And the Evolutionary Beat Goes On . . .

"If you met a human from 10,000 years ago," Pritchard quipped, "they may look a little different, but if you dressed them right, they would probably blend in. Gould's talking about changes in body plan and broader changes."

To spot natural selection at work, Pritchard and Bruce Lahn, also a geneticist at the University of Chicago who has conducted independent research in the same area, first look for places along the human genome to identify sites that show changes in some people but not in others. Then they look at the genetic material surrounding the changed part.

If the surrounding area looks very different from one person to the next, the particular change probably occurred a long time ago, because the general area has had time to accumulate other changes in the DNA. If there are not many differences in the surrounding genetic sequence, that indicates the particular change is relatively new.

Then scientists figure out how widespread that particular change is in large populations. Changes that are both new and widespread reveal the hand of natural selection -- since advantageous genetic changes will quickly spread through the population.

Next, scientists try to guess what the genetic change is accomplishing. If the change is in a part of the genome known to be involved in the immune system, the change may have something to do with responding to new diseases. Other changes may have to do with brain functioning or skin color.

Europeans, for example, show strong changes over the past 10,000 years in genes that affect skin color -- as humans moved into northern Europe, where there was less ultraviolet light, there was a strong evolutionary advantage to having lighter skin to allow in more ultraviolet light, which is needed to synthesize Vitamin D.

Lahn found changes in two genes, dubbed ASPM and MCPH1, that are known to be involved in brain development. He published his results recently in the journal Science.

While genetic changes, especially related to the brain, may prompt people to think different populations are evolving different mental abilities, both Lahn and Pritchard pooh-poohed this idea. For one thing, they pointed out, biology is complex, and the same genes often play multiple roles in the body. A gene that affects brain development may also play a role in the immune system, so it is not possible to say with certainty that natural selection has favored the change because of its effect on the brain.

Besides, Pritchard added, scientists found about the same number of changes in all three groups they studied, suggesting that evolution is taking place everywhere, adapting different groups to the particulars of their ecological niches.

Come to think of it, the late Stephen Jay Gould might have been upset with the above illustration. Contrary to the popular imagination, evolution is not a linear process that culminates in the triumphal ascent of humans at the top of the genetic heap. The process is analogous to a bush, where twigs and leaves push out in every direction.

When biologists talk about evolution and the survival of the fittest, they do not necessarily mean the strongest, fastest or smartest. Fitness is whatever works in a particular environment, and the new research shows that as environments change, notions of fitness change, too.