Human-Borne Diseases May Have Contributed to Neanderthal Extinction

Humans have become the most dominant hominid species on the planet, but it wasn't always that way. Some 45,000 years ago, anatomically modern humans migrated from Africa into Europe and Asia, infringing on the territory of their Northern cousins, the Neanderthals. While the hominids briefly interbred, the net outcome of human expansion turned out to be disastrous for Neanderthals. Their numbers dwindled rapidly relative to the interlopers, and were edged into extinction roughly 40,000 years ago.

Multiple theories attempt to account for why intermingling with humans spelled doom for the Neanderthals, with explanations ranging from competition over resources to all-out interspecies warfare.

Now, new research published in the American Journal of Physical Anthropology suggests that human-borne diseases may have also played a major role in the downfall of Homo neanderthalensis. While there is no empirical evidence for disease transmission between the groups—yet—it stands to reason that our forebears carried pathogens from their native African turf with them as they swept through Europe. All sorts of fun infectious creatures like tapeworms, tuberculosis, and ulcer-inducing bacteria likely hitchhiked straight into Neanderthal range via their human hosts.

"Humans migrating out of Africa would have been a significant reservoir of tropical diseases," said Cambridge University infectious disease expert Charlotte Houldcroft, co-author of the new study, in a statement. "For the Neanderthal population of Eurasia, adapted to that geographical infectious disease environment, exposure to new pathogens carried out of Africa may have been catastrophic."

Indeed, having acquired no resistance to these exotic African ailments, Neanderthals would have been particularly vulnerable to epidemics. The scenario is somewhat analogous to modern pandemics, like the devastating death toll among indigenous New World populations in the wake of European colonization, though Houldcroft cautioned that the latter example was much more extreme.

"[I]t is unlikely to have been similar to Columbus bringing disease into America and decimating native populations," she said. "It's more likely that small bands of Neanderthals each had their own infection disasters, weakening the group and tipping the balance against survival."

Even so, the fact that the two hominids regularly knocked each other up demonstrates that they were in very close contact, making disease propagation more likely. Indeed, Houldcroft and human evolution researcher Simon Underdown, who co-authored the new research, speculate that humans may have also gifted Neanderthals with sexually transmitted diseases like genital herpes.

"The 'intermediate' hominin that bridged the virus between chimps and humans shows that diseases could leap between hominin species," Houldcroft noted. "The herpes virus is transmitted sexually and through saliva. As we now know that humans bred with Neanderthals, and we all carry [two to five percent] of Neanderthal DNA as a result, it makes sense to assume that, along with bodily fluids, humans and Neanderthals transferred diseases."

It's probable that a combination of pressures were ultimately responsible for the extinction of the Neanderthals—a death of 1,000 anthropogenic cuts, if you will. Genomic analysis will continue to elucidate precisely how human-borne diseases contributed to the die-off, but suffice it to say, they likely had a significant effect on our ill-fated, bygone cousins.