The increase in human population from millions to billions in the last 10,000 years accelerated the rate of evolution because ''we were in new environments to which we needed to adapt,'' Harpending adds. ''And with a larger population, more mutations occurred.''
In another example, the researchers noted that in China and most of Africa, few people can digest fresh milk into adulthood. Yet in Sweden and Denmark, the gene that makes the milk-digesting enzyme lactase remains active, so almost everyone can drink fresh milk, explaining why dairy farming is more common in Europe than in the Mediterranean and Africa, Harpending says.
The researchers studied 3.9 million gene snippets from 270 people in four populations: Han Chinese, Japanese, Africa's Yoruba tribe and Utah Mormons who traced their ancestry to northern Europe.
Richard Potts, director of the human origins program at the Smithsonian Institution's National Museum of Natural History, said he thinks the researchers reasoning regarding rapid adaptive change is plausible.
The study mainly points to an overall expansion in the human population over the past 40,000 years to explain the genetic data.
''Yet the archaeological record also shows that humans increasingly divided themselves into distinct cultures and migrating groups — factors that seem to play only a small role in their analysis. Dividing the human population into finer units and their movement into new regions — the Arctic, Oceania, tropical forests, just to name some — may have also forced quicker adaptive evolution in our species,'' Potts said.
Potts, who was not part of the research team, added that he liked the report ''because it points to how genetic data can be used to test a variety of ideas about recent human adaptation.''
Two years ago Harpending and colleague Gregory M. Cochran published a study arguing that above-average intelligence in Ashkenazi Jews — those of northern European heritage — resulted from natural selection in medieval Europe, where they were pressured into jobs as financiers, traders, managers and tax collectors.
Those who were smarter succeeded, grew wealthy and had bigger families to pass on their genes, they suggested. That evolution also is linked to genetic diseases such as Tay-Sachs and Gaucher in Jews.
The new study was funded by the Department of Energy, the National Institute of Mental Health, the National Institute of Aging, the Unz Foundation, the University of Utah and the University of Wisconsin.