US:THE AIDS virus has been affecting people for about 100 years, decades longer than scientists had thought, according to a new study.
Genetic analysis pushes the estimated origin of HIV back to between 1884 and 1924, with a more focused estimate at 1908.
Previously, scientists had estimated the origin at around 1930. Aids was not recognised formally until 1981, when it attracted the attention of US health officials.
The new result is "not a monumental shift, but it means the virus was circulating under our radar even longer than we knew", said Prof Michael Worobey of the University of Arizona, an author of the study.
The results appear in the current issue of Nature. Researchers note that the newly calculated dates fall during the rise of cities in Africa, and suggest urban development may have promoted HIV's establishment and early spread.
Scientists say HIV descended from a chimpanzee virus that jumped to humans in Africa, probably when people butchered chimps. Many individuals were probably infected that way, but so few others caught the virus that it failed to gain a foothold, researchers say.
But the growth of African cities may have changed that by locating clusters of people close together and by promoting prostitution, Prof Worobey suggested. "Cities are kind of ideal for a virus like HIV," he said.
Perhaps a person infected with the Aids virus in a rural area went to what is now Kinshasa, Democratic Republic of Congo, "and now you've got the spark arriving in the tinderbox", he said.
Key to the new work was the discovery of a HIV sample that had been taken from a woman in Kinshasa in 1960. It was only the second such sample to be found from before 1976; the other was from 1959, also from Kinshasa.
Researchers took advantage of the fact that HIV mutates rapidly. This means two strains from a common ancestor quickly become less alike over time. That allows scientists to "run the clock backward" by calculating how long it would take for strains to become as different as they are observed to be. That would indicate when they both sprang from their most recent common ancestor.
The new work used genetic data from the two old HIV samples plus more than 100 modern samples to create a family tree going back to these samples' last common ancestor. Researchers got various answers under various approaches for when that ancestor virus appeared, but the 1884 to 1924 bracket is probably the most reliable, Prof Worobey said. - (AP)