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Our ancestors may have doomed Neanderthals by giving them herpes and tapeworms

An exhibit shows the life of a Neanderthal family in a cave in the new Neanderthal Museum in Krapina, Croatia. (Nikola Solic/Reuters)

Neanderthals went extinct about 40,000 years ago. We're not exactly sure what led to their demise, but climate change and competition over prey from Homo sapiens all may have played roles.

Another thing that may have helped kill off the Neanderthals in Europe? Infectious diseases, carried by humans who left Africa and made their way to Europe.

That's according to researchers at Cambridge and Oxford Brookes universities who analyzed ancient DNA and pathogen genomes. They published their findings Sunday in the American Journal of Physical Anthropology.

Humans and Neanderthals may have interbred 50,000 years earlier than previously thought

"Humans migrating out of Africa would have been a significant reservoir of tropical diseases," lead author Charlotte Houldcroft of Cambridge's Division of Biological Anthropology, said in a release. "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."

There's no hard evidence so far showing that our ancestors passing these diseases on to Neanderthals. Rather, the researchers assert that the diseases must have been transferred, given the timeline and geography of human migration and what pathogen genomes tell us about the ancestry of disease. There's also evidence showing that humans and Neanderthals interacted, and even mated, with each other at least 60,000 years ago.

"As we now know that humans bred with Neanderthals, and we all carry 2 to 5 percent of Neanderthal DNA as a result, it makes sense to assume that, along with bodily fluids, humans and Neanderthals transferred diseases," Houldcroft said.

Bad heart to bad habits: Blame your Neanderthal DNA

Tapeworms, the genital herpes virus and tuberculosis may have all made their way into Europe, courtesy of humans, according to the paper.

The research suggests some infectious diseases are actually much older than previously believed. Helicobacter pylori, which causes stomach ulcers, is estimated to have first infected humans in Africa at least 88,000 years ago and first arrived in Europe 52,000 years ago. Herpes simplex 2 was transmitted from unknown hominins to humans 1.6 million years ago, the virus's genome suggests.

As agriculture developed roughly 8,000 years ago, humans stayed put in larger groups living closely with animals -- which led to an explosion of infectious diseases. But these researchers say that the ancestors of pathogens preceded this disease explosion.

Neanderthals went extinct before this transition into an agrarian lifestyle. As hunter-gathers, they tended to live in groups of 15 to 30, Houldcroft said. "So disease would have broken out sporadically, but have been unable to spread very far," she said. "Once agriculture came along, these diseases had the perfect conditions to explode, but they were already around."

The way disease affected these European Neanderthals probably wasn't like how disease carried by Christopher Columbus and early Europeans wiped out native populations in the Americas. A combination of factors contributed to the Neanderthal downfall, "and evidence is building that spread of disease was an important one," Houldcroft 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," Houldcroft said.