AIDS has been around for 100 years, longer than anyone realised until now. Old medical specimens from Africa reveal that HIV crossed from chimps to humans around 1908. It then kept circulating in people because central Africa鈥檚 first-ever cities were created at the same time.
Until now, the only known sample of HIV from before 1976 (the first known case in today鈥檚 global pandemic) came from a patient in 1959, in what is now Kinshasa, in the Democratic Republic of the Congo. Because viruses accumulate mutations over time, the genetic differences between the 1959 sample and more recent ones were used to estimate that HIV started spreading in people around 1930. But the estimate was uncertain because scientists had just one pre-1976 virus.
Then, after Micheal Worobey at the University of Arizona in Tucson and colleagues developed a technique to track HIV鈥檚 invasion of the Americas using wax-embedded tissue samples from the 1960s, they recovered HIV in one taken in Kinshasa in 1960.
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The genes showed that both the 1959 and 1960 viruses came from the same chimp virus that initially invaded humans. But because the differences between them were greater than those between HIV strains that have been diverging for 40 years, the team deduced that these two had been in humans for longer than expected (Nature, ).
鈥淥ur best estimate for when HIV entered humans is 1908, but it could have been from 1884 to 1924,鈥� says Worobey. HIV has long been thought to have repeatedly crossed from chimps to scattered, rural humans over the centuries, but never spread between people until it encountered cities, where an infected person might infect more than one other person, as required for an epidemic. When the team looked at the region鈥檚 political history, they were struck by parallels between HIV鈥檚 spread and population expansion.
The first major cities 鈥� Kinshasa, Douala, Brazzaville, Yaounde, Bangui 鈥� were founded by European colonialists in the late 1800s. Their populations started booming around 1910. 鈥淚 was stunned by the timing,鈥� says Worobey. 鈥淚 would bet that cities, and the high-risk [sexual] behaviours found in them, are necessary to allow one of these sporadic viral jumps to get a toehold in the human population.
鈥淗igh-risk sexual behaviours in cities are necessary to allow one of the viral jumps from chimps to get a toehold in humans鈥�
鈥淭he good news is that this suggests HIV lives right on the edge of extinction,鈥� he says. If education, testing, drugs and prevention efforts make the virus less likely to spread, this might snuff out the epidemic 鈥� maybe even without the elusive HIV vaccine.
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