Scientists have identified a hominim species with a smallish brain and dish-like face that was likely responsible for spreading genital herpes to humans between three and 1.4 million years ago.
Two herpes simplex viruses infect primates from unknown evolutionary depths.
In modern humans these viruses manifest as cold sores (HSV1) and genital herpes (HSV2).
Unlike HSV1, however, the earliest proto-humans did not take HSV2 with them when our ancient lineage split from chimpanzee precursors around seven million years ago.
Somewhere between three and 1.4 million years ago, HSV2 jumped the species barrier from African apes back into human ancestors - probably through an intermediate hominin species unrelated to humans. Hominin is the zoological tribe to which our species belongs.
Scientists from University of Cambridge and Oxford Brookes University in UK believe may have identified the culprit - Parathropus boisei, a heavyset bipedal hominin with a smallish brain and dish-like face.
In a study published in the journal Virus Evolution, researchers suggest that P boisei most likely contracted HSV2 through scavenging ancestral chimp meat where savannah met forest - the infection seeping in via bites or open sores.
Hominins with HSV1 may have been initially protected from HSV2, which also occupied the mouth, until HSV2 adapted to a different mucosal niche in the genitals, researchers said.
Close contact between P boisei and our ancestor Homo erectus would have been fairly common around sources of water, such as Kenyas Lake Turkana.
This provided the opportunity for HSV2 to boomerang into the human bloodline.
The appearance of Homo erectus around 2 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 Charlotte Houldcroft, a virologist from Cambridge.
"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," she said.
"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," she added.
