Early theories on the evolutionary origin of the nervous system argued for a three-stage process: first, the development of non-nervous “independent effectors,” such as muscle cells; second, the appearance of non-nervous receptors responding to certain modalities in a receptor-effector mechanism; and finally, the formation of a “protoneuron,” from which primitive nerves and ganglia evolved.

Development of the central nervous system continues for many years after birth. Synapses form and new connections appear, increasing in number throughout childhood and into adulthood. Only synapses and pathways that are used survive into adulthood; the process of synaptic pruning allows unused synapses to be eliminated.