Box Extension 15.1

Genomics and the Evolution of Nervous Systems

Did nervous systems evolve only once? All multicellular animals except sponges have neurons and nervous systems, but perhaps not all nervous systems are homologous. Genomic analyses have found many genes important for nervous system organization to be present in primitive, unicellular protists, and could have been employed in evolution of neurons on more than one occasion. Box Extension 15.1 describes the organization and evolution of nervous systems in different animal groups.

The simplest and presumably most primitive form of nervous system is termed a nerve net. In a nerve net, neurons are dispersed in a thin layer, not clustered into nerves or a central nervous system (CNS), and they are positioned seemingly at random relative to each other. Nerve nets occur in cnidarians, such as jellyfish, anemones, and Hydra, as principal elements of the nervous system (Part 1 of the figure). Nerve nets also occur as subsidiary elements of the nervous system, in peripheral parts of the body in many invertebrate groups and in the intestines of vertebrates. The simplest nerve nets are unpolarized. That is, transmission of impulses is bidirectional rather than unidirectional: Neurons make synaptic contacts at points where they cross, but either neuron can be presynaptic and either can be postsynaptic. Moreover, action potentials initiated in the postsynaptic neuron spread in both directions along its processes, and then spread along multiple, diffuse paths in the nerve net. Even in cnidarians, considered the most primitive phylum having a nervous system, more elaborately organized elements may augment unpolarized nerve nets. Some jellyfish, for instance, have a more directionally oriented (polarized) network of through-conducting neural pathways with primitive integrative centers, as well as the unpolarized net.

Genomic studies indicate that the molecular components of nervous systems preceded neuronal evolution. The voltage-gated channels necessary for generation and propagation of action potentials have homologues in prokaryotes, indicating that their evolution predates the origins of nervous systems. In addition, many molecular components of synapses evolved well before the origin of nervous systems.

Genomic studies also suggest that the nervous system of ctenophores evolved independently of nervous systems of other metazoa. The phylum Ctenophora (comb jellies) had previously been linked with Cnideria (jellyfish, sea anemones, Hydra) as the Coelenterata, but they are clearly distinct and probably a sister group to all other animals. Ctenophores therefore separated from both Cnideria and Bilateria before sponges, which lack neurons and nervous systems. It is likely that ctenophore nervous systems evolved independently, rather than sponges having lost a nervous system. Genomic studies strongly support this conclusion: A recently sequenced ctenophore genome lacks genes for most small-molecule neurotransmitter pathways and many other neuron-specific genes of other animals; instead it contains many ctenophore-specific genes that suggest an independent evolutionary path.

Figure A Nervous systems of different phyla (1) The sea anemone (phylum Cnidaria) has a nerve net. (2) The nervous system of a sea star (phylum Echinodermata) is radially symmetrical. Flatworms (phylum Platyhelminthes) (3), squids (phylum Mollusca) (4), earthworms (phylum Annelida) (5), and humans (phylum Chordata) (6) all display CNSs that feature brains.

Cnidarian nervous systems appear to be not very centralized, with fibers running in all directions and little apparent organization into central integrating areas (see Part 1 of the figure). Cnidarians have radial symmetry, a body form with no front or back and with apparently limited potential for the evolution of nervous system centralization. Echinoderms, evolutionarily closer to vertebrates but having secondarily evolved radial symmetry, also have relatively simple and uncentralized nervous systems (Part 2 of the figure). In contrast, all groups with bilateral symmetry (Parts 3–6 of the figure) show evolutionary trends of increasing centralization and complexity of nervous system organization.

Nervous Systems of Animals with Bilateral Symmetry Exhibit Centralization and Cephalization

Two major trends characterize the evolution of nervous systems in the bilaterally symmetrical phyla of animals: centralization and cephalization. Centralization of nervous systems refers to a structural organization in which integrating neurons are collected into central integrating areas rather than being randomly dispersed. Cephalization is the concentration of nervous structures and functions at one end of the body, in the head. Both trends can be seen even in flatworms, which belong to the phylum Platyhelminthes, considered the most ancient phylum to have bilateral symmetry (see Part 3 of the figure). Apparently the presence of a distinct anterior end and the development of a preferred direction of locomotion in bilateral animals have been important in the evolution of centralized, cephalized nervous systems.

In flatworms and animals of more complex bilaterally symmetrical phyla, centralization is anatomically evident by the presence of longitudinal nerve cords, discrete aggregations of neurons into longitudinally arranged clusters and tracts to constitute a distinct CNS. Motor neurons extend out from the CNS to effectors, and sensory neurons extend from the periphery of the body into the CNS. Increasing numbers of interneuronsneurons that are neither sensory nor motor and are confined to the CNS—make their appearance as nervous systems become more complex. The interneurons enhance capacities for centralized integrative processing in the nervous system. The peripheral nervous system (PNS) also is increasingly consolidated in bilaterally symmetrical animals. Instead of a random meshwork of processes running in all directions in an unpolarized nerve net, the peripheral sensory and motor processes are coalesced into nerves, discrete bundles of nerve axons running between the CNS and the periphery (see Parts 3–6 of the figure).

Cephalization, the other general evolutionary trend in nervous system organization, involves varying degrees of anterior concentration of nervous system organization. In the most primitive of centralized nervous systems, each region of the CNS largely controls just its own zone or segment of the body (see Parts 3 and 4 of the figure); indeed, elements of such segmental or regional organization persist in all phyla, including vertebrate chordates. In most bilaterally symmetrical animals, however, the most anterior part of the CNS exerts a considerable degree of domination and control over other regions. This anterior part, typically larger and containing more neurons than other parts, is called the brain (see Parts 3–6 of the figure). Brain is a general term for an anterior enlargement of the CNS.

Cephalization is thought to be an evolutionary adaptation resulting from the tendency of bilaterally symmetrical animals to move forward, so that information about newly encountered parts of the environment impinges first on the front of the animal. As a correlate of forward motion, most groups with bilateral symmetry have evolved anterior placement of many of their major sense organs. The anterior placement of the brain allows it to receive environmental information from these sense organs with a minimum of neural connection lengths and corresponding delays. The relative importance of the anterior brain then led to varying degrees of its dominance over the rest of the CNS.

Among vertebrates, a trend somewhat analogous to cephalization occurs within the brain: The relative size of the forebrain (anterior part of the brain) increases successively in reptiles, nonprimate mammals, and primates. Along with this development, functions formerly controlled by the spinal cord or brainstem come increasingly under forebrain control.

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