  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.
