Analysis of roughly 16,000 ancient and modern genomes revealed an increase in the frequency of genes linked to lighter skin pigmentation, , resistance to HIV and leprosy (Hansen’s disease), and a decrease in the frequency of genes tied to male‑pattern baldness and higher susceptibility to .
How the researchers tracked natural selection across millennia
In a paper published in Nature, lead author Ali Akbari, a research fellow at Harvard University, explains that the team developed a new statistical approach called AGES (Ancient Genome Selection) that can detect signals of natural selection over time spans up to about 18,000 years in thousands of genomes from Western Eurasia (Europe and parts of Western Asia, including Turkey).
Evolutionary change can happen in several ways: mutations, natural selection (when traits that improve survival get passed to descendants), gene flow (mixing genetic material between populations), and genetic drift (random changes in gene frequencies). Akbari says that when researchers looked only at modern genomes in the past, they often concluded that directional selection was rare. But large datasets and methods that separate selection signals from other evolutionary processes now make it possible to detect small but consistent changes over time.
What the study found
- Researchers found evidence of natural selection in 479 genetic variants in the sample of Western Eurasian genomes — about 60% of those variants correlate with known traits in modern humans.
- Strong positive selection appeared for linked to lighter skin tone, red hair, blood group B, and increased resistance to HIV and leprosy.
- At the same time, the team observed selection that reduced the frequency of genes associated with male‑pattern baldness and greater susceptibility to rheumatoid arthritis.
The data indicate that these variants were advantageous in the evolution of modern Western Eurasian populations, but the DNA markers themselves don’t answer why those traits proved useful. For example, researchers explain the rise in alleles associated with lighter skin pigmentation as likely related to enhanced vitamin D synthesis in regions with low sunlight. The increase in red hair is harder to explain: the red hair color itself may not have provided a direct advantage, and the genes for red hair could have hitchhiked alongside a more important adaptation.
Selection priorities shifted across different eras
The authors also documented that selection on specific variants could change across different time periods. For example:
- Genes that increased susceptibility to tuberculosis rose in frequency for a long period and then began to decline around 3,500 years ago.
- Variants associated with susceptibility to multiple sclerosis increased in frequency until about 2,000 years ago and then decreased.
Akbari says those fluctuations likely reflect changes in the environment and selective pressures over time — for instance, the emergence of new pathogens or shifts in lifestyle and diet.
Next step: broader regional coverage
The researchers have made the data and the AGES method publicly available so other scientists can apply the approach to different regions. The team has already published a study on East Eurasia that found similar overall patterns.
“Differences between regions will probably be less about whether selection occurred and more about how local environments and cultural changes shaped it — through diet, pathogens, climate, and so on,” Akbari said. Applying the method to other historical and geographic contexts will help clarify which factors influenced human biology in different places and times.
“Human evolution hasn’t slowed — we were just missing the signal,” Ali Akbari told Live Science.
The study offers a new way to evaluate the pace and nature of recent and shows that even small, consistent changes over millennia leave a clear mark on the genome.
Photo: Unsplash
