The place, most likely: southern Cameroon. The estimated time: the 1920s. There and then, a chimpanzee virus hopped into some unlucky person, before making its way to the city now known as Kinshasa. In a populous and growing hub with a multitude of hosts, the new virus gained ground, eventually spawning lineages that spread around the world.

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This long history means that many people must have been infected with HIV before anyone knew what was making them sick. Samples of their tissues were collected by pathologists, treated with a chemical fixative, embedded in wax, examined under microscopes (likely to no avail), and then stored in drawers. That process (and especially the fixative) does horrible things to the RNA molecules that comprise HIV’s genome, “but not horrible enough to destroy them completely,” Worobey told me. “It turns out that you can retrieve [RNA] more than 50 years later, even if the thing has been sitting in a drawer at ambient temperature. Which can be pretty warm in Kinshasa.”

But since HIV hadn’t even been identified, there’s no easy way of telling which old sample might contain traces of the virus. Finding those traces is like looking for a largely corroded needle in a thousand haystacks. Understandably, then, despite two decades of searching, scientists have only twice found traces of HIV from its prediscovery period in Kinshasa. David Ho from Rockefeller University found one in a blood sample from 1959; Worobey identified another in a lymph node from 1960.

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In both cases, barely anything of the virus’s RNA remained—just 1 percent of the total. Those pieces were enough to prove that HIV was circulating in Kinshasa decades before its discovery, and to sketch the outlines of the virus’s history. But since fragmentary evidence can be misleading, Worobey wanted to get a complete historical genome.

His colleagues Sophie Gryseels and Tom Watts developed more sensitive tools for extracting RNA and applied them to more than 1,600 tissue samples from the University of Kinshasa. They found just one with traces of HIV and spent five years pulling every piece of RNA they could from it. “There are very sophisticated evolutionary models you can use to trace back what has happened through history, but they’re still models,” Gryseels told me. “With old genetic material, you can see what reality was actually like.”

The old virus most closely resembled those from subtype C—the most prevalent lineage of HIV, and one that dominates in southern Africa today. But the 1966 virus wasn’t actually part of that group. It was more of a distant cousin, and it suggests that what we see of HIV today is just a small fraction of the total diversity that existed in Kinshasa in the 1960s. Only a few of those historical viruses then broke out, to become global problems.