Science fiction writers have suggested a future Earth populated by a blend of all races into a common human form.
In real life, the reverse seems to be happening. People are evolving more rapidly than in the distant past, with residents of various continents becoming increasingly different from one another, researchers say.
"I was raised with the belief that modern humans showed up 40,000 to 50,000 years ago and haven't changed," explained Henry C. Harpending, an anthropologist at the University of Utah. "The opposite seems to be true."
"Our species is not static," Harpending added in a telephone interview.
That doesn't mean we should expect major changes in a few generations, though, evolution occurs over thousands of years.
Harpending and colleagues looked at the DNA of humans and that of chimpanzees, our closest relatives, they report in this week's online edition of Proceedings of the National Academy of Sciences.
If evolution had been proceeding steadily at the current rate since humans and chimps separated 6 million years ago, there should be 160 times more differences than the researchers found.
That indicates that human evolution had been slower in the distant past, Harpending explained.
"Rapid population growth has been coupled with vast changes in cultures and ecology, creating new opportunities for adaptation," the study says. "The past 10,000 years have seen rapid skeletal and dental evolution in human populations, as well as the appearance of many new genetic responses to diet and disease."
And they found that different changes are occurring in Africans, Asians and Europeans.
Most anthropologists agree that humans first evolved in Africa and then spread to other areas, and the lighter skin color of Europeans and Asians is generally attributed to selection to allow more absorption of vitamin D in colder climates where there is less sun.
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.