New genetic evidence suggests that evolution has continued to shape our species powerfully over the past 100,000 years. By looking for signals based on how much DNA mutates over generations, researchers found clues that as much as 10 percent of the human genome may be linked to these recent adaptive genetic changes.
Cornell University population geneticist Scott Williamson and colleagues analyzed over a million genetic variations in DNA samples from 24 individuals, including African Americans, European Americans, and Chinese. They were looking for regions in the genome where a beneficial mutation is carried by everyone in a population. Then, by looking at the variability in the DNA surrounding the mutation, the team could figure out how long ago the mutation spread through the population.
More than a hundred sites in the genome showed strong evidence of recent selection, including genes that affect muscle tissue, hair, hearing, immune-system function, skin pigmentation, sense of smell, and the body’s response to heat stress.
For some of the traits, it’s easy to identify evolutionary pressures that could have favored certain mutations. Immune-function genes are logical targets for selection because, as Williamson explains, “If an individual carries a mutation that provides disease resistance, that confers a clear selective advantage.”
Changes to skin pigmentation pathways probably reflect selective pressures related to sunlight exposure that humans experienced as they spread out from humanity’s origins in Africa to other parts of the world and adapted to local environments. In other cases, such as the hair follicle genes, the forces driving our recent evolution remain a mystery.
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