New research from the University of Cambridge and Oxford Brookes University predicts which species acted as an intermediary between the ancestors of Homo sapiens and those of chimpanzees to carry the herpes simplex virus 2 (HSV2) — a human herpesvirus found worldwide that causes genital lesions and more rarely causes encephalitis — across the species barrier.

Two herpes simplex viruses infect primates from unknown evolutionary depths. In modern humans these 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 7 million years ago. Humanity dodged the genital herpes bullet — almost.

Somewhere between 3 and 1.4 million years ago, HSV2 jumped the species barrier from African apes back into human ancestors.

Now, Dr. Charlotte Houldcroft of the University of Cambridge and co-authors believe they may have identified the culprit: the Plio-Pleistocene hominin Paranthropus boisei, also known as the ‘Nutcracker Man’ because of his enormous, flat, thickly enameled cheek teeth, a robust skull and massive, powerful chewing muscles.

“Paranthropus boisei most likely contracted HSV2 through scavenging ancestral chimp meat where savannah met forest,” the researchers said.

“Hominins with HSV1 may have been initially protected from HSV2, which also occupied the mouth. That is until HSV2 adapted to a different mucosal niche.”

Close contact between Paranthropus boisei and our ancestor Homo erectus would have been fairly common around sources of water, such as Kenya’s Lake Turkana. This provided the opportunity for HSV2 to boomerang into our 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 HSV2 — only this time it was Paranthropus boisei being devoured.

“Herpes infect everything from humans to coral, with each species having its own specific set of viruses,” Dr. Houldcroft said.

“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.”

“By modeling the available data, from fossil records to viral genetics, we believe that Paranthropus 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.”

The study is published in the journal Virus Evolution.

_____

Simon J. Underdown et al. 2017. Network analysis of the hominin origin of Herpes Simplex virus 2 from fossil data. Virus Evolution 3 (2): vex026; doi: 10.1093/ve/vex026