In this week’s Science, researchers led by Michael Worobey of the University of Arizona and Preston Marx of the Tulane National Primate Research Center looked at the history of simian immunodeficiency virus (SIV)—the primate precursor to HIV—and found that the disease may be thousands of years older than scientists originally suspected. The new study estimates that SIV has been in monkeys and apes in Africa for at least 32,000 years—not just a few hundred years, as many had suspected. That means that human beings have likely been exposed to the virus for a long time as well, every time hunters killed primates for food (something that’s sadly still done). And the new information raises a new question in turn: if human beings had encountered, and likely even been infected, by SIV for thousands of years, why did the virus only move full-bore into humans and mutate into the mass killer HIV in the 20th century? What changed?
For such a big question, the study itself was simple. The team tested 79 monkeys from Bioko, an island off the West African coast, chosen because the island was cut off from the African mainland by rising seas 10,000 years ago at the end of the last ice age. Of the six monkey species that had developed in isolation from the mainland in the years since, four—drills, red-eared guenons, Preuss’s guenons and black colobuses—has individuals who carried SIV. (SIV is less deadly to monkeys than HIV is to human beings.) Though the four different SIV strains the four species carried were genetically different from each other—meaning they hadn’t been transmitted from monkeys carried over from the African mainland by human beings. But because each strain was similar to SIV strains infecting monkeys of the same genus on the mainland, the Science team inferred that the SIV strains had existed before Bioko was cut off from the African mainland. Using a molecular clock—which measures how fast a virus mutates—they estimated SIV emerged 32,000 to 78,000 years ago, as Marx told Donald McNeil of the New York Times:
When we only had 25 years of data, we were dating from the tip at the end of a branch of the evolutionary tree…I knew that what we had before couldn’t be right, because the virus had spread from the Atlantic to the Indian Ocean to the southern end of the continent, and it couldn’t have done that in a couple of hundred years.
If HIV had established itself in human beings hundreds of years ago—at least in its current lethal form—we would have known about it, and some of the millions of Africans forcibly taken into slavery would have carried the disease to the Americas. That didn’t happen, and the disease didn’t appear in people until the 20th century, with the first confirmed case found in blood drawn in 1959 from a man in Kinshasa. As Marx went on to say:
Something happened in the 20th century to change this relatively benign monkey virus into something that was much more potent and could start the epidemic. We don’t know what that flashpoint was, but there had to be one.
What happened? Marx told McNeil that he believes, ironically, that a key event might have been the introduction of cheap syringes into Africa during campaigns to wipe out other infectious diseases during the middle of the 20th century. The syringes were often reused, and infected blood could have easily moved from one person to another—just as shared needles have spread HIV among drug users.
But Arizona’s Worobey believes HIV likely emerged earlier in the 20th century, well before syringes were common in Africa. In this case, the difference might have been the growth of crowded cities in Africa. Before 1910—around when Worobey believes the virus emerged—no town in Central Africa had more than 10,000 people. That has changed incredibly over the past century as Africans have moved from the countryside into cities—Lagos, the capital of Nigeria, has a metro population of more than 15 million, and it’s growing ten times faster than New York. Megacities mean more people coming into contact with each other—and it means the chance for more red-light districts and risky behaviors of the sort that aren’t easy to carry out in rural areas.
Of course, urban areas have always played an outsized role in the spread of infectious disease—as former TIME editor Karl Greenfeld writes in China Syndrome, his book on the SARS, it wasn’t until the late 19th century that cities infectious disease deaths in cities dropped below the replacement rate. (Before then, cities like London would have shrunk due to disease deaths without the constant influx of new residents.) Add in the spread of air travel networks—which played a major part in the spread of both HIV and SARS—and you can see why new infectious disease could become more frequent in the future. A new disease is much more likely to find new hosts—and with those hosts, spread further—in an urbanized, globalized world than ever before.