Beneath the complexity of the human brain are molecular principles shaped by evolution explaining the origins of the behavioral repertoire. The role of the nervous system is to provide a repertoire of behaviors allowing the animal to respond and adapt to changing environments during the course of its life. Multiprotein complexes in the postsynaptic terminal of synapses control adaptive and cognitive processes in metazoan nervous systems. These multiprotein complexes are organized into molecular networks that detect and respond to patterns of neural activity. Combinations of proteins are used to build different complexes and pathways producing great diversity. These complexes evolved from an ancestral core set of proteins controlling adaptive behaviors in unicellular organisms known as the protosynapse. Later expansion in numbers and interactions resulted in more complex synapses in invertebrates and vertebrates. The resultant combinatorial complexity has contributed to the neuroanatomical, neurophysiological, and behavioral diversity in these species. Mutations in genes encoding the complexes result in many human diseases of the nervous system. This general mechanism of cognition provides a useful template for studying evolution of behavior in all animals.