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Evolution Of The Nervous System

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There are two main origin theories on the evolution of the nervous system. These two concepts are monophyly and polygenesis. Monophyly states that, all neurons came from a single ancestry, while polygenesis states that there is more than one independent ancestry of neurons among species. The concept of monophyly is the groundwork of all biology (Moroz, 2009). The lineage of the nervous system present in all animals can be traced back to a common ancestor (Ghysen, 2003).

Nervous system in animals evolved once, because of it’s shared common ancestry. In the recent studies it has been shown that the ctenophores Mnemiopsis leidyi is the last common ancestor of all metazoans (Bucher 2015). Ghysen presented a resemblance between the inner ear
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In the single origin of neurons, comb jellies are said to the oldest in the clade coming before Porifera, Placozoa, jelly and bilateria (Bucher 2015). In the process of evolution the sponges and Placozoa lost their ability to send rapid signals and their neurons. The loss of their nervous system is still uncertain, however there could be an environmental role that could have played a role in the evolutionary process. The phylum Placozoa only has one species called Trichoplax adhaerens. This particular species have been identification with six somatic cell types containing proteins that are normally in neurosecretory cells. Which includes FMRFamid-like peptide, opening the door to the possibility of neuronal like cell existence in the phylum (Bucher 2015). Only Hexactinellida also recognized as the glass sponges practice electrical impulses that send brisk signals to the current. Primary cilia are present in the cell of vertebrate, invertebrate and also in sponges. Primary cilia’s function is similar to one another. Similarly, the cilium are involved in sensing chemical activity, sending signals through calcium wave. By removing the cilia or the osculum, sponges lose their ability to ‘sneeze”, which is used by freshwater sponges to unload their waste. Cilia is present in the osculum of all sponges that have been examined, displays the common sensory organ in Porifera (Leys 2015). The nervous system did evolve once through the common ancestry, however sponges and Placozoa lost their nervous system in their development
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