<P> The evolution of nervous systems dates back to the first development of nervous systems in animals (or metazoans). Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single - celled and colonial eukaryotes . Simple nerve nets seen in animals like cnidaria evolved first, followed by nerve cords in bilateral animals--ventral nerve cords in invertebrates and dorsal nerve cords supported by a notochord in chordates . Bilateralization led to the evolution of brains, a process called cephalization . </P> <P> Action potentials, which are necessary for neural activity, evolved in single - celled eukaryotes . These use calcium rather than sodium action potentials, but the mechanism was probably adapted into neural electrical signaling in multicellular animals . In some colonial eukaryotes such as Obelia electrical signals do propagate not only through neural nets, but also through epithelial cells in the shared digestive system of the colony . </P>

The earliest evolution of the nervous system (a nerve net) occurred in