What nervous systems do: early evolution, input–output, and the skin brain thesis

Nervous systems are standardly interpreted as information processing input–output devices. They receive environmental information from their sensors as input, subsequently process or adjust this information, and use the result to control effectors, providing output. Through-conducting activity is here the key organizational feature of nervous systems. In this paper, we argue that this input–output interpretation is not the most fundamental feature of nervous system organization. Building on biological work on the early evolution of nervous systems, we provide an alternative proposal: the skin brain thesis (SBT). The SBT postulates that early nervous systems evolved to organize a new multicellular effector: muscle tissue, the primary source of animal motility. Early nervous systems provided a new way of inducing and coordinating self-organized contractile activity across an extensive muscle surface underneath the skin. The main connectivity in such nervous systems runs across a spread out effector and is transverse to sensor-effector signaling. The SBT therefore constitutes a fundamental conceptual shift in understanding both nervous system operation and what nervous systems are. Nervous systems are foremost spatial organizers that turn large multi-cellular animal bodies into dynamic self-moving units. At the end, we briefly discuss some theoretical connections to central issues within the behavioral, cognitive and neurosciences.