Fig. 9. Evolution of the vertebrate brain – a hypothesis. On the phylogenetic tree of the chordates, hypothetical evolutionary events are positioned according to the present findings in the lamprey. In the common ancestor of chordates, the dorsal nerve chord was patterned anteroposteriorly by some regulatory genes. Segmentation of the brain had not appeared even after the divergence of the amphioxus lineage. The present study suggests that the common ancestor of the vertebrates had already acquired rhombomeric segmentation and at least three longitudinal subdivisions in the forebrain. The most fundamental event in the establishment of the ancestral vertebrate brain is assumed here to have involved cell lineage restriction of neuroepithelial cells, both along the dorsoventral and the anteroposterior axes, to develop compartmentalized polygonal subdivisions. Note that lampreys and hagfishes are considered to form a monophyletic group in this figure, based on recent molecular data from Kuraku et al. (Kuraku et al., 1999) and Mallat and Sullivan (Mallat and Sullivan, 1998). After the divergence of the agnathans and gnathostomes, the evolution of the pallidum may have evolved specifically in the lineage of the latter.