As populist politicians—and the neocreationists who love them—continue to challenge the teaching of evolution in public schools, scientists at the University of Chicago have found evidence that not only indicates that the human genome is a product of evolution, but also suggests it's actually still undergoing evolutionary change.
The researchers identified hundreds of areas of the human genome that appear to have been positively selected over the past 10,000 years but have not yet reached "fixation"—the point where they are shared in the same form by all members of a population. The study, published in the online journal PLoS Biology, confirms that evolutionary mechanisms have continued to operate on humans in the relatively recent past.
Advertisement
Some examples of the favored gene regions are areas known to affect skin pigmentation, help the body resist malarial infection and aid in metabolizing carbohydrates and digesting the lactose in milk. The scientists speculate that these genes were favored because they helped humans adapt to different regional challenges as they spread out of Africa, a migration that first began about 100,000 years ago.
"Each of these kinds of changes likely resulted in powerful selective pressures for new genotypes that were better suited to the novel environments," the team wrote in their paper.
The researchers looked at genetic data from a sample of 209 people, drawn from three genetically distinct populations: East Asia, Africa and Europe. They found that in each group, different sets of genes had been selected for, with only 20% of the total number of favored genes overlapping across all three.
"We're identifying genetic variances that produce some kind of phenotypic change," said Jonathan Pritchard, a geneticist at the University of Chicago and the lead author of the study. "They must be fairly important to have been selected for."
According to Pritchard, the shift from hunter-gatherer societies to farming and herding communities may have been the most powerful force causing selection in people's genes. Where animal husbandry was common, genes allowing adults to digest lactose conferred an advantage on carriers. Humans with this adaptation are unusual among mammals in general, most of which lose their ability to process lactose after they reach maturity.
Besides forcing humans to digest new types of foods, farming and herding also caused them to live closer together, Pritchard said, increasing the risk of epidemic and therefore the benefits of genetic resistance to disease.
"Increased population density promoted the spread of infectious diseases, as did the new proximity of farmers to animal pathogens," the scientists wrote in their paper.
The difference in skin pigmentation is also thought to have arisen in our recent evolutionary past. Pritchard hypothesized that as human populations moved north from Africa and the Middle East, paler skin became preferable because it allowed people to produce vitamin D where there was less exposure to the sun.
Pritchard acknowledges the anthropological significance of his study, but he's more interested in its present day medical applications. More information on what exactly these recent, positively selected genes do would help improve doctors' understanding of the human body.
"There are medical implications to understanding the genome and identifying genetic variance that produces some kind of phenotypic change," said Pritchard. "We're creating a catalog of these genes, but now they need to be examined more closely to learn what their consequences are."