US researchers say the deadly AIDS virus first began spreading among humans at the turn of the 20th century in sub-Saharan Africa.
Their finding, reported in the journal Nature on Wednesday, pushes back the origin of the human immunodeficiency virus by several decades.
Researchers think the growth of cities - and high-risk behavior associated with urban life - may have helped the virus to flourish.
There is no cure for AIDS, which is most commonly transmitted through sexual contact.
Prior estimates put the origin of HIV at 1930. But Michael Worobey of the University of Arizona in Tucson now believes HIV began infecting humans between 1884 - 1924.
The research is based on gene fragments dug from a wax-embedded lymph node from a woman in Kinshasa in the Democratic Republic of Congo, formerly Zaire.
The 1960 sample is the second-oldest genetic sequence of HIV-1 group M, the main strain of the virus responsible for the AIDS pandemic.
The oldest sequence came from a 1959 blood sample given by a man in Kinshasa, formerly known as Leopoldville.
Putting the two samples together with dozens of other previously known HIV-1 genetic sequences, the researchers constructed family trees for this strain of HIV.
The ancestor of the pandemic strain of the AIDS virus is estimated as originating in 1908, plus or minus about 20 years.
Beginnings in southern Cameroon
Research from chimpanzee droppings suggests the virus first spread from chimps to humans in southeastern Cameroon.
It is thought the disease spread slowly among the local population until one of the infected people went to Kinshasa, where it had more opportunity to spread.
By the 1960s, several thousand people may have been infected with HIV.
By 1981, the rest of the world began to recognise the pandemic, which has now infected 33 million people and killed 25 million.
