"The frontal lobes of the human brain are the most highly developed part relative to other animals and it has long been thought that the evolution of the frontal lobes parallels the development of human communication and civilization," Walsh adds. "The GPR56 gene is only found in higher animals that possess a frontal lobe. It has undergone significant changes, even among these species, suggesting it may be a target of evolution.

"Being able to access the complete sequence of the human genome has allowed us to identify increasing numbers of genes that are required for cortical development," he adds. "Although these genes cause mental retardation, by studying the biological function of their gene products we also gain insight into the normal development and function of the human brain."

In addition to Walsh and Piao, study co-authors include BIDMC researchers R. Sean Hill, Adria Bodell, and Bernard S. Chang; and A. James Barkovich of the University of California, San Francisco, as well as many clinicians from around the world who provided DNA samples. The study was funded by the Howard Hughes Medical Institute and by grants from the National Institutes of Health, the March of Dimes, and the McKnight Foundation. Beth Israel Deaconess Medical Center is a major patient care, teaching, and research affiliate of Harvard Medical School, ranking third in National Institutes of Health funding among independent hospitals nationwide.