ter in the human lineage, the study also uncovered two dozen "outlier" genes that might have made important contributions to the evolution of the human brain. These outlier genes were identified by virtue of the fact that their rate of change is especially accelerated in the human lineage, far more so than the other genes examined in the study. Strikingly, most of these outlier genes are involved in controlling either the overall size or the behavioral output of the brain -- aspects of the brain that have changed the most during human evolution.

According to graduate student Eric Vallender, a coauthor of the article, it is entirely possible by chance that that two or three of these outlier genes might be involved in controlling brain size or behavior. "But we see a lot more than a couple -- more like 17 out of the two dozen outliers," he said. Thus, according to Lahn, genes controlling the overall size and behavioral output of the brain are perhaps places of the genome where nature has done the most amount of tinkering in the process of creating the powerful brain that humans possess today.

There is "no question" that Lahn's group has uncovered evidence of selection, said Ajit Varki of the University of California, San Diego. Furthermore, by choosing to look at specific genes, Lahn and his colleagues have demonstrated "that the candidate gene approach is alive and well," said Varki. "They have found lots of interesting things."

One of the study's major surprises is the relatively large number of genes that have contributed to human brain evolution. "For a long time, people have debated about the genetic underpinning of human brain evolution," said Lahn. "Is it a few mutations in a few genes, a lot of mutations in a few genes, or a lot of mutations in a lot of genes? The answer appears to be a lot of mutations in a lot of genes. We've done a rough calculation that the evolution of the human brain probably involves hundreds if not thousands of
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Contact: Jim Keeley
keeleyj@hhmi.org
301-208-6560
Howard Hughes Medical Institute
28-Dec-2004

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