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."

"The HIV virus evolves incredibly quickly," said geneticist Bette Korber of Los Alamos National Laboratory in New Mexico, who did an analysis in 2000. "Those mutations get passed on to the next individual. So we have that evolutionary pace to enable a look backward."

Korber's analysis compared the 1959 blood sample and modern samples. She traced their common ancestor to roughly 1931.

The new analysis, published in the journal Nature on Wednesday, added lymph node tissue from a woman who died in 1960 in the Belgian Congo. The tissue specimen was one of more than 800 preserved in ice-cube-size blocks of paraffin at the University of Kinshasa.

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.