New perspectives on the evolution of protochordate sensory and locomotory systems, and the origin of brains and heads

Cladistic analyses generally place tunicates close to the base of the chord ate lineage, consistent with the assumption that the tunicate tail is primi tively simple, not secondarily reduced from a segmented trunk. Cephalochord ates (i.e. amphioxus) are segmented and resemble vertebrates in having two distinct locomotory modes, slow for distance swimming and fast for escape, that depend on separate sets of motor neurons and muscle cells. The sense o rgans of both amphioxus and tunicate larvae serve essentially as navigation al aids and, despite some uncertainty as to homologies, current molecular a nd ultrastructural data imply a close relationship between them. There are far fewer signs of modification and reduction in the amphioxus central nerv ous system (CNS), however, so it is arguably the closer to the ancestral co ndition. Similarities between amphioxus and tunicate sense organs are then most easily explained if distance swimming evolved before and escape behavi our after the two lineages diverged, leaving tunicates to adopt more passiv e means of avoiding predation. Neither group has the kind of sense organs o r sensory integration centres an organism would need to monitor predators, yet mobile predators with eyes were probably important in the early Palaeoz oic. For a predator, improvements in vision and locomotion are mutually rei nforcing. Both features probably evolved rapidly and together, in an 'arms race' of eyes, brains and segments that left protochordates behind, and ult imately produced the vertebrate head.