The herpes simplex virus affects us modern humans in two forms — as cold sores (HSV1) and as the more insidious genital herpes (HSV2). But during the early stages of our evolution about 7 million years ago, the branch of hominids that eventually led to Homo sapiens had left behind the HSV2 strain completely.

How, then, did this now-widespread sexually transmitted virus (over one of every six people between the ages of 14 and 49 in the United States has genital herpes, according to the Centers for Disease Control and Prevention) manage to become so prevalent among modern humans? In a paper published Sunday, scientists think the answer lies with an extinct distant ancestor called Paranthropus boisei.

That P. boisei is not even from the genus Homo is enough evidence that this proto-human was not directly a part of the lineage that led to our own species. However, we share almost 99 percent of our DNA with chimpanzees, and P. boisei evolved roughly 2.7 million years ago, long after our ancestors had already split from the branch that went on to evolve into our closest living relatives.

Scientists use the unofficial term hominins to refer to all the early proto-human species and those that followed them following the split from what were at the time ancestors of modern chimpanzees. So, by that classification, P. boisei is, like H. sapiens, a hominin. Hominids, on the other hand, is a collective that refers to all the great apes taken together, which includes us and all other extinct species of the genus Homo, chimpanzees and bonobos, gorillas, and orangutans.

P. boisei walked on two feet, was heavyset and had a brain that was roughly 40 percent the size of a modern human’s brain. The British researchers who published the study suggest in their paper it contracted HSV2 while feasting on meat of chimpanzee ancestors, the infection carrying over either through bites or through open sores on the body.

However, HSV2 must have been restricted to the mouth mostly, where HSV1 already resided in many hominins, making them resilient to HSV2. But that changed when HSV2 found a more conducive home for itself and “adapted to a different mucosal niche” in the genitals. The actual human ancestors around at the time, likely H. erectus, probably came in frequent contact with P. boisei in places like watering holes. And that contact — undoubtedly sexual in nature on occasion — allowed HSV2 to make its way back into the lineage, the scientists suggest, adding that H. erectus likely hunted and consumed P. boisei, ingesting HSV2 in the process.

“Herpes infect everything from humans to coral, with each species having its own specific set of viruses. For these viruses to jump species barriers they need a lucky genetic mutation combined with significant fluid exchange. In the case of early hominins, this means through consumption or intercourse — or possibly both,” senior author of the study Charlotte Houldcroft, a virologist from the University of Cambridge’s Department of Archaeology, said in a statement Sunday.

To arrive at their conclusion, the researchers modeled a large number of possibilities for the transmission of HSV2 from chimpanzees to the many hominins that roamed Africa at the time. They looked at climate data, changes in topography, fossil locations and geography to finally narrow down on P. boisei as the culprit who is indirectly responsible for giving genital herpes to anyone who has it today.

The paper also suggests another transmission route. A direct Homo ancestor, H. habilis, may have been first infected by HSV2 from ancient chimpanzees, primarily through butchering and consumption of meat. It was H. habilis that, mainly through sexual contact, passed the virus on to P. boisei, which in turn infected H. erectus.

“Once HSV2 gains entry to a species it stays, easily transferred from mother to baby, as well as through blood, saliva and sex. HSV2 is ideally suited to low-density populations. The genital herpes virus would have crept across Africa the way it creeps down nerve endings in our sex organs — slowly but surely,” Houldcraft said.

The paper, titled “Network analysis of the hominin origin of Herpes Simplex virus 2 from fossil data,” was published online in the journal Virus Evolution.