Evolution of nervous systems

The evolution of nervous systems dates back to the first development of nervous systems in animals (or metazoans). Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Primitive systems, like those found in protists, use chemical signalling for movement and sensitivity; data suggests these were precursors to modern neural cell types and their synapses.^[1] When some animals started living a mobile lifestyle and eating larger food particles externally, they developed ciliated epithelia, contractile muscles and coordinating & sensitive neurons for it in their outer layer.^[2][3]

Simple nerve nets seen in acoels (basal bilaterians) and cnidarians are thought to be the ancestral condition for the Planulozoa (bilaterians plus cnidarians and, perhaps, placozoans). A more complex nerve net with simple nerve cords is present in ancient animals called ctenophores^[4] but no nerves, thus no nervous systems, are present in another group of ancient animals, the sponges (Porifera). Due to the common presence and similarity of some neural genes in these ancient animals and their protist relatives, the controversy of whether ctenophores or sponges diverged earlier, and the recent discovery of "neuroid" cells specialized in coordination of digestive choanocytes in Spongilla, the origin of neurons in the phylogenetic tree of life is still disputed.^[5][6][7] Further cephalization and nerve cord (ventral and dorsal) evolution occurred many times independently in bilaterians.^[5]