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. When some animals started living a mobile lifestyle and eating larger food particles externally, they developed ciliated epithelia, contractile muscles, and coordinative and sensitive neurons for it in their outer layer.

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 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. Further cephalization and nerve cord (ventral and dorsal) evolution occurred many times independently in bilaterians.

Action potentials, which are necessary for neural activity, evolved in single-celled eukaryotes. These use calcium rather than sodium action potentials, but the mechanism was probably adapted into neural electrical signaling in multicellular animals. In some colonial eukaryotes, such as Obelia, electrical signals propagate not only through neural nets, but also through epithelial cells in the shared digestive system of the colony. Several non-metazoan phyla, including choanoflagellates, filasterea, and mesomycetozoea, have been found to have synaptic protein homologs, including secretory SNAREs, Shank, and Homer. In choanoflagellates and mesomycetozoea, these proteins are upregulated during colonial phases, suggesting the importance of these proto-synaptic proteins for cell to cell communication. The history of ideas on how neurons and the first nervous systems emerged in evolution has been discussed in a 2015 book by Michel Antcil. In 2022 two proteins SMIM20 and NUCB2, that are precursors of the neuropeptides phoenixin and nesfatin-1 respectively have been found to have deep homology across all lineages that preceded creatures with central nervous systems, bilaterians, cnidarians, ctenophores, and sponges as well as in choanoflagellates.

Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction. Other ways sponge cells communicate with neighboring cells is through vesicular transport across highly dense regions of the cell membranes. These vesicles carry ions and other signaling molecules, but contain no true synaptic function.

Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.

The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, endoderm and ectoderm. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.

The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Cryogenian period, 700–650 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an especially large ganglion at the front, called the "brain".

Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain.