Scientists have identified the ancient hominin species that gave early humans genital herpes two million years ago.
Parathropus boisei was a heavyset human-like species that walked on two legs with a smallish brain and dish-like face.
It likely contracted the virus after eating infected ancestral chimpanzees, and then passed the pathogen onto us when hunted by Homo erectus for food.
Close contact between P. boisei and our ancestor Homo erectus would have been fairly common around sources of water, such as Kenya's Lake Turkana, the researchers found.
This provided the opportunity for the genital herpes virus to shift onto our bloodline.
"Once this virus gains entry to a species it stays, easily transferred from mother to baby, as well as through blood, saliva and sex," said study coauthor Dr Charlotte Houldcroft, from the University of Cambridge.
"The genital herpes virus would have crept across Africa the way it creeps down nerve endings in our sex organs - slowly but surely."
Somewhere between three and 1.4 million years ago, genital herpes jumped the species barrier from African apes into human ancestors.
Scientists have previously speculated that the virus made the leap via an intermediate hominin species unrelated to humans.
Now, a team of scientists from Cambridge and Oxford Brookes universities have found that Parathropus boisei is to blame.
They suggest P. boisei most likely contracted the virus while eating ancestral chimp meat in an area where the African Savannah met surrounding forest.
The infection, which at the time came through the mouth rather than the genitals, probably seeped in via bites or open sores.
Hominins infected with cold sores may have initially been protected from this virus, known as HSV2.
But it quickly "adapted to a different mucosal niche' to survive, the scientists said.
That mucosal niche was found in the genitals, causing the virus to move away from mouth-based infections toward the STI we know today.
The appearance of Homo erectus around two million years ago was accompanied by evidence of hunting and butchery.
Once again, consuming 'infected material' would have transmitted the virus - only this time it was P. boisei being devoured.
"Herpes infect everything from humans to coral, with each species having its own specific set of viruses," said Dr Houldcroft.
"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."
Dr Houldcroft and her team used data ranging from fossil finds to herpes DNA and ancient African climates to come to their conclusion.
They input this data into a computer programme that modelled HSV2 transmission probabilities for the hominin species that roamed Africa three million years ago.
The researchers found the species with the highest transmission probability was Parathropus boisei - a genetic fit virally who was found in the right places to be the herpes intermediary.
"By modelling the available data, from fossil records to viral genetics, we believe that Parathropus boisei was the species in the right place at the right time to both contract HSV2 from ancestral chimpanzees, and transmit it to our earliest ancestors, probably Homo erectus."