Abstract
The nervous system of multicellular animals has evolved through several phases： emergence of a nerve cell, centralization of nerve cells, formation of brain in bilateral animals, and most dramat-ically, neural tube in chordates. To understand the evolution of nervous system at the molecular level, we compared the genes expressed in nervous system among the animals. From the comparison of planarian that is the typical animal without central nervous system, we found that more than 95% of expressed genes in planarian nervous system were shared among bilateral animals including human. These common genes could be thought as "ancestral gene set" of nervous system that emerged before the divergence of protostomes and deuterostomes. On the other hand, the comparison of the expressed genes in human with other animals indicated that more than 60% of genes were newly appeared in chordates. This indicates that well-developed nervous system in chordates could be emerged by the newly-appeared genes. To know the detail of the genes that brought the nervous system to chordates, we are focusing on one of the ancestral deuterostome, sea urchin, and investigating the expressed genes in nervous system.