The earliest animals needed a simple brain - a brain stem - to control things like breathing, blood pressure, temperature regulation, consciousness, eye movements, facial sensation, facial movement, hearing, swallowing, and movement of the trunk (e.g., a fish). Once animals got a little more sophisticated, developing arms and legs, new neurologic structures were added over this brain stem. These motor control systems work well for gross movement for limbs, but were too primitive for delicate motor control of fingers. So, reptiles depended on the old motor control system, but primates were given a more sophisticated "pyramidal" tract to control fine finger movements. The old system lies right on top of the brain stem and is called the basal ganglia because it is at the base of the brain. Ganglia is any collection of nerve cell bodies (where the nucleus of the nerve cell is housed).

Reptiles, perhaps, have one emotion: anger. Mammals needed a system of nerves to express a wider range of emotions, hence the "limbic system," an old section of the brain located in the temporal lobe and deep areas of the brain.

As mammals became smarter, making more sophisticated judgments involving planning, hunting, nurturing, and adapting to new environments, the brain had to be expanded to accommodate these new demands. The cell bodies controlling these advanced tasks began to cluster over the surface of the brain, ending up in man as a rind of cell bodies, called the cerebral cortex, covering the entire surface of the brain. This rind turns gray after being stored in formaldehyde, while the axons extending south from the cortical surface remain white, hence the terms gray and white matter. To give more surface area to the cerebral cortex, the cortex was made into a corrugated surface.

Certain areas of the brain developed special functions. For example, the back of the brain, the occipital cortex, translates the retinal signals into a visual image. The temporal lobes interpret the meaning of sounds. The left temporal lobe translates sounds into speech. The parietal lobes handle incoming sensations while the frontal lobes, which constitute nearly half the brain, control important functions like reasoning, judgment, planning, proper social behavior, and so on. Memories are logged into the brain through the temporal lobes intermingled with smell and emotional centers, explaining why we remember things that have emotional significance and why animals use their noses so much.