The AIDS virus has been circulating among people for about 100 years, decades longer than scientists had thought, a new study suggests. 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 wasn’t recognized formally until 1981 when it got the attention of public health officials in the United States.

The new result is “not a monumental shift, but it means the virus was circulating under our radar even longer than we knew,” says Michael Worobey of the University of Arizona, an author of the new work.

The results appear in Thursday’s issue of the journal Nature. Researchers note that the newly calculated dates fall during the rise of cities in Africa, and they suggest urban development may have promoted HIV’s initial 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 other people caught the virus that it failed to get a lasting foothold, researchers say.

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