennia since, humans would have been exposed to it countless times as they butchered monkeys for food. However, only between the 1800s and 1959 did a human infection from a chimpanzee virus spread widely enough to eventually evolve into the present HIV epidemic.
Previously, it had been thought SIV was a much younger virus, perhaps only a few hundred years old. In the current study, researchers examined SIV in monkey species that had developed in isolation on Bioko, a land mass that was separated from West Africa some 10,000 years ago as sea levels rose.
Four species on Bioko had SIV, but each strain was genetically very distinct, suggesting they did not come from a recently imported monkey with SIV in the last few centuries. Each virus also was close to the strain infecting monkeys of the same four genuses on the mainland, suggesting the SIV strains existed before Bioko was cut off.
Given that 10,000-year window, recalculating SIV's "molecular clock" by how fast it mutates places the common ancestor to all SIV strains at between 32,000 and 78,000 years old. That ancestor virus may have existed for millions of years.
If HIV had been in humans before the 20th century, it would have arrived in the Americas via the slave trade, said Dr. Preston A. Marx, a virologist at the Tulane primate center and co-author of the study. The immediate ancestor to HIV came from chimpanzees, and SIV still causes illness and death in chimps, but not quickly, suggesting a relatively recent adaptation.
Marx believes the introduction into Africa of millions of inexpensive, mass-produced syringes in the 1950s allowed a human infection by chimpanzee virus to spread more widely. Campaigns to eradicate yaws, syphilis, malaria, smallpox, and polio required many syringes, and their re-use was often officially approved. The devices also became status symbols in non-medical settings.
However, University of Arizona virologist Michael Worobey, who co-authored the study, and University of Alabama virologist Dr. Beatrice Hahn suspect the growth of colonial cities helped spark the epidemic. Before 1910, no central African city had more than 10,000 people, and syringes were handmade, expensive and rare. Later urban migrations fueled sexual contacts and prostitution. The earliest confirmed HIV infection in a human to date was traced to blood drawn from a man in Kinshasa in 1959.
The full study, "Island Biogeography Reveals the Deep History of SIV," was published in Science (2010;329(5998):1487).