Humans migrating out of Africa likely spread disease to Native Europeans and Asians, contributing to their extinction, according to a new study.
Similar to how Native Americans had no evolved immunity for diseases brought over by Europeans, Neanderthals and other archaic humans from Europe and Asia are believed to have had no evolved immunity for diseases brought over by migrating anatomically modern humans from Africa.
"Humans migrating out of Africa would have been a significant reservoir of tropical diseases," co-author Charlotte Houldcroft said in a press 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."
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Houldcroft, from the University of Cambridge's Division of Biological Anthropology, and her colleagues reviewed the latest evidence gleaned from pathogen genomes as well as from ancient bone DNA. The scientists concluded that some infectious diseases are likely to be many thousands of years older than previously believed. Their findings are reported in the American Journal of Physical Anthropology.
The longstanding view about the evolution of major human diseases has been that most emerged and spread with the dawn of agriculture around 8,000 years ago. The scientists, however, determined that there was a much longer "burn in period" for human diseases that pre-dates agriculture.
Houldcroft explained that genetic data shows many infectious diseases have actually been "co-evolving with humans and our ancestors for tens of thousands to millions of years."
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What's more, numerous diseases traditionally thought to be "zoonoses," (transferred from animals into humans), turn out to have human origins.
The scientists suspect that the migrating humans brought with them tapeworm, tuberculosis, stomach ulcers and types of herpes that could have all spread to Neanderthals. Prior research shows that modern humans interbred with Neanderthals, proving that individuals from these groups were in close contact with each other at least for certain periods.
While these illnesses are not necessarily fatal, the researchers believe that the Neanderthals lacking immunity could have grown weak and less able to find food, which could have catalyzed their extinction.
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It does not appear that Neanderthals spread disease to the humans from Africa. It was just the opposite. The interbreeding benefited our species since we received genetic components through interbreeding that protect against types of bacterial sepsis (blood poisoning occurring from infected wounds) and encephalitis caught from ticks that inhabit Siberian forests.
The interbreeding means that a small percentage of Neanderthal DNA lives in all people today of European and Asian heritage. As a result, Neanderthals were absorbed into the modern human population. As a physically and culturally distinct group, however, the Neanderthals died out.
Recent studies indicate that some early Native American lineages might have died out too, following European colonization of the Americas. Comparisons to what happened to the Native Americans only go so far, though, given that Neanderthals are thought to have lived in small groups spread out over large distances.
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"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.
In addition to shedding light on what could have happened to the Neanderthals, the research also indicates what sort of health troubles are early ancestors must have battled. Helicobacter pylori, a bacterium that causes stomach ulcers, is estimated to have first infected humans in Africa 88 to 116 thousand years ago, and arrived in Europe after 52,000 years ago.
Herpes simplex 2, the virus that causes genital herpes, also was around then. The genetic investigations reveal that it probably first transmitted to humans in Africa 1.6 million years ago from another, currently unknown human species.