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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, Worobey said.
The new work is "clearly an improvement" over the previous estimate of around 1930, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. His institute helped pay for the work.
Fauci described the advance as "a fine-tuning."
Experts say it's no surprise that HIV circulated in humans for about 70 years before being recognized. An infection usually takes years to produce obvious symptoms, a lag that can mask the role of the virus, and it would have infected relatively few Africans early in its spread, they said.
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