The simplest nervous system is found in cnidarians, radially symmetrical organisms which have a nerve net and no centralization. As evolution proceeded, most sensory organs developed at the anterior end of bilaterally symmetrical organisms because this is the part of the animal that most through the environment first. Therefore, the anterior end of the nervous system became larger and larger, eventually developing into the brain and the spinal cord to connect it to the rest of the organism's body. Cephalization is the development of the brain; organisms that exhibit complex behavior and have complex sensory organs (such as vertebrates) require an extremely cephalized nervous system.

The nervous system evolved and advanced with increasingly advanced behavior. For instance, it was originally made to be in response to reflexes and simple stimuli, and later developed control of memory, learning, and thought. This is why only the most complex organisms have the ability to go through with these processes.

With most complex processes to worry about, brains of complex organisms (such as vertebrates) became divided in order to divy up the labor. This is why lobes of the brain developed.