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. Simple nerve nets seen in animals like Cnidaria (jellyfish) evolved first, consisted of polymodal neurons which serve a dual purpose in motor and sensory functions. Cnidarians can be compared to Ctenophores (comb jellyfish), 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 (sponges), 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 invertebrates and dorsal nerve cords supported by a notochord in chordates evolved with a central nervous system that was around a central region, a process known as cephalization.
- 1 Neural precursors
- 2 Sponges
- 3 Nerve nets
- 4 Invertebrate Neural Induction
- 5 Vertebrate Neural Induction
- 6 Molecular Signaling of Neural Induction
- 7 Homeobox (HOX) Genes in Invertebrates
- 8 Homeobox Genes in Vertebrates (Rhombomeres)
- 9 Nerve cords
- 10 Evolution of central nervous systems
- 11 See also
- 12 References
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 do 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 recent book.
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.
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.
Invertebrate Neural Induction
Neural induction represents the initial step in the generation of the nervous system and begins with the segregation of neural and glial cells from other types of tissues. Experiments and research pertaining to neural induction are focused on invertebrates, specifically C. Elegans and Drosophila as well as vertebrates, specifically frogs. Invertebrates are much more powerful genetic systems however due to how easily researchers can screen for they are looking for. This process is called forward genetics. Frogs on the other hand are not as good as the aforementioned invertebrates because of their slower life cycle and their tetraploid genes, which are much more difficult to manipulate. One benefit that studying vertebrates such as frogs brings however are their big eggs, in which cellular changes can be observed.
The neurogenic region of invertebrates begins at the ventrolateral regions of the embryo. In Drosophila Melanogaster, development begins once the ventral furrow folds into the embryo interior. The invaginated cells become the mesoderm, and the neurogenic region becomes more ventral. The closure of the furrow creates a midline that will become the site of neurogenesis. The neuroblasts of the ectoderm enlarge and squeeze away the epithelium layer through a process called delamination. Delamination occurs in 5 total waves, called niches, each creating about 60 neuroblasts. These neuroblasts undergo cell division to produce the "ganglion mother cell" (GMC). The GMC divides only once to produce neurons or glia.
Vertebrate Neural Induction
After fertilization, the egg is polarized into a vegetal hemisphere and an animal hemisphere. The animal hemisphere, at the top of the egg (the neurogenic region) has smaller cells than the rest of the egg. Following this polarization, a Blastula is formed after the egg undergoes multiple division, with a blactocoel being the outcome. The blastocoel differs from the blastula because of the tiny pocket or space that is created. Following this, the gastrula is formed via the process of gastrulation which leads to the creation of the neurula. Through this process, the neurogenic regions turns in to the neural plate, which in the precursor to the neural tube, which later becomes the brainstem.
The creation of the neural tube occurs once the neural plate folds inwards. Along with the neural tube, the neural crest is also created at this time, and it is the space in between the neural tube and the ectoderm. The neural crest produces the neurons and glia that lie outside of the central nervous system, i.e the peripheral nervous system.
Molecular Signaling of Neural Induction
When discussing neural induction, there are several major pathways which ultimately regulate gene expression. Two signaling cascades which appear to effect gene expression early on are the BMP signaling pathway and the Wnt signaling pathway. The BMP pathway starts with BMPs binding to a receptor composed of type 1 and type 2 subunits. This receptor is a major determiner in setting up epidermal cells. When no BMP is present the removal of the animal cap leads to the creation of neurons. When BMP is present, the removal of the animal cap leads to epidermal cells. When the receptor is bound, the type 2 subunit phosphorylates the type 1 subunit. The phosphorylation of the type 1 subunit causes further phosphorylation of RSMAD protein. These phosphorylated RSMADs for a complex with coSMADs forming a RSMAD::coSMAD complex. This complex then moves into the nucleus. Once in the nucleus, the complex binds to DNA sequences called BMP response elements which are present in the promotor regions of genes. This initiates transcription.
The Wnt process begins when the Wnt protein binds to a receptor called Frizzled. When Wnt is bound to the receptor, a second protein called β-catenin binds with several other proteins. Another protein called Disheveled prevents degradation of the formed complex only when Wnt is bound to Frizzled. As β-catenin accumulates, some of it moves into the nucleus where it complexes with TCF. The newly formed TCF β-catenin complex binds to DNA and activates transcription.
In frogs, involuting mesodermal cells of the involuting marginal zone release Chordin, Noggin, and Follistatin to ihibit BMP, causing the induction of neural tissue
Nervous system development is quite similar among thousands of different species, demonstrating an evolutionary connection of some kind. One of the primary examples of this are orthologs. Orthologs are any two or more homologous gene sequences found in different species that are related by linear descent. As stated previously, BMP inhibits neural differentiation in vertebrates. Drosophila Melanogaster possess a molecule known as dpp, which also inhibits neural differentiation in a similar way. Sonic hedgehog (shh) is the morphogen of vertebrates that induces neural crest development by inhibiting BMP. Drosophila has sog, which inhibits dpp, causing similar results.
Homeobox (HOX) Genes in Invertebrates
After the formation of the regionally specified anterior-posterior axis, genes must be turned "on" to form unique structures in specific regions. This is orchestrated via the homeobox (Hox) transcription factor signaling pathway. First identified in Drosophila by Edward Lewis who won a Nobel Prize for the discovery.
Homeobox genes are organized in Drosophila along two complexes [Antennapedia complex (ANT-C) and Bithorax Complex (BX-C)]. A total of 8 genes are organized in the cluster based upon anterior-posterior expression. Homeobox proteins are transcription factors composed of a highly conserved 60 amino acid protein sequence present in many organisms. Homeobox proteins bind to specific sequences of the DNA of genes to regulate their expression.
HOX clusters like the ones in Drosophila have been identified in vertebrates. Although there are more HOX genes in vertebrates, the position of the HOX genes in relation to its expression along the A-P axis is conserved among species.
Homeobox Genes in Vertebrates (Rhombomeres)
Most experiments into the role of HOX genes in vertebrate nervous system development has came from studies about hindbrain formation. The hindbrain forms a segmented pattern reminiscent of the segments within the Drosophila embryo. In vertebrates, these segments of the hindbrain are referred to as rhombomeres.
The rhombomeres are numbered from the anterior most unit, r1, which is just posterior to the midbrain, to the posterior most unit, r8, at the border between the hindbrain and the spinal cord. Each rhombomere gives rise to a unique set of motor neurons that controls different muscles in the head. For example, r2 and r3 make the trigeminal motor neurons that innervate the jaw.
Example: The facial nerve motor neurons are mainly produced in r4 and the abducens motor neurons are produced in r5. Loss of the Hoxa1 gene in mice results in a complete loss of r5 and a reduction in r4. This causes severe shrinking of the facial nerve and a otal loss of the abducens nerve.
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 Ediacaran period, 550–600 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.
Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups (superphyla) called protostomes and deuterostomes. Deuterostomes include vertebrates as well as echinoderms and hemichordates (mainly acorn worms). Protostomes, the more diverse group, include arthropods, molluscs, and numerous types of worms. There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Some anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline. But recent molecular data from different protostomes and deuterostomes reject this scenario.
As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two ganglia at the head end function similar to a simple brain. Photoreceptors on the animal's eyespots provide sensory information on light and dark.
The nervous system of one very small worm, the roundworm Caenorhabditis elegans, has been mapped out down to the synaptic level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons.
Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly. Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles. Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain.
In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil, in the interior.
Iodine and T4 stimulate the spectacular apoptosis (programmed cell death) of the cells of the larval gills, tail and fins, and also stimulate the evolution of the nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting. Contrary to amphibian metamorphosis, thyroidectomy and hypothyroidism in mammals may be considered a sort of phylogenetic, metabolic and neurologic regression to a former stage of reptilian life. Indeed, many disorders that seem to afflict hypothyroid humans have reptilian-like features, such as a general slowdown of nervous reflexes with lethargic cerebration, metabolism, digestion, heart rate, hypothermia and a dry, hairless, scaly, cold skin. 
Evolution of innate behaviors
Evolution of central nervous systems
Evolution of the human brain
There has been a gradual increase in brain volume as the ancestors of modern humans progressed along the human timeline of evolution (see Homininae), starting from about 600 cm3 in Homo habilis up to 1736 cm3 in Homo neanderthalensis. Thus, in general there is a correlation between brain volume and intelligence. However, modern Homo sapiens have a smaller brain volume (brain size 1250 cm3) than neanderthals; women have a brain volume slightly smaller than men, and the Flores hominids (Homo floresiensis), nicknamed "hobbits", had a cranial capacity of about 380 cm3, about a third of the Homo erectus average and considered small for a chimpanzee. It is proposed that they evolved from H. erectus as a case of insular dwarfism. In spite of their threefold smaller brain there is evidence that H. floresiensis used fire and made stone tools as sophisticated as those of their proposed ancestor, H. erectus. Iain Davidson summarizes the opposite evolutionary constraints on human brain size as "As large as you need and as small as you can".
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