Life-sized sculpture of Neanderthal female at National Archeological Museum of Madrid. Photo: Juan Aunion

Human papillomavirus (HPV) comes in hundreds of different strains, and infects almost everyone at some point in their lifetimes. For most people, the infection passes by without symptoms, due to the evolutionary mechanisms Homo sapiens have developed to combat the virus.

But a particular variety called HPV16 can cause cervical and other cancers. A new study contends that the strain actually hopped to modern humans during sex with Neanderthals and Denisovans, our ancient cousins.

The massive genetic-sequencing project indicates that the microbiology of the virus could tell us a bit about humanity’s family tree, the authors write in the latest issue of Molecular Biology and Evolution.

“Our results suggest that ancestral HPV16 already infected the ancestor of H. sapiens and H. neanderthalensis half a million years ago, and that two main HPV16 lineages codiverged with either human lineage,” they write. “When a population of modern humans migrated out of Africa some 60 to 120 thousand years ago… and interbred with Neanderthal/Denisovan populations in Europe and in Asia, a transfer of sexually transmitted pathogens occurred, in parallel with the genomic introgression.”

The theory was based on 118 full sequences of HPV16 and five main lineage subtypes of it. They reconstructed the evolution of the virus over thousands of generations using computer algorithms, to understand how it changed over time.

One of the best pieces of evidence of the transmission of the cancer-causing HPV from ancient hominids to modern humans is the near-absence of the HPV16A strain in sub-Saharan Africa. Since the Neanderthals and Denisovans apparently spread the germ back to humans after they left Africa, the non-presence of the strain where modern humans began their rise fits the theory, said the scientists, from the Catalan Institute of Oncology and the French National Center for Scientific Research.

The limitation of the method is that there is no viral DNA from ancient HPV strains. The viruses never infect the bones of hominids. The two scientists plan instead to trace further HPV sequences in ancient human skin that may exist in the evidentiary record.

“Oncogenic viruses are very ancient,” said Ignacio Bravo, of the French National Center for Scientific Research. “The history of humans is also the history of the viruses we carry and we inherit. Our work suggests that some aggressive oncogenic viruses were transmitted by sexual contact from archaic to modern humans.”

Interbreeding between modern humans and Neanderthals was first identified in 2010, with the first sequencing of an entire Neanderthal genome from a bone. Since then, the evidence pointing toward an extended period of sex between the two species has emerged. Our extinct cousins currently make up as much of five percent of our genetic makeup – and have been blamed for a litany of modern problems, from depression to allergies.