We are unlikely to ever know all the details of the birth of the AIDS epidemic. But a series of recent genetic discoveries have shed new light on it, starting with the moment when a connection from chimp to human changed the course of history.
We now know where the epidemic began: a small patch of dense forest in southeastern Cameroon. We know when: within a couple of decades on either side of 1900. We have a good idea of how: A hunter caught an infected chimpanzee for food, allowing the virus to pass from the chimp’s blood into the hunter’s body, probably through a cut during butchering.
As to the why, here is where the story gets even more fascinating, and terrible. We typically think of diseases in terms of how they threaten us personally. But they have their own stories. Diseases are born. They grow. They falter, and sometimes they die. In every case these changes happen for reasons.
For decades nobody knew the reasons behind the birth of the AIDS epidemic. But it is now clear that the epidemic’s birth and crucial early growth happened during Africa’s colonial era, amid massive intrusion of new people and technology into a land where ancient ways still prevailed. European powers engaged in a feverish race for wealth and glory blazed routes up muddy rivers and into dense forests that had been traveled only sporadically by humans before.
The most disruptive of these intruders were thousands of African porters. Forced into service by European colonial powers, they cut paths through the exact area that researchers have now identified as the birthplace of the AIDS epidemic. It was here, in a single moment of transmission from chimp to human, that a strain of virus called HIV-1 group M first appeared....
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....