Human evolution is speeding up - and humans on different continents are becoming increasingly different, researchers say.
The pace of evolutionary change has accelerated since humans wandered out of Africa thousands of years ago, with evidence in the way people in different parts of the world digest milk, make vitamin D from sunlight and fight off diseases such as HIV, according to researchers at the University of Utah.
Scientists examined 3.9 million snippets of DNA from 270 people representing four different groups - Han Chinese, Japanese, Africa's Yoruba tribe and Northern Europeans - and found patterns showing evidence of recent natural selection in 1,800 genes.
But what genetic differences might be triggered by the specific variations remain a mystery because the roles of the genes are still unknown, they say in a new report.
"With a lot of these genes, we don't have the whole story yet," said Henry Harpending, a University of Utah anthropology professor and senior author of the report, published online yesterday in Proceedings of the National Academy of Sciences.
But it means up to 7 percent of the human genome is rapidly evolving to local conditions - a rate consistent with findings reported last year by another research team, the scientists say.
Experts say natural selection has been driven by increases in population and the diaspora of humans to all kinds of habitats since they left Africa about 50,000 years ago. The more people that produce offspring, the more genetic mutations arise from adaptations to changing climates, varied diets and emerging diseases.
"We are still evolving, and there is local adaptation going on," said Sarah Tishkoff, an evolutionary biologist at the University of Maryland.
Harpending said the findings show that humans are evolving differently on different continents and much faster than 6 million years ago, when they separated from a common ancestor with the chimpanzee.
"We are getting less alike, not merging into a single, mixed humanity," he said.
Natural selection is believed to be why pale-skinned Northern Europeans, who live in areas with less sunshine than other areas, can produce vitamin D from so little sunlight - and why they sunburn so easily.
Future generations might favor genes that provide resistance to malaria and HIV, Harpending said.
Researchers say the results shed light on how humans continue to respond to evolutionary pressures. But Tishkoff expressed skepticism that recent changes are outpacing those in our evolutionary past.
"There have been some dramatic changes in the past 10,000 years, but have we just been starting out on this path since 5 million years ago, since we evolved from chimpanzee ancestors? I'd doubt it," she said. "I'm sure there were diseases that wiped out huge numbers of people millions of years ago and genetic responses to those diseases."
The DNA samples were obtained from the International HapMap Project, a partnership of scientists and agencies developing a genetic database to help in the search for genes associated with diseases and responsiveness to new drugs.
The report is one of several recently published by researchers who scanned the human genome to look for evidence of selective pressures among the thousands of specific sites where variations have occurred.
In June, researchers from Cornell University found evolutionary pressures among 100 genes obtained from Chinese, African-American and European-American volunteers.
Last year, scientists at the University of Chicago found evidence of recent selection pressures among 700 genetic variants in Africans, Europeans and East Asians, and Tishkoff found three separate mutations among 43 ethnic groups in East Africa that enabled them to tolerate lactose. One of the variations might have arisen only 3,000 to 7,000 years ago, she said.
A major factor in human evolutionary history was the transformation about 10,000 years ago from being hunters and gatherers to living as farmers, according to Harpending.
"A really big factor was definitely the rise of agriculture," he said.
Farming meant selection of genes that favored offspring who could digest the cereal grains that were harvested and the raising of cattle meant genes that favor the ability to digest lactose in milk. The researchers note that in China and most of Africa, where dairy cattle have been rare, few people can digest fresh milk.
The findings come from the same researchers who argued in 2005 that predisposition to genetic diseases such as Tay-Sachs and a trend toward above-average intelligence found in Jews from Northern Europe were the result of natural selection from when they were pressured into jobs as financiers and tax collectors in medieval Europe.
Genetic analysis that highlights different traits among different groups should never be used as a basis for discrimination, Harpending said. But as to work on identifying the genetic underpinnings of differences among people from different parts of the globe, he said, "I don't think it needs to be controversial at all."