I thought this was interesting -- it fits with Jared Diamond's argument about malaria.
The researchers compared that sample with modern strains to determine its mutation rate. Then they matched that rate with the 1959 sample, tracing their common ancestor to between 1884 and 1924.
The researchers surmised that the creation of colonial cities around the turn of the century was the catalyst that allowed the virus to take hold.
Jim Moore, an anthropologist at UC San Diego who was not associated with the study, said the fact that the virus could have spread unnoticed for decades is no surprise, given the mortality rates in Africa during the colonial period.
"The conditions then were horrendous in terms of how Africans were treated," he said. "People dying of AIDS would have been part of the background."