In the past 100,000 years, modern humans have colonized the far corners of the globe, adapting to new environments as they migrated. Researchers have long assumed that these dramatic transitions resulted in a sort of accelerated evolution in which genes for traits such as skin color and stature changed rapidly to allow humans to survive in their new habitats. Now, a team of French and Spanish researchers has found powerful new evidence to support this idea, identifying 582 genes that have evolved differently in different populations in the past 60,000 years, including a dozen that protect people from obesity, diabetes, hypertension, and other diseases.
The team, led by population geneticist Lluis Quintana-Murci of the Pasteur Institute and Centre National de le Recherche Scientifique in Paris, analyzed DNA of 210 individuals from the database of Phase II of the International HapMap Project, an effort to identify variations in human genes that cause disease. The researchers analyzed 2.8 million single nucleotide polymorphisms (SNPs)--mutations in a single nucleotide in a genome that varies between individuals or populations--from Europeans, Africans, and Asians. Then they sorted the mutations by type, focusing on 15,259 nonsynonymous mutations, which alter amino acids and thus a gene's function.
Using statistical analysis, the researchers found that some mutations occurred at such high frequencies compared to other SNPs in the same populations that they must have improved survival and reproductive success and been the result of strong positive selection pressure. These mutations varied tremendously between populations, which counters a popular view that many of the differences between populations arose by chance or were genetic variants that hitchhiked along with other genes that improved reproductive success, says biological anthropologist Henry Harpending of the University of Utah, Salt Lake City, and co-author of another study of accelerated evolution.
Although the researchers don't know the function of most of the 582 genes that were under such intense positive natural selection, they have identified about 50 that appear to be responses to diseases or changes in diet or environment. Some examples include mutations that alter how adults regulate insulin, digest sugars and starches, metabolize ethanol and zinc, transport fats, regulate the immune response to pathogens, and repair and replicate DNA. "New mutations that 'protect' people from diabetes and obesity have been selected probably because they significantly improved peoples' ability to handle agricultural diets," says biological anthropologist John Hawks of the University of Wisconsin, Madison, who collaborates with Harpending. For example, he says, new dependence on a few cereal grains required efficient digestion of starches.
The study, reported online 3 February in Nature Genetics, is the latest in a series of recent reports to identify genes that are still evolving or have evolved recently in different human populations (ScienceNOW, 10 December 2007). "I think it is clear there's quite a bit of recent selection going on," says population geneticist Jonathan Pritchard of the University of Chicago in Illinois.