The pace of human evolution has been increasing at a stunning rate since our ancestors began spreading through Europe, Asia and Africa 40,000 years ago, quickening to 100 times historic levels after agriculture became widespread, according to a study published today.
By examining more than 3 million variants of DNA in 269 people, researchers identified about 1,800 genes that have been widely adopted in relatively recent times because they offer some evolutionary benefit.
Until recently, anthropologists believed that evolutionary pressures on humans eased after the transition to a more stable agrarian lifestyle. But in the past few years, they realized the opposite was true — diseases swept through societies in which large groups lived in close quarters for a long period.
Altogether, the recent genetic changes account for 7 percent of the human genome, according to the study, published in the Proceedings of the National Academy of Sciences.
The advantage of all but about 100 of these genes remains a mystery, said University of Wisconsin at Madison anthropologist John Hawks, who led the study. But the research team was able to conclude that infectious diseases and the introduction of new foods were the primary reasons that some genes swept through populations with such speed.
“Dietary changes, disease changes — those create circumstances where selection can happen,” Hawks said.
One of the most famous examples is the spread of a gene that allows adults to digest milk.
Although children were able to drink milk, they typically developed lactose intolerance as they grew up. But after cattle and goats were domesticated in Europe and yaks and mares were domesticated in Asia, adults with a mutation that allowed them to digest milk had a nutritional advantage over those who didn’t. As a result, they were more likely to have healthy offspring, prompting the mutation to spread, Hawks said.
The mechanism also explains why genetic resistance to malaria has spread among Africans — who live where disease-carrying mosquitoes are prevalent — but not among Europeans or Asians.
Researchers found that the more the population grew, the faster human genes evolved. That’s because more people created more opportunities for a beneficial mutation to arise, Hawks said.
Among the fastest-evolving genes are those related to brain development, but the researchers aren’t sure what made them so desirable, Hawks said.
There are other mysteries too.
“Nobody 10,000 years ago had blue eyes,” Hawks said. “Why is it that blue-eyed people had a 5 percent advantage in reproducing compared to non-blue-eyed people? I have no idea.”
