The ornately folded outer layer of the human brain, the cerebral cortex, has long received nearly all the credit for our ability to perform complex cognitive tasks such as composing a sonata, imagining the plot of a novel or reflecting on our own thoughts. One explanation for how we got these abilities is that the cortex rapidly expanded relative to body size as primates evolved -- the human cortex has 10 times the surface area of a monkey's cortex, for example, and 1,000 times that of a mouse. But the cortex is not the only brain region that has gotten bigger and more complex throughout evolution. Nestled beneath the cortex, a pair of egg-shaped structures called the thalamus has also grown, and its wiring became much more intricate as mammals diverged from reptiles. The thalamus -- from the Greek thalamos, or inner chamber -- transmits 98 percent of sensory information to the cortex, including vision, taste, touch and balance; the only sense that doesn't pass through this brain region is smell.