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ADAPTIVE SELECTION · Accelerated mutation rate produced humans' large brain

BY KEVIN DAVIES


Feb. 11, 2005 | A bioinformatics survey of evolutionary changes in genes central to brain development suggests that the human brain developed as a result of accelerated gene mutation that is unique to humans. Lead investigator Bruce Lahn, a Howard Hughes Medical Institute investigator at the University of Chicago, says his study "offers the first genetic evidence that humans occupy a unique position in the tree of life."


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Dorus, S. et al. "Accelerated evolution of nervous system genes in the origin of Homo sapiens." Cell 119, 1027-40; 2004.
In findings published in Cell, Lahn's group compared DNA sequences of 214 genes known to be involved in brain development from humans, macaques, rats, and mice. By comparing the rate of change of bases that leave protein sequences unaffected with those that produce a coding change, the Chicago team gauged the effect of natural selection on those genes. More than half of the genes surveyed showed greater selective pressure in primates compared to rodents, accruing several additional amino-acid changes per gene product than expected. Those changes are particularly evident in the evolutionary lineage leading to Homo sapiens.

Interestingly, 17 of the 24 most highly mutated genes have been implicated in controlling either brain size or behavior. Lahn believes many of these mutations were selected precisely for their effect on cognitive ability. "The making of the large human brain is not just the neurological equivalent of making a large antler. Rather, it required a level of selection that's unprecedented," he says.

The authors conclude their Cell paper by noting: "As first recognized by Charles Darwin, adaptive evolution must have played a key role in driving the acquisition of greater cognitive powers in humans ... We argue that accelerated protein evolution in a large cohort of nervous system genes ... represents a salient genetic correlate to the profound changes in brain size and complexity during primate evolution."* 










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