The neuralians, animals with nervous systems (Nielsen, 2008), first began to populate the earth during the Cambrian era. The function of these nervous systems was to sense and relay information about surroundings. Neuronal networks comprise both sensory cells and communicating cells, which interact via chemical or electrical synapses (Westfall, 1996). Many signalling molecules and transcription factors that are critically implicated in neural development are highly conserved among animals, suggesting a common ancestor. The development of nervous systems is one of the most prominent events in animal evolution as it allows an animal to process multiple messages simultaneously and respond appropriately (Watanabe et al., 2009). This essay will discuss theories as to why neuralians evolved during the Cambrian and not before or after. The origins of modern nervous systems will also be investigated.
During the Cambrian era there was a large increase in both the diversity and disparity of animal life (Marshall, 2006). This 'explosion' of animals appears to have been the culmination of several specific ecological, environmental, and developmental events. Environmentally, the planet first needed to be able to maintain large levels of life. It is thought that sufficient atmospheric oxygen levels were the most important factor necessary for this (Knoll and Carroll, 1999). This is not to say that atmospheric oxygen levels were unable to support large amounts of life prior to the Cambrian, just that the ability to maintain life of this magnitude was required. Ideal temperature ranges may have also have been important in sustaining life. Prior to the Cambrian 'explosion' there is thought to have been an increase in continental shifting. This caused methane from the earth's core to be released into the atmosphere. Atmospheric methane absorbs heat and is therefore thought to have produced an increase in the planets temperature (Kirschvink and Raub, 2003). Prior to this the earth was much colder, with the oceans extensively freezing at several points between 550 million years ago (mya) and 750 mya (Hoffman et al., 1998).
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Immediately prior to the Cambrian, 543mya, there was a mass extinction detected in the fossil record by negative carbon isotope anomalies (Knoll and Carroll, 1999; Marshall, 2006). This is thought to have removed the Ediacaran biota, which had flourished following a similar mass extinction 580 mya produced by the Gaskiers glaciations - a partial freezing of the planets oceans. However unlike the Gaskiers, it is unknown as to the exact cause of the Precambrian extinction. This extinction theory as to why the Cambrian led to both increases in diversity and disparity is supported by Valentine's (1980) niche space model, which explains the increases in both diversity and disparity ecologically. Valentine reasons that the probability of a major evolutionary innovation or a number of smaller innovations being successful is dependent on there being limited competition at the time the morphological changes occurred. This allows for the creature to multiply and adjust to its ecological niche. As the planets niche spaces filled during the beginning of the Cambrian, it appears to have become progressively more difficult for new body plans to become established. This theory has been adapted by Kauffman (1993) who argues that, rather than theoretical 'niche spaces', disparity is dependent on the number of needs required for survival. This causes the potential number of morphologies to be limited until new needs arise. In this manner the large increase in disparity is thought to have occurred due to an 'arms race' situation (Cadée, 2005; Kauffman, 1993; Niklas, 1994). As the late Ediacarans/ early Cambrians developed defensive and/ or predation mechanisms, there became a large increase in survival requirements. Dzik (2007) examined trace fossils from the Ediacaran/ Cambrian boundary which show a variety of species burrowing into the sediment, yet feeding above the surface. As the animals were not gaining nutrients from the soil, it is suggested that the development of predatory organisms targeting the larger metazoans must have forced them to seek protection. This defensive step in evolution occurred concurrently with the mass skeletonization of smaller creatures (Dzik, 2007; Vermeij, 1989) further implicating the advent of predation in the initial development of filter feeder defences.
The genetic structures of animals needed to be in place prior to the Cambrian explosion to allow for morphological changes to occur. Both the genes themselves and the specific combinations of those genes during development needed to be present (Marshall, 2006). As new niches opened and survival demands grew, these genes were provided with opportunities to develop functionality. The major inhibiting factor of morphological evolution is only changes that essentially change nothing can be tolerated (Ghysen, 2003). All changes are screened for compatibility with the current developmental system throughout the developmental program. Once a coherent organization has been produced, it is further stabilized by the progressive accumulation of additional mechanisms. This stabilization would produce 'canalization', an ever increasing robustness of the system in which it becomes progressively harder to modify genetic pathways produced earlier in evolution (Marshall, 2006). By progressively reinforcing the developmental process, individuals will be protected from any effects of mutation, potentially leading to increased genetic variation amongst the entire population (Kirschner and Gerhart, 1998). Populations with increased genetic variation will have increased resilience to environmental and ecological changes, improving the success of the species. Once this stage of neural development had been reached, and once the radiation had become possible, the entire evolutionary process would be unlikely to reoccur. Consequently the major radiation occurred only once, and only from one ancestor; the one whose neural development had reached a level of complexity and stability robust enough to withstand significant changes (Ghysen, 2003).
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To examine the origin of nervous systems it is necessary to examine the nervous systems of modern animals. Bilaterians, which include over 99% of current animal species, are bilaterally symmetrical and triploblasts, meaning they contain three germ layers; the ectoderm, mesoderm and endoderm (Marshall, 2006). Bilaterians such as insects and vertebrates have substantially different nervous systems, both anatomically and developmentally. Until recently it was believed that these differences were far too great for the evolutionary origin of the brains to be linked. Molecular and genetic analysis of mouse and fly nervous systems has shown that the two brain types are actually very similar (Arendt and Nubler-Jung, 1999). In the developing vertebrate central nervous system (CNS), the basic units correspond to intersections on a grid (Figure 1). Each unit originally comprises a discrete, developmentally independent compartment, which expresses a unique combination of genes. As neural development proceeds, specific neuroanatomical features develop within and at the interfaces between these units. The embryonic Drosophila CNS is patterned by a closely comparable grid system with genes that are homologous to those of vertebrates. Specifically the Otx, Emx and Hox genes have been shown to be essential for the correct development and patterning of both brain types (Sprecher and Reichert, 2003). It is now generally accepted that the nervous systems of triploblasts do share many essential features. This implies that they must have a single common ancestor that already enjoyed a fairly sophisticated nervous system. This first common ancestor of all bilateria has been named Urbilateria (De Robertis and Sasai, 1996)
Figure 1. Compartmentalised arrangement of genes in the developing CNS. Figure adapted from (Carroll et al., 2005)
Cnidarians are regarded as the first class of organisms in animal evolution to exhibit a nervous system (Holland, 2003; Watanabe et al., 2009). The cnidarians have radial symmetry and comprise of organisms such as hydras and jellyfishes. Histologically, cnidarians are usually considered to consist of two tissue layers, the epidermis and endoderm. Neurons are found in both the epidermis and endoderm, although they are much more abundant in the epidermis (Westfall and Elliott, 2002). These neurons form independent nerve nets, and each cnidarian may have several nerve nets throughout its body. Each nerve net is a two-dimensional meshwork of bipolar and multipolar neurons with neurites that can conduct in either direction. Basiepidermal nerve nets comprise neuronal cell bodies which communicate with interneurons, motor neurons, and epidermal sensory cells (Westfall and Elliott, 2002).
The ancestral cnidarian nerve net, which was pervasive and basiepidermal, became localized in the basal groups of bilaterally symmetrical animals. This is thought to be due to the development of antineural activity which limited the CNS to a specific area (Holland, 2003). Further evidence of a cnidarian ancestor has been found in genetic screening studies (Ball et al., 2007; de Jong et al., 2006; Mazza et al., 2007). Many of the genes necessary for bilaterian nervous system development are also expressed in cnidarians. However both Hox and Otx homologues, while being expressed in cnidarians, do not correspond to the anterior-posterior patterning seen in bilaterians. This suggests that in early metazoans these genes may not yet have been incorporated into nervous system development (Watanabe et al., 2009).
While cnidarians were the first entire class to display nervous systems, the very first nervous system developed in a more primitive creature. It is therefore necessary to examine the creatures from which cnidarians evolved from to find the last non-neuralian and the first neuralian. The sponges are considered to be the most primitive group of animals, from which cnidarians and bilaterians evolved from. They are multicellular but have a small number of cell types, they also do not have occluding cell junctions or Hox genes (Richelle-Maurer et al., 2006; Tyler, 2003). However sponges do have most of the genes required for the postsynaptic scaffold although the sponges lack both a nervous system and synapses. This genetic screening study also suggests that only a small number of extra genes may be needed to complete the network characteristics of the synapse exhibited in more advanced creatures (Sakarya et al., 2007). The first common ancestor of creatures with nervous systems is thought to be a modified gastrula (Figure 2); it has therefore been named neurogastraea (Nielsen, 2008). Neurogastrea had a nervous system with sensory cells, communicating networks, a coordinating centre and organized electrical and chemical synapses. Neurogastrea is also thought to have developed Hox genes (Nielsen, 2008), suggesting that the first stages of neurodevelopmental patterning occurred in these creatures. Cnidarians and neurogastraea are very similar. Cnidarians have several homologous genes and are both morphologically and developmentally similar to neurogastraea. This suggests that both cnidarians, and therefore, as described above, bilaterians, are descendant from a sponge (Neilsen, 2008).
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Figure 2. Proposed evolution from sponge to neurogastraea. Sponge larvae began reproducing without reaching the adult sponge stage (G), leading to the loss of the adult stage and a new independent species, gastraea (H). The first nervous system is then thought to have evolved from a modified gastraea (I), which gained specialized sensory organs, chemical synapses, electrical synapses, septate junctions and a short Hox-cluster. Figure adapted from Neilsen (2008).
The analysis of basic nervous systems has already helped in the understanding of more complicated nervous systems. Genetic studies have discovered that cnidaria and bilaterians share a variety of genes and signalling molecules, some of which have been found to be essential to neurodevelopment. Further studies will be able to provide a greater understanding of the first nervous systems and how they developed. This will in turn help to explain the evolution of nervous systems, from cnidarians to humans, by providing insights into the crucial genes and molecules required to change the morphology at each evolutionary point.
In summary, the neuralains began to appear on earth due to the large increase in survival requirements following the mass extinction at the Ediacaran-Cambrain boundary. While ediacarans had the genes to develop nervous systems, the ecological conditions at the time did not require creatures to discard a large amount of energy in exchange for a nervous system. The large extinction detected by the carbon isotopic anomaly the planets ecology changed (Knoll and Carroll, 1999). Food for filter feeders became scarcer as many smaller creatures were wiped out. This led to the need to purposefully find food in order to survive, producing the first predators. Predation started an 'arms race' which led to the formation of prey defensive mechanisms such as skeletonization and burrowing. There is also possibility that biomineralization occurred prior to predation, making easily digestible food even harder to find following the extinction and forcing the filter feeding animals to adapt. Gastrea, a modified sponge larvae, was the last ancestral species to not have a nervous system. Neurogastrea evolved from gastrea, developing the first nervous system which contained similar genes and structures to that of modern bilaterian nervous systems. The incorporation of nervous systems ultimately allowed predators to sense which prey was most valuable, when it was worth striking, and where the highest prey densities were located. These factors allowed for increased survival of the predator species, making nervous systems very valuable for survival.