WASHINGTON - Humans are evolving more rapidly than in the distant past, with residents of various continents becoming increasingly different from one another, researchers reported yesterday.
"I was raised with the belief that modern humans showed up 40,000 to 50,000 years ago and haven't changed," said Henry C. Harpending, an anthropologist at the University of Utah. "The opposite seems to be true. Our species is not static."
That doesn't mean we should expect major changes in a few generations. Evolution takes thousands of years.
Harpending and colleagues looked at the DNA of humans and chimpanzees. If evolution had been proceeding at the current rate since the two lines separated six million years ago, there should be 160 times more differences than were found, they said.
That indicates evolution had been slower in the distant past.
"Rapid population growth has been coupled with vast changes in cultures and ecology, creating new opportunities for adaptation," the researchers write in this week's online edition of Proceedings of the National Academy of Sciences. "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."
The study found different changes in Africans, Asians and Europeans.
Most anthropologists agree that humans first evolved in Africa and then spread. 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 population increase 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 said. "And with a larger population, more mutations occurred."
In another example, the researchers noted that in China and most of Africa, few adults can digest fresh milk. 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, they said.
The study examined 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.
The authors mainly pointed to an overall expansion in population over the last 40,000 years to explain the genetic data.
Two years ago, Harpending cowrote a study that said above-average intelligence in Ashkenazi Jews - those with ancestors in Germany and other parts of Europe - resulted from natural selection in medieval times, when they were forced into jobs as bankers, traders and managers.
The smarter ones 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.