Conventional wisdom holds that if you could bring back someone from 40,000 years ago, he or she would blend perfectly well with today's population.

After all, the fossils show that our ancestors were "anatomically modern" by 100,000 years ago, and by 40,000 B.C., they were creating complex tools and art.

It was easy to assume our species hadn't evolved much since then.

Now molecular biology is overturning that assumption.

Evidence for more recent evolution is coming not from fossils but from patterns seen in the DNA of contemporary people. Genes show that blue eyes, for example, apparently didn't exist until 6,000 years ago, and the ability to digest milk goes back just 7,000 years.

Scientists have compared genes from different ethnic groups and found more recent genetic mutations are changing the way some people metabolize food, store fat, grow hair, and fight disease.

One new mutation that appears only in Asians may improve hearing and balance.

Some of the more recent changes have not spread through the world uniformly - affecting only those of European, African, or Asian ancestry. Would any future findings suggest that some group had traits that might be considered superior to others?

Already, for example, some scientists have theorized that recent genetic changes have endowed Ashkenazi Jews with higher average intelligence.

To deal with such potentially inflammatory ideas, the National Institutes of Health assembled a meeting in 2008 on the social and ethical implications of natural selection in humans. "There was a spirited debate about whether we should be engaged in this research at all," said Josh Akey, a geneticist from the University of Washington. Some argued that no good could come of it, he said.

Akey and other scientists who do this work say they hope to advance medicine by revealing why some people are more vulnerable than others to HIV, malaria, flu, autoimmune diseases, allergies, diabetes, alcoholism, and obesity.

The scientists argue that none of this work points to the superiority of one group or person over another. It's an old misconception that evolution is elevating humanity up some ladder of perfection, he said, or that one group could be more evolved than another.

"When we talk about natural selection, people think about survival-of-the fittest," Akey said, "but that's not really how it works."

Local environments bend the course of evolution. Traits that would make someone "fit" in one place could prove to be liabilities in another.

Several genetic mutations protect against malaria, for example, though they can also cause diseases including sickle cell anemia. A few researchers have speculated that conditions known as torsion dystonia and Gaucher's disease are associated with higher intelligence.

New genetic anomalies could be seen as diseases or advantages, depending on the context.

These new "footprints" of recent evolution would have been invisible even a few years ago, before an explosive growth in the ability to analyze DNA and map genes.

Much of that DNA was collected through a project called HapMap - which included samples from 90 Nigerians, 90 Americans of European ancestry, 45 people from Tokyo, and 45 from Beijing. The collection is not meant to be all-inclusive but to provide a sample of the world's diversity.

What has been revealed so far is that some altered versions of genes are spreading fast, as if they were incurring some advantage to their owners. The biologists can tell how fast a gene is spreading by how much extra DNA it's trailing along with it, said Sharon Grossman, a geneticist at Harvard.