A Behavioural Perspective on the Early Evolution of Nervous Systems: A Computational Model of Excitable Myoepithelia

How the very first nervous systems evolved remains a fundamental open question. Molecular and genomic techniques have revolutionized our knowledge of the molecular ingredients behind this transition but not yet provided a clear picture of the morphological and tissue changes involved. Here we focus on a behavioural perspective that centres on movement by muscle contraction. Building on the finding that molecules for chemical neural signalling predate multicellular animals, we investigate a gradual evolutionary scenario for nervous systems that consists of two stages: A) Chemically transmission of electrical activity between adjacent cells provided a primitive form of muscle coordination in a contractile epithelial tissue. B) This primitive form of coordination was subsequently improved upon by evolving the axodendritic processes of modern neurons. We use computer simulations to investigate the first stage. The simulations show that chemical transmission across a contractile sheet can indeed produce useful body scale patterns, but only for small-sized animals. For larger animals the noise in chemical neural signalling interferes. Our results imply that a two-stage scenario is a viable approach to nervous system evolution. The first stage could provide an initial behavioural advantage, as well as a clear scaffold for subsequent improvements in behavioural coordination.