In Harpending's view, the other factor that accelerated evolution was the invention of farming. Suddenly, people had to adapt to different diets, different ways of getting food, and different social structures.
Starvation and malnutrition got worse in farming groups, he said, because people could no longer move around and change food sources. Farming groups were dependent on just a few crops that could be wiped out by pests, disease, or drought.
Famines would have culled farming populations, favoring those who could survive better on a newly starchy diet, lower in protein and vitamins than the wild foods eaten by hunter-gatherers.
Natural selection also might have favored those who could handle wine and beer without becoming alcoholics, he said.
The implication for today, he said, is that people whose ancestors were hunter-gatherers or recent converts to farming are less adapted to modern life and more likely to suffer from diabetes and other diet-related problems as well as alcoholism.
The human immune system is changing too, he said, adapting to diseases that began to sweep through populations once farming enabled life in crowded cities.
Harpending makes the even more contentious case that farming also reshaped the human mind - favoring traits that allow people to deal with repetitive work and hierarchical society. In some ways, he said, farming people were tamed and domesticated along with their animals.
Individuals who were unruly, violent, or disruptive were killed off.
Evolution through domestication can happen in just a few centuries, said Washington's Akey, who also studies genetics of domestic dogs. He's comparing breeds to see where the DNA is separating the Great Danes from the dachshunds, the border collies from the poodles.
Oddly, he said, he has found a parallel - at the DNA level, dogs and humans are showing some of the same changes. It's possible, he said, that both dogs and humans needed to acquire new disease-resistance genes in response to crowding. Or it could be a reflection of the taming and domestication of both species.
Dog brains are smaller than those of their ancestors, the wolves, and likewise our brains are apparently smaller than those of our ancestors 20,000 years ago, Harpending said. The human brain hit its peak and then began shrinking.
It may mean our ancestors needed more brainpower to survive in the wild, or it could be that our brains have become better organized and more compact - just as computers have done, he said.
It's a mystery, but perhaps there's an answer somewhere in our DNA.
Contact staff writer Faye Flam at 215-854-4977 or fflam@phillynews.com.
