The strain of virus responsible for nearly all cases of AIDS first appeared in the 1930s, say scientists who used a "molecular clock" to date the origin of HIV. The finding, reported in the 9 June issue of Science, casts doubt on the controversial idea that contaminated polio vaccines seeded the AIDS pandemic in the late 1950s.
The origin of AIDS is much clearer today than it was in the early 1980s, when the new disease blindsided the scientific establishment. Virologists have identified the chimpanzee virus that probably gave rise to HIV, while epidemiologists have rummaged through old samples and forgotten medical records to identify early cases. Still, the trail of direct evidence stops with an HIV-positive blood sample drawn in 1959 in what is now the Democratic Republic of Congo. How and when the chimpanzee virus entered humans remains uncertain.
To look farther into HIV's murky past, theoretical biologist Bette Korber of Los Alamos National Laboratory in New Mexico and her colleagues used a statistical model to clock the evolution of HIV's envelope gene, which codes for one of the spiky proteins jutting from the virus's surface. With help from a supercomputer, Korber's group compared 159 gene sequences from viruses in the so-called M group, which causes the majority of AIDS cases worldwide. Assuming that differences between genes accumulate at a constant rate, the researchers calculated that the common ancestor of all the M group viruses arose between 1915 and 1941--most likely in 1931. They believe the virus had already jumped to humans at this time; a later invasion would require that multiple viral strains entered people before spreading from person to person--a scenario Korber calls very unlikely.
The study impresses other scientists in the field. "It's the best analysis by a long way that's been carried out in this area," says molecular evolutionist Paul Sharp of the University of Nottingham in the United Kingdom. If confirmed, the early date would squash the hypothesis--described recently in a book called The River (Science, 12 November 1999, p. 1305)--that the AIDS pandemic was triggered by an oral polio vaccine inadvertently contaminated with the chimpanzee virus.