Evolutionary convergence of higher brain centers spanning the protostome-deuterostome boundary.

Currently available evidence supports a single origin for the centralized nervous system of bilaterally symmetrical animals. Beneath the staggering diversity of protostome and deuterostome nervous systems lies a fundamental groundplan consisting of a tripartite brain and a nerve cord divided into distinct antero-posterior and medio-lateral zones. As divergent lineages have taken independent paths towards increased encephalization, complex brain centers have arisen that serve multiple levels of sensory processing and advanced behavioral coordination and execution. Many questions arise as one surveys the distribution of these brain centers across the bilaterian phylogenetic trees. What environments did these lineages encounter that promoted the acquisition of energetically expensive brain centers composed of thousands, millions or even trillions of neurons? What novel behavioral capabilities did these brain centers in turn give rise to? Comparative studies within vertebrate clades have revealed instances of parallelism and convergence that have been instructive in associating evolutionary changes in brain structure and function with specific behavioral ecologies. The present account reviews these findings and extends them to invertebrate animals that have independently evolved higher brain centers. By expanding the scope of comparative studies across phyla, it will be possible to uncover structural and functional constraints imposed by deep homology, and to better understand the environmental pressures that have given rise to brain and behavioral complexity.