A new study suggests that the deadly AIDS virus has been circulating in the human population for around 100 years, which is decades longer than scientists initially believed. Previous estimates put the origin of HIV back to 1930, but research has now put its origins between 1884 and 1924, with a more focused estimate at 1908.

AIDS was not formally recognized until 1981 when it attracted the attention of public health officials in the United States.

Results of the genetic analysis of the virus appear in Thursday's issue of the journal Nature. Researchers note that the new origin dates also fall during the rise of cities in Africa, and suggest that urban development may have helped promote HIV's initial establishment and early spread.

Researchers say that HIV descended from a chimpanzee virus that jumped to humans in Africa, which is likely to have occurred when people butchered chimps. They believe that many people were infected this way, but that it didn't take hold in others until the growth of African cities, when lots of people were put close together and prostitution became promoted.

Essential to the new study was the discovery of an HIV sample taken from a woman in Kinshasa in 1960. This was only the second sample to be found before 1976. The other was from 1959, and was also from Kinshasa.

Scientists were able to take advantage of the fact that HIV tends to mutate rapidly. Two strains from a common ancestor became less and less alike in genetic material over time, which allows scientists to "run the clock backward" by calculating how long it would take various strains to become as different as they are observed to be. This would then indicate when they both sprang from their most recent common ancestor.

The work included genetic data from the two old HIV samples, as well as more than 100 modern samples to create a family tree that goes back to these samples' last common ancestor.

Experts indicate that it is no surprise that HIV circulated in humans for 70 years before becoming recognized. Infections usually take years to produce obvious symptoms, and it would have infected few Africans in its early years.