They report that with microcephalin, a new allele arose about 37,000 years ago, although it could have appeared as early as 60,000 or as late as 14,000 years ago. About 70 percent of people in most European and East Asian populations carry this allele of the gene, but it is much rarer in most sub-Saharan Africans.

With the other gene, ASPM, a new allele emerged 14,100 to 500 years ago, the researchers favoring a midway date of 5,800 years. The allele has attained a frequency of about 50 percent in populations of the Middle East and Europe, is less common in East Asia, and is found at low frequency in some sub-Saharan Africa peoples.

The Chicago team suggests that the new microcephalin allele may have arisen in Eurasia or as the first modern humans emigrated from Africa some 50,000 years ago. They note that the ASPM allele emerged about the same time as the spread of agriculture in the Middle East 10,000 years ago and the emergence of the civilizations of the Middle East some 5,000 years ago, but say that any connection is not yet clear.

Dr. Lahn said there might be a fair number of genes that affect the size of the brain, each making a small difference yet one that can be acted on by natural selection. "It's likely that different populations would have a different makeup of these genes, so it may all come out in the wash," he said. In other words, East Asians and Africans probably have other brain-enhancing alleles, not yet discovered, that have spread to high frequency in their populations.

He said he expected that more such allele differences between populations would come to light, as have differences in patterns of genetic disease. "I do think this kind of study is a harbinger for what might become a rather controversial issue in human population research," Dr. Lahn said. But he said his data and other such findings "do not necessarily lead to prejudice for or against any particular population."

A greater degree of concern was expressed by Francis S. Collins, director of the National Human Genome Research Institute. Dr. Collins said that even if the alleles were indeed under selection, it was still far from clear why they had risen to high frequency, and that "one should resist strongly the conclusion that it has to do with brain size, because the selection could be operating on any other not yet defined feature." He said he was worried about the way these papers will be interpreted.

Sarah Tishkoff, a geneticist at the University of Maryland and a co-author of both studies, said the statistical signature of selection on the two genes was "one of the strongest that I've seen." But she, like Dr. Collins, said that "we don't know what these alleles are doing" and that specific tests were required to show that they in fact influenced brain development or were selected for that reason.