We've evolved faster and faster as world population has boomed

The pace of human evolution has been increasing at a stunning rate since our ancestors began spreading through Europe, Asia and Africa 40,000 years ago, quickening to 100 times historic levels after agriculture became widespread, according to a study published today.

The findings support the notion that the planet's exploding population is a living laboratory for genetic trial and error, researchers say.

For instance, just 10,000 years ago — the blink of an eye in evolutionary time — fewer people carried the gene for an enzyme called lactase, which allows humans to digest cow's milk. A bigger proportion of people had the dark skin of our African ancestors, and no one had blue eyes.

"Blue eyes are new," says lead author John Hawks of the University of Wisconsin.

Hawks says the findings, in the journal Proceedings of the National Academy of Sciences, are merely "breaking the ice" in a rapidly expanding effort to use genetics to explain how people have changed since the first humanlike creatures appeared 7 million years ago.

"I think it's a very important paper," says Clark Larsen, chairman of anthropology at Ohio State University. He says his own study of physical evidence supports the researchers' notion that there was "a ton" of biological change in the past 10,000 years. "It's still going on. Right before our eyes," he said.

By examining more than 3 million variants of DNA in 269 people, the researchers identified about 1,800 genes that have been widely adopted in relatively recent times because they offer some evolutionary benefit.

"I was raised with the belief that modern humans showed up 40,000 to 50,000 years ago and haven't changed," explained Henry Harpending, a co-author and anthropologist at the University of Utah. "The opposite seems to be true."

Altogether, the newer genetic changes account for 7 percent of the human genome, the study says. The advantage of all but about 100 of these genes remains a mystery, Hawks said. But the research team was able to conclude that infectious diseases and the introduction of new foods were the primary reasons that some genes swept through populations.

Some evolving traits are simple to track. Blue eyes today are common, and millions of people in northern climates now carry the lactose gene, along with about half of those living along the Mediterranean. Why did the lactose gene, which allows adults to digest milk, spread so rapidly? Because it helps people survive and bear children, Harpending says.

"If you can metabolize milk sugar rapidly, it gives you an advantage when food is scarce," he says. People who survive are more likely to pass a gene along to offspring, who also are better able to survive.

This also explains why genetic resistance to malaria has spread among Africans — who live where disease-carrying mosquitoes are prevalent — but not among Europeans or Asians.

Most of the genetic changes the researchers identified were found in only one geographic group or another. Races as we know them today didn't exist until fewer than 20,000 years ago, when genes involved in skin pigmentation emerged, Hawks said. Paler skin allowed people in northern latitudes to absorb more sunlight to make vitamin D.

Hawks and his colleagues from the Irvine campus of the University of California, the University of Utah and Santa Clara, Calif.-based gene chip maker Affymetrix examined genetic data collected by the International HapMap Consortium, which cataloged single-letter differences among human DNA in people of Nigerian, Japanese, Chinese and European descent.

The researchers found that the more the population grew, the faster human genes evolved. That's because more people created more opportunities for a beneficial mutation to arise, Hawks said.

Among the fastest-evolving genes are those related to brain development, but the researchers aren't sure what made them so desirable.

There are other mysteries, too.

"Nobody 10,000 years ago had blue eyes," Hawks said. "Why is it that blue-eyed people had a 5 percent advantage in reproducing compared to nonblue-eyed people? I have no idea."

Information from The Associated Press and USA Today is included in this report.