The arrival of colonial cities in sub-Saharan Africa at the dawn of the 20th Century may have sparked the spread of HIV.
US experts analysed one of the earliest samples of the virus ever found, in the Democratic Republic of Congo in 1959.
The study, published in the journal Nature, suggests the virus may have crossed from apes to humans between 1884 and 1924.
They believe newly-built cities may have allowed the virus to thrive.
Aids, the illness caused by HIV, was first reported by doctors in 1981, but the virus had been around for many decades before that.
HIV is not a single virus – there are a number of different strains and subtypes of strains, some sharing the same "founder event" in history, in which a single human was infected.
Scientists believe that these "founder events" may have involved eating monkeys infected with a similar virus.
Research published last year found the viral ancestor of a subtype of HIV responsible for most modern cases in the US and Europe in a blood sample taken in Leopoldville, the capital of Belgian Congo – now Kinshasa, the capital of the Democratic Republic of Congo.
Now the same team, from the University of Arizona at Tucson, has found another sample containing a different subtype in a 1960 sample from a different patient in the same city.
By analysing the genetic differences between the two viruses, and calculating the amount of time these differences would take to evolve, they now say that the two probably have a common ancestor dating from at least 50 years earlier.
Dr Michael Worobey, who led the research, said: "Now, for the first time, we have been able to compare two relatively ancient HIV strains.
"That helped us to calibrate how quickly the virus evolved and make some really robust inferences about when it crossed into humans, how the epidemic grew from that time, and what factors allowed the virus to enter and become a successful human pathogen."
HIV was and remains a "relatively poorly transmitted" virus, he said, so the key to the success of the virus was possibly the development of cities such as Leopoldville in the early 1900s.
The large numbers of people living in close proximity would have allowed more opportunity for new infections.
"I think the picture that has emerged here, is that changes the human population experienced may have opened to the door to the spread of HIV," he said.
Professor Paul Clark, a researcher into evolutionary history at the University of Edinburgh, said that while the finding was mainly of "historical interest", it might provide more clues about how the virus changed over time.
He suggested that it was likely that all of the early cases of "group M" HIV-1 – the strain causing 19 out of 20 modern infections – happened in the Leopoldville area.
He said: "We can now paint a remarkably detailed picture of the time and place of origin of HIV-1 group M viruses and their early diversification, and thus of the prehistory of the AIDS pandemic."
(From BBC news report on Wednesday, 1 October 2008 with the website of http://news.bbc.co.uk/2/hi/health/7646255.stm)
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