How can the development of an ordered array of neuronal connections be encoded in the genome? Results on the establishment of sensory connections in insects indicate that this programming is a multi-stepped process which begins as soon as the first axons develop. Because each step relies on the previous level of organization, the first steps of the process are subject to intense structural constraints, and therefore have been largely conserved through evolution. What is known of the molecular biology of some essential steps, like the differentiation of excitable cells, their aggregation in nerve cords, and the diversification of a periodic structure, supports the idea that the basic organization of the CNS evolved before the divergence between the chordate and the arthropod/annelid lineage.