Genital herpes infects about one in six American adults. But who was patient zero, the individual responsible for this irritating scourge? Researchers in England believe they’ve found him, or at least his species: Paranthropus boisei, a heavyset, bipedal hominin likely passed the first case of genital herpes to our ancient ancestors.

The new study, published in Virus Evolution, has identified when and how the first human ancestor likely contracted genital herpes. The team used fossil data showing where and when ancient people were likely present in Africa, the estimated range for ancestral chimpanzees in the tropical rainforest, and viral genetics to model the history of the virus.

They already knew that HSV2, the virus responsible for genital herpes, probably entered early humans before they left Africa. And that initial entry would have enabled its spread to wherever they migrated. But no one knew exactly when or how the virus traveled from chimpanzees to humans. 

Our ancient hominin ancestors likely had HSV1, the virus associated with mouth sores, for far longer. That virus was a hand-me-down from when humans first separated from chimpanzees. But genital herpes, caused by HSV2 didn't make the leap at that time.

Extensive data crunching led them to P. boisei, a now extinct hominin—an ancient group that included Homo sapiens (i.e.., us)—as the vessel for the virus's entry into humans. The initial infection would have occurred between 3 and 1.4 million years ago. “Paranthropus boisei was the most critical intermediate host for transmitting HSV2 between anc-chimps and the ancestors of Homo sapiens,” the study authors write. 

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Simon Underdown, an anthropologist at Oxford University and lead study author, told Newsweek that just one infected ancient human ancestor could have caused this virus to spread throughout the entire species. “We know a lot of these species did not have large population sizes and from a biological point of view it would only need one infection to jump across,” said Underdown.

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But how did the virus, stuck among our ancient chimpanzee cousins, make the leap into humans (especially considering that today HSV2 is usually transmitted through sexual intercourse)? Underdown thinks that eating infected meat was the culprit. He and his team theorize that P. boisei must have become exposed through hunting or scavenging infected chimp meat—one particular scavenger may have had open wounds or been bitten.

And P. boisei was probably spending time in the same places as Homo erectus, our ancestor, which emerged about 2 million years ago. Lake Turkana, in Kenya, has evidence of both populations from around the same time. Homo erectus did a great deal of hunting and butchering, and are the first known species that looked recognizably human.  Underdown and colleagues think that Homo erectus hunted and ate an infected P. boisei, setting the stage for many early-relationship confessions to come. 

Underdown doesn't think the initial infection was sexually transmitted because. It's highly unlikely that humans would have found the P. boisei “sexually appealing,” he says. 

Following the initial infection, HSV2 likely spread from the mouth to the genitals through touch, perhaps from urinating or scratching. And once the virus found a home with humans, it stayed.

The study offers a unique look at how the most common sexually transmitted disease first entered our population. The disease is particularly widespread in central and east Africa, a region with close ties to human origins. Although most individuals with HSV2 infections never show any symptoms, the virus can sometimes cause painful blisters in the genital area. There is no cure for the virus, but symptoms can be managed with medication.

The work also offers a glimpse into the lives of our ancient ancestors. “What we’ve been able to do is essentially reconstruct an event that happened in the day of life of one of our ancestors,” says Underdown. “It really fleshes out what our ancestors were like.”

Genital herpes is likely no one’s favorite topic, but the researchers aren't stopping there. With their innovative method, they also hope to tracing the origins of pubic lice—introduced through contact with gorillas more than 3 million years ago—and human papillomavirus, which may have come from Neanderthals.