Europeans, in their conquest of the world, brought disease to the Americas. Along with war and enslavement, this proved too much for some civilizations, nearly or completely wiping them out.

Remarkably, the same fate may have befallen Neanderthals, who were wiped out as our ancestors spread out of Africa and made their home in Europe. As a new study in the American Journal of Physical Anthropology reveals, Homo sapiens probably brought with them diseases that would have at the very least contributed to the demise of their evolutionary cousins.

The ultimate reason, or reasons, for the disappearance of the Neanderthals remains one of the greatest scientific mysteries of all time, and everything, including being outsmarted by H. sapiens, has been suggested as a possible explanation. It’s remarkable to think that, as the last of them reached their end around 40,000 years ago, they could have been pushed into extinction with the help of human-harbored pathogens.

“Humans migrating out of Africa would have been a significant reservoir of tropical diseases,” Dr. Charlotte Houldcroft, a geneticist at Cambridge's Division of Biological Anthropology and co-author of the study, said 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.”

As humans began to migrate out of Africa and up towards the Levant and mainland Europe, they would have brought with them diseases they would have naturally evolved some resistance to. By looking at skeletal, archaeological and genetic evidence from modern humans and our migrating ancestors, the team of researchers think they have discovered which ones may have made their way from Africa up to the Neanderthal homelands.

^{An electron micrograph image of H. pylori. Yutaka Tsutsumi/Wikimedia Commons; Copyrighted Free Use}

One commonplace bacterial species was Helicobacter pylori, which is responsible for stomach ulcers. Evidence suggest that its first human infection likely occurred in Africa around 100,000 years ago. The herpes simplex 2 virus, which was likely transmitted to humans in Africa 1.6 million years ago from another currently mysterious hominin species, would also have migrated northwards along with humanity.

Although Neanderthals likely put up some resistance to the emergence of humans in what is now the Middle East, H. sapiens eventually managed to infiltrate the continent, co-existing with Neanderthals and even breeding with them. This would have facilitated the spread of both pathogens to pockets of Neanderthal populations whose immune systems had not yet experienced these diseases. Over time, this would have led to a reduction in their numbers.

Although the study yields no direct evidence of human-to-Neanderthal transmission of these diseases, the probability of this occurring, according to the authors, is overwhelmingly likely.

“However, it is unlikely to have been similar to Columbus bringing disease into America and decimating native populations [suddenly],” Houldcroft noted. Neanderthals lived in small groups, so once one group died from infection, it could not spread any further; this meant that the effect of human-borne disease would have been very gradual.

By the time agriculture proliferated around 8,000 years ago after the decline of the last ice age, these types of diseases spread effortlessly between interconnected human populations. By this point, however, Neanderthals were long gone, although disease looks likely to have played a role in their extinction.