Fig. 2.

The polygenesis of neuronal centralization. The hypothetical scenario outlining multiple origins of neuronal centralization in Bilateria. The ancestral urbilateria had diffuse, possibly only epidermal nerve nets and did not possess central nervous systems. Centralization of neuronal elements and the formation of CNSs and complex brains occurred independently several times in the course of animal evolution (see also Figs 1, 4 and 5). The inversion of the ventral to dorsal axis took place during earlier stages of Chordate evolution. Ctenophore and Cnidaria nervous systems may have evolved in parallel and, at least in part, may not necessarily be related to bilaterian neural organization. Red, orange and green colors schematically illustrate different neuronal structures. Open white circles indicate the position of the mouth. the author supports this hypothesis of neuronal evolution; see text and [54] for details