"Humans migrating out of Africa would have been a significant reservoir of tropical diseases," said lead author, Charlotte Houldcroft from the University of Cambridge's Division of Biological Anthropology, in a statement. "For the Neanderthal population of Eurasia, adapted to that geographical environment, exposure to new pathogens carried out of Africa may have been catastrophic."
Researchers at Cambridge and Oxford Brookes universities analyzed DNA from ancient bones and pathogen genomes. They concluded that some infectious diseases are probably thousands of years older than previously believed.
There is evidence that humans caught viruses from other hominins before moving out of Africa. There is even evidence that our human ancestors mated with Neanderthals
thousands of years ago and exchanged disease-related genes. So researchers argue that it's safe to say humans could have passed diseases on to Neanderthals when they moved into Europe.
"As we now know that humans bred with Neanderthals, and we all carry 2% to 5% of Neanderthal DNA as a result, it makes sense to assume that, along with bodily fluids, humans and Neanderthals transferred diseases," said Houldcroft.
Infections likely to have been passed from humans to Neanderthals include tapeworm, tuberculosis, stomach ulcers and types of herpes, according to the study.
The researchers suggest that herpes simplex 2, which is the virus that causes genital herpes, was passed to humans in Africa about 1.6 million years ago. Helicobacter pylori, a strain of bacteria that causes stomach ulcers, likely infected humans for the first time in Africa at least 88,000 years ago and first arrived in Europe about 52,000 years ago. But recent evidence suggests that Neanderthals became extinct about 40,000 years ago, supporting this new theory that modern humans are responsible for killing off Neanderthals.
Infectious diseases were previously thought to have spread quickly when agriculture developed about 8,000 years ago and humans began living in larger groups surrounded by animals. But new research argues that diseases existed much earlier than previously thought.
"Once agriculture came along, these diseases had the perfect conditions to explode, but they were already around," said Houldcroft.
Researchers say that the infections would not have all happened at once, as they did when Christopher Columbus and other Europeans arrived in the Americas and eradicated indigenous populations. Instead, they would have spread between small groups of about 15 to 30, weakening them and making it nearly impossible to survive.
Many theories exist to explain the demise of the Neanderthal population, including climate change, human cognitive superiority, competition for food due to humans hunting with dogs and wolves, and direct competition and violence between humans and Neanderthals, said Houldcroft.
She believes that the extinction is probably due to a combination of factors.
"Infectious diseases exchanged between humans and Neanderthals are likely to have been just one of many factors making it harder for Neanderthals to survive in Europe alongside modern humans," said Houldcroft. "I don't think any single factor was solely responsible, and we may never know which theory is correct, although we can continue to look for more evidence and try to test different theories."