Scientists had long known that a blood sample, preserved from 1959, showed that HIV had been circulating in Kinshasa, the capital of Congo, for several decades before the virus first drew international attention in the 1980s. In 2008, evolutionary biologist Michael Worobey sharpened that picture when he reported in the journal Nature the discovery of a second sample of the virus, trapped in a wax-encased lymph node biopsy from 1960.
By comparing these two historic pieces of virus and mapping out the differences in their genetic structures in his lab at the University of Arizona, Worobey determined that HIV-1 group M was much older than anyone had thought. Both samples of the virus appeared to have descended from a single ancestor at some time between 1884 and 1924. The most likely date was 1908.
Taken together, these two discoveries offered the clearest clues to the birth and early life of the epidemic.
Not far from where HIV-1 group M was born was a major river, the Sangha, flowing toward the heart of Central Africa. This section of the Sangha was not ideal for navigation because of its ribbons of sandbars and the dense vegetation along its banks.
In the especially treacherous middle section, near where Hahn and Sharp’s team found the viral ancestor of HIV, few major human settlements ever developed. But there were numerous communities on the Sangha’s more accessible stretches. And due south, past riverside trading towns, was the mighty Congo River itself, the superhighway of Central Africa.
Once the virus made the jump from chimp to human, a single infected person could have carried HIV down the Sangha, onto the Congo River and into Kinshasa. The Belgians had founded the city in 1881, during what historians call “The Scramble for Africa,” when colonial powers carved up the continent into areas of influence. By the early 20th century Kinshasa, then called Leopoldville, was the biggest city in Central Africa, fueled by the dizzying growth of trade with the outside world.
A final, powerful bit of evidence supported the theory that Kinshasa lay at the heart of the epidemic’s early movements.
Scientists studying HIV-1 group M already had found many related varieties, what scientists call subtypes, each with slightly different genetic structures and paths through the world. One, scientists discovered, had traveled east from Kinshasa toward Lake Victoria. One went south to Zambia, Botswana and South Africa. One hopped all the way across the ocean to Haiti, then to the United States and Europe.
Many others traveled not very far at all, staying in the Congo Basin. But as scientists plotted out the genetic histories of these varieties and built an extensive family tree for HIV, they all appeared to have spread from a single explosion, a big bang of the AIDS epidemic: Ground Zero was Kinshasa.