Four scenarios for evolution of central nervous systems in bilaterians. In scenario 1, the urbilaterian had multiple nerve cords, one of which evolved into the dorsal central nervous system (CNS) of chordates, while another nerve cord evolved into the ventral CNS of protostomes. In scenario 2, the CNSs of protostomes and deuterostomes evolved independently from an ectodermal nerve net in the bilaterian ancestor. In scenario 3, the chordate and protostome nerve cords evolved from a ventral nerve cord in the urbilaterian ancestor. A dorso/ventral (D/V) inversion occurred at the base of the deuterostomes; the dorsal nerve cord of hemichordates is thus homologous to the chordate CNS and to the protostome ventral nerve cord. In scenario 4, the protostome and chordate nerve cords evolved from the CNS of an urbilaterian ancestor, but a D/V inversion occurred at the base of the chordates. Thus, the ventral nerve cord of a hemichordate is homologous to the chordate and protostome CNSs. Scenarios after [1, 3, 7, 23, 26–29].