Disorders of the human brain, such as neurological or psychiatric diseases, have long been known to run in families, suggesting that they are passed down from generation to generation.
In accordance with this hypothesis, genetic risk factors for the development of these diseases have been identified. However, fundamental questions about evolutionary factors have remained elusive. In other words, why aren’t genetic variants that increase disease risk removed during evolution?
New discoveries about events in the deep human past have given scientists new tools to begin to unravel these mysteries: When modern humans moved out of Africa 60,000 years ago, they met and mixed with other humans archaic, such as the Neanderthals.
About 40% of the Neanderthal genome is still found in non-Africans today, and each individual carries an additional 2% of Neanderthal DNA.
2% of Neanderthal DNA, still present in our DNA
Some of the archaic genetic variants may have conferred benefits at some point in our evolutionary past. Today, scientists can use this information to learn more about the impact of these genetic variants on human behavior and the risk of developing disease.
Using this approach, a new study by an international team led by researchers from the University of Tartu, Charité Berlin and Amsterdam UMC analyzed the associations of Neanderthal DNA with a wide variety of brain conditions and traits such as sleep, smoking or drinking of alcohol, with the goal of narrowing down the specific contribution of DNA to variation in the behavioral characteristics of humans today.
The study found that while Neanderthal DNA showed a disproportionate number of associations with several traits that are associated with diseases of the central nervous system, the diseases themselves did not show a significant number of associations with Neanderthal DNA, they write EurekAlert.
Among the traits with the strongest contribution from Neanderthal DNA are smoking habits, alcohol consumption and sleeping habits. Using data from other cohorts several of these results could be replicated. Of particular note were two independent Neanderthal variants at maximum risk for a positive smoking status.
Neanderthals, to blame for smoking?
“Our results suggest that Neanderthals carried multiple variants that substantially increase the risk of smoking in humans today. It remains unclear what phenotypic effects these variants had. However, the results provide interesting candidates for further functional tests and will help us in the future to better understand the specific biology of our ancestors,” said Michael Dannemann, associate professor of evolutionary genomics at the University of Tartu and lead author of this study.
“Significant associations of Neanderthal DNA with drinking and smoking habits could help us unravel the evolutionary origin of addictive and reward-seeking behaviors,” added Stefan M Gold, professor of neuropsychiatry at Charité, Berlin. who co-directed this study, published in Translational Psychiatry.
“It is important to note that sleep problems, alcohol consumption and nicotine have been consistently identified as common risk factors for a number of neurological and psychiatric disorders. On the other hand, there are some intriguing findings from anthropology that have suggested some social benefits of greater tolerance to these substances in hunter-gatherers. Thus, our findings support the hypothesis that it is not brain diseases themselves that have evolutionary explanations, but that natural selection shapes the traits that make us vulnerable to them in the modern context.”