A trajectory of increasing activity and the elaboration of chemosensory modality: a new perspective on vertebrate origins.

This article reviews recent advances in comparative biological studies of vertebrate origins, with the aim of revisiting the long-standing controversy concerning these origins. Since early vertebrate evolution is paralleled by an evolutionary trend towards increasing activity, I focus on the evolution of respiratory and circulatory systems and discuss their potential roles in early vertebrate evolution. I give particular attention to the nasohypophyseal duct, an orifice characteristically found in agnathan vertebrates, and hypothesize that this duct originally functioned to convey oxygen dissolved in seawater to the respiratory gills. The chemosensory cell population that originated from the wall of the duct became the incipient olfactory organ and played a role in the organization of feeding behavior. An increase in chemosensory receptor genes via large-scale genomic evolution in the vertebrate lineage caused the repertoire of chemosensory cells to diversify and led to the appearance of the integrative center, including telencephalic structures typically lacking in protochordates.