Brains of Primitive Chordates

Craniates (which include the sister taxa vertebrata and hyperotreti, or hagfishes) represent the most complex organisms in the chordate phylum, particularly with respect to the organization and function of the central nervous system. How brain complexity has arisen during evolution is one of the most fascinating questions facing modern science, and it speaks directly to the more philosophical question of what makes us human. Considerable interest has therefore been directed toward understanding the genetic and developmental underpinnings of nervous system organization in our more ‘primitive’ chordate relatives, in the search for the origins of the vertebrate brain in a common chordate ancestor. We describe some of the principal features of the central nervous systems of hemichordates, cephalochordates, and urochordates – taxa of increasing genetic relatedness to craniates. The central nervous systems of these taxa show a variety of forms, from a hardly specialized basiepithelial nerve plexus (hemichordates) to a few small ganglia with a tail nerve cord (urochordates), a swimming spinal cord with a barely recognizable cerebral vesicle (cephalochordates), and a fully developed brain and spinal cord (craniates). It is important to emphasize that although the term primitive may be construed as implying a lower rank in evolutionary sequence, we use it strictly with respect to extant brain complexity. Each of these groups has in fact existed for substantial periods of evolutionary time and represents highly adapted lifeforms in their own right. We focus here on shared aspects of central nervous system structure and development that may relate to a common evolutionary origin but also indicate some of the specialized structural features that bear witness to the degree of divergence and range of derived characters that have arisen in the several hundred million years since these groups split from that origin.