The researchers say there is evidence that many infectious diseases may have been co-evolving with humans and our ancestors for tens of thousands to millions of years.
So concludes a study published in the American Journal of Physical Anthropology by the universities of Cambridge and Oxford Brookes in the UK.
Tapeworm, tuberculosis, some types of herpes, stomach ulcers and other chronic diseases would have weakened the Neanderthals, impaired their fitness as hunter-gatherers and contributed to their demise, the researchers suggest.
First author Dr. Charlotte Houldcroft, of the Division of Biological Anthropology at Cambridge, says:
“Humans migrating out of Africa would have been a significant reservoir of tropical diseases. 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.”
Both fossil and genetic evidence suggest that Neanderthals and modern humans (Homo sapiens) evolved from a common ancestor between 500,000 and 200,000 years ago.
Neanderthals dominated Europe for tens of thousands of years until during one very cold period, when modern humans spread across Europe. Over just a few thousand years afterward, Neanderthal numbers dwindled to the point of extinction.
Some infectious diseases are much older than we thought
After reviewing the latest evidence from pathogen genomes and DNA from ancient bones, Dr. Houldcroft and colleagues conclude that some infectious diseases are probably many thousands of years older than previously thought.
Fast facts about Neanderthals
- Neanderthals are our closest extinct human relative
- While their bodies were shorter and stockier than ours, their brains were just as large, and often larger
- Neanderthals made and used diverse sophisticated tools, controlled fire, lived in shelters, made and wore clothing, buried their dead and marked their graves with flowers.
Add this to evidence that our human ancestors bred with Neanderthals and swapped disease genes, and that the modern human caught viruses from other hominins before migrating out of Africa, then it follows, argue the authors, that humans relayed the diseases to Neanderthals.
New techniques allow us to study the history of disease from two angles: by analyzing the genetic code of modern diseases, and by looking for disease genes in DNA extracted from fossils of our early ancestors.
The traditional view of infectious diseases is that they arose when humans began farming and living in denser populations alongside livestock some 8,000 years ago.
But Dr. Houldcroft and colleagues say the newer evidence suggests many infectious diseases may have been “co-evolving with humans and our ancestors for tens of thousands to millions of years.”
Also, it is likely that many diseases, such as tuberculosis, thought to have crossed from herd animals into humans may have traveled in the other direction – started in humans and then moved to animals.
Instead, the authors suggest the advent of agriculture and dense, settled populations provided the perfect conditions for diseases that were already present but sporadic in the Neanderthals – who as hunter-gatherers lived in small foraging groups – to spread quickly.
Herpes passed to humans 1.6 million years ago
The researchers suggest herpes simplex 2 – the virus that causes genital herpes – was one of the diseases that modern humans passed to Neanderthals. Information preserved in the viral genome suggests it passed to humans in Africa around 1.6 million years ago.
Another example is Helicobacter pylori, a bacterium that causes stomach ulcers. This likely passed to humans in Africa 88-116 thousand years ago, arriving in Europe with waves of human migration some 52,000 years ago, note the authors. Recent evidence suggests the Neanderthals became extinct some 40,000 years ago.
Some of the newer theories suggest the Neanderthals died out because of climate change, or because humans teamed up with wolves and dominated the food chain. But Dr. Houldcroft concludes:
“It is probable that a combination of factors caused the demise of Neanderthals, and the evidence is building that spread of disease was an important one.”
Earlier this year, Medical News Today also learned that interbreeding with Neanderthals may have strengthened the human immune system.
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