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STUDIES HINT THAT BRAIN MAY STILL BE EVOLVING
The discovery adds weight to the view that human evolution is still a work in progress, since previous instances of recent genetic change have come to light in genes that defend against disease and confer the ability to digest milk in adulthood. It had been widely assumed until recently that human evolution more or less stopped 50,000 years ago. The new finding, reported in today's issue of Science by Bruce T. Lahn of the University of Chicago, and colleagues, could raise controversy because of the genes' role in determining brain size. New versions of the genes, or alleles as geneticists call them, appear to have spread because they enhanced brain function in some way, the report suggests, and they are more common in some populations than others. But several experts strongly criticized this aspect of the finding, saying it was far from clear that the new alleles conferred any cognitive advantage or had spread for that reason. Many genes have more than one role in the body, and the new alleles could have been favored for some other reason, these experts said, such as if they increased resistance to disease. Even if the new alleles should be shown to improve brain function, that would not necessarily mean that the populations where they are common have any brain-related advantage over those where they are rare. Different populations often take advantage of different alleles, which occur at random, to respond to the same evolutionary pressure, as has happened in the emergence of genetic defenses against malaria, which are somewhat different in Mediterranean and African populations. If the same is true of brain evolution, each population might have a different set of alleles for enhancing function, many of which remain to be discovered. The Chicago researchers began their study with two genes, known as microcephalin and ASPM, that came to light because they are disabled in a disease called microcephaly. People with the condition are born with a brain much smaller than usual, often with a substantial shrinkage of the cerebral cortex, that seems to be a throwback to when the human brain was a fraction of its present size. Last year, Dr. Lahn, one of a select group of researchers supported by the Howard Hughes Medical Institute, showed that a group of 20 brain-associated genes, including microcephalin and ASPM, had evolved faster in the great ape lineage than in mice and rats. He concluded that these genes might have had important roles in human evolution. As part of this study, he noticed that microcephalin and ASPM had an unusual pattern of alleles. With each gene, one allele was much more common than all the others. He and his colleagues have now studied the worldwide distribution of the alleles by decoding the DNA of the two genes in many different populations. 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.
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