'Human evolution didn't slow down; we were just missing the signal': Large DNA study reveals natural selection led to more redheads and less male-pattern baldness

Over the past 10,000 years, natural selection has contributed to the evolution of nearly 500 genes in the DNA of West Eurasians, affecting their looks and susceptibility to different illnesses, a new study finds.

Natural selection in this group led to an increased frequency of light skin tone, red hair, and resistance to HIV and leprosy (also called Hansen's disease), and it decreased the frequency of male-pattern baldness and rheumatoid arthritis susceptibility, the new study of 16,000 genomes reveals. This discovery contradicts the long-standing view that recent human evolution was limited.

"Human evolution didn't slow down; we were just missing the signal," study first author Ali Akbari, a staff scientist at Harvard University, told Live Science in an email.

Evolutionary change can occur through a variety of mechanisms, including mutation; natural selection, in which traits that are advantageous to survival are passed to offspring; gene flow, in which genetic material is mixed between populations; and genetic drift, in which the frequency of a gene in a population changes due to random chance.

In a study published Wednesday (April 15) in the journal Nature, Akbari and colleagues developed a new statistical method to identify natural selection over an 18,000-year period in thousands of ancient and modern genomes from people living in West Eurasia, an area encompassing Europe and parts of western Asia, like Turkey.

"Previous work, based on the scars that natural selection leaves in present-day genomes, led to the view that directional selection was rare," Akbari said. But with large datasets like the one that the researchers amassed and methods that can separate the signal of natural selection from other evolutionary processes, "we can now detect small, consistent changes over time," he explained.

The researchers found evidence of natural selection in 479 gene variants in the West Eurasian genome dataset, 60% of which correspond with known traits in present-day people. Some of the gene variants that were strongly positively selected for are involved in expressing traits such as light skin tone, red hair, resistance to HIV and leprosy infections, and the B blood type. They also discovered genes related to a lower chance of male-pattern baldness and a lower risk of rheumatoid arthritis.

The results suggest that all of these variants were useful in the evolution of modern West Eurasian people — but the DNA does not help to explain exactly why these traits were useful. The increase in the frequency of light skin pigmentation probably reflects selection for increased synthesis of vitamin D in regions of low sunlight, the researchers wrote in the study. But it's harder to explain the rise in redheads. It is possible that red hair itself was not beneficial but rather that the genes for the trait are also associated with a more important adaptation.

Some traits were positively or negatively selected at different times, the researchers found. For several millennia, genes for tuberculosis susceptibility increased in frequency and then decreased around 3,500 years ago. Similarly, genes for susceptibility for multiple sclerosis increased until about 2,000 years ago and then decreased in frequency.

"This likely reflects changes in environment or selective pressures over time; for example, the introduction of new pathogens," Akbari said.

The researchers have made their data and methods — called AGES (Ancient Genome Selection) — freely available so that other scientists can expand on this work. Akbari said the research team now plans to explore other groups outside West Eurasia to better understand how the global human population evolved. They have already posted a preprint of a study investigating East Eurasia, which looked at people with East Asian ancestry; this research found similar patterns, Akbari said.

"What is likely to differ across regions is not whether selection occurred, but how local environments and cultural changes shaped it, including factors like diet, pathogens, and climate," Akbari said. "Extending this approach more broadly will help us understand how different historical pressures influenced human biology in different settings."

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