]The human brain weights 1.35kg(3 lbs). It consists of the forebrain (prosencephalon), the midbrain (mesoncephalon), and the hindbrain ( rhombencephalon). All vertebrate brains contain these three structures.

Natural selection not only sifted out physical characteristics, but also selected mental characteristics for future survival. The brain's evolution was carried out in three steps. The first mental aspect to evolve was its increase in volume. The relative size of a brain of a mammal and a reptile are proportionately different. For example, a 100g mouse contains a much larger brain than a 100g frog. The second aspect of the brain that evolved was compartmentalization of the brain. With time, distinct areas in an organism's brain carried out distinct functions. For example, the cerebellum is involved with the coordination of body parts. The third and most astounding evolutionary trait of the vertebrate brain is the development of the forebrain. When amphibians began venturing on land from water senses such as vision and hearing became more important in their survival. Hence, natural selection chose organisms with a larger midbrain and hindbrain. More complex behaviors parallel the growth of the forebrain, or the cerebrum.

The hindbrain and the midbrain make up the brainstem and they form a cap on the spinal chord that extends to about the middle of the brain. Development stems from these two areas which is located anterior end of the spinal chord. It consists of three parts that serve to maintain homeostasis, balance movement coordination, and signal conduction. The pons and the medulla oblongata both serve to carry out autonamic and homeostatic functions. These include breathing, heard and blood vessel activity, swallowing, vomiting, and digestion.

The midbrain, or the upper portion of the brainstem contains centers for processing sensory input. All fibers involved with hearing either terminate or pass in the inferior colliculi, and the superiors colliculi is involved with visual input. The major nuclei in the midbrain are involved with a sector called the reticular formation which regulates arousal.

The most intricate section in the brain is the forebrain where most thought such as emotion, learning, and memory occur. Integrating motor and sensory pathways allow image perception. The two major portion of the forebrain are the diencephalon, and the telencephalon. The diencephalon contains the thalamus and the hypothalamus. The upper portion of the telencephalon contains the cerebrum