One of the first things people notice about the human brain is its intricate landscape of hills and valleys. These convolutions derive from the cerebral cortex, a two- to four-millimeter-thick mantle of gelatinous tissue packed with neurons sometimes called gray matter that mediates our perceptions, thoughts, emotions and actions. Other large-brained mammals such as whales, dogs and our great ape cousins have a corrugated cortex, too each with its own characteristic pattern of convolutions. But small-brained mammals and other vertebrates have relatively smooth brains. The cortex of large-brained mammals expanded considerably over the course of evolution much more so than the skull. Indeed, the surface area of a flattened human cortex equivalent to that of an extra-large pizza is three times larger than the inner surface of the braincase. Thus, the only way the cortex of humans and other brainy species can fit into the skull is by folding.