APA Dictionary of Psychology

the concept that the brains of complex animals have evolved over many millions of years from a network of simple nerve fibers connecting various body areas, as in primitive multicellular animals. At a more advanced stage, a neural axis developed to connect and integrate peripheral neurons and to house cell bodies; this axis became a spinal cord. Still later, collections of neurons with control functions developed at the head end of the spinal cord, as in the brains of higher invertebrates, birds, fish, and reptiles. From those concentrations of brain tissue evolved the forebrain, with its highly convoluted cerebral cortex, of mammals, especially prominent in whales, great apes, and Homo sapiens. See also cephalization; encephalization.