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The evolution of nervous systems dates back to the first development of nervous systems in animals. Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Many primitive systems, like those found in complex protozoa, use non-electrical signaling for motility and other aspects necessary for survival. Data suggests that these systems, which use a chemical gradient for messaging, evolved into the electrical-signaling cells known today.
Simple nerve nets seen in animals like Cnidaria evolved first, consisted of polymodal neurons which serve a dual purpose in motor and sensory functions. Cnidarians can be compared to Ctenophores , which although are both jellyfish, have very different nervous systems. Unlike Cnidarians, Ctenophores have neurons that use electrochemical signaling. This was perplexing because the phylum Ctenophora was considered to be more ancient than that of Porifera , which have no nervous system at all.
This led to the rise of two theories which described how the early nervous system came about. One theory stated that the nervous system came about in an ancestor basal to all of these phylum, however was lost in Porifera. The other theory states that the nervous system arose independently twice , one basal to Cnidarians and one basal to Ctenophores.
Bilateral animals – ventral nerve cords in protostomes and dorsal nerve cords in hemichordates and chordates – evolved with a central nervous system that was around a central region, a process known as cephalization.