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People with red hair who have put up with teasing or “fiery” stereotypes may be pleased to learn that they appear to be winners from an evolutionary perspective. A large genetics study has revealed that, in Europe, the gene for red hair has been actively selected for more than 10,000 years.
The study did not aim to uncover the reasons for the trend, but focused on the broader question of whether human evolution has plateaued since the advent of agriculture. By analysing DNA from nearly 16,000 ancient human remains and more than 6,000 living individuals, the scientists provided compelling evidence that, in fact, biological evolution has continued apace.
The scientists identified 479 genetic variants that appear to have been favoured by natural selection. Genes linked to red hair, fair skin, susceptibility to coeliac disease and variants that lower the chance of diabetes, baldness and rheumatoid arthritis have all become more common in our recent history.
“Perhaps having red hair was beneficial 4,000 years ago, or perhaps it came along for the ride with a more important trait,” the researchers said. Previous research has shown that people with red hair and fair skin can produce vitamin D more efficiently, for instance, which may have conferred survival benefits in northern climates.
Previously only about 21 instances had been identified of genetic traits being multiplied by natural selection, including genes linked to the ability to digest milk in adulthood. The dearth of evidence suggested directional selection had been rare since modern humans arose in Africa about 300,000 years ago and began to split into different population groups around the world.
The latest study, which used an unprecedented number of ancient DNA samples combined with sophisticated computational techniques, revealed that evolutionary selection has driven the spread or decline of hundreds of genes in West Eurasia, and that that selection has accelerated since people transitioned from hunter-gatherer lifestyles to farming.
“With these new techniques and large amount of ancient genomic data, we can now watch how selection shaped biology in real time,” said Dr Ali Akbari, the first author of the study at Harvard University.
The potential benefits of some of the genes selected for appear intuitive. Genes linked to red hair and fair skin “plausibly reflects selection for increased synthesis of vitamin D in regions of low sunlight in farmers with little of it in their diets”, the scientists concluded.
Other trends were harder to interpret. A mutation that is a big risk factor for coeliac disease appeared 4,000 years ago and has became ever more common since. People with this gene variant appear to have had better odds of surviving and passing on their genes to the next generation despite being at risk of an autoimmune disorder.
Similarly, an immune gene called TYK2 that dramatically raises the risk of tuberculosis steadily grew in frequency between 9,000 and 3,000 years ago, before going into decline again. One possibility is that these disease-risk genes may have been helpful in protecting against pathogens that became more common in particular time periods.
The study also found negative selection for combinations of genes that promote high body-fat percentage, which they put down to the “thrifty genes” hypothesis. This suggests that a genetic adaptation to store fat, which would have been helpful for survival during periods of scarcity for hunter-gatherers, became a disadvantage once agriculture led to more reliable food availability.
“This work allows us to assign place and time to forces that shaped us,” said Prof David Reich, a professor of genetics at Harvard Medical School and senior author of the study.
The study focused on evolutionary trends in West Eurasia, where the DNA samples originated, but did not address the question of whether these trends were unique to these populations or were occurring worldwide. The findings are published in Nature.
