Fig. 1.

Parallel evolution of neuronal centralization in Deuterostomes and Protostomes. The diagram shows types of neural organization among all major Deuterostome lineages. The presented reconstruction of phylogenetic relationships is based upon recent large-scale phylogenomic analysis among all bilaterians [63]. Filled circles indicate possible events of neural centralization from diffuse Nerve Net type of organization in a common ancestor of all Deuterostomes and Protostomes (open circles). Note that nerve nets in Deuterostomes have only superficial similarities and might not be genealogically related to each other. See text for details. Two major superclades of Protostomes (Ecdysozoa and Lophotrochozoa) might also have evolved complex brains independently from Chordates