Recent research in neuroscience and genetics has led to the idea that the neuron,
previously thought to have only evolved once, has gone through two separate
spurts of evolution. By examining the genome and neuromuscular structure of
ctenophores, researchers are finding that the nervous system and immune system
of this clade of animals has come around almost entirely independent from other
animal groups - from the closely related proiferans to our own bilaterian
grouping. The researchers examined the genomes of two model ctenophores,
Pacific sea gooseberries and comb jellies (pictured above), to examine how
these species expressed genes involved in their muscular system, nervous
system, and immune system. The researchers used the genomes to place
ctenophores at the base of the phylogenetic tree (pictured below), adding to
the idea that ctenophores likely evolved separately from the rest of animal
phylogeny. The examination of the immune system genetics found that the
ctenophore immune system differs greatly from bilaterians, sponges, and cnidarians,
lacking rather important pattern recognition markers. Genes involved in body
patterns and axis formations were also found lacking in the ctenophores,
despite being present in all metazoans. The examination of the nervous system
of ctenophores found that ctenophores use practically none of the
neurotransmitters known to be used in cnidarians and bilaterians (i.e.
serotonin, adrenaline, dopamine, glycine, acetylcholine), suggesting these
neurotransmitters are adaptations of the later cnidarian and bilaterian lines.
While there are some neuron-related genes in the ctenophore that are shared by
bilaterians, the neurons of the ctenophore do not express these shared genes.
The similarity between ctenophores and later metazoans along with the stark
differences in the nervous, muscular, and immune system as well as the genetic
identification of ctenophores as a basal group supports the idea that
ctenophores evolved independent from and parallel to the later metazoans.
In
easier terms, the findings of this research give way to the idea that the
ctenophore phylum evolved separately from other phylums. This contrast previous
ideas of ctenophores as an ancestor to later species and groups. This finding
is especially interesting in the scope of neuroscience, as the previous way of
thinking had the ctenophore nervous system as an ancestral system for later
species, including bilateria. This research leads us to believe that, instead
of a single, continual development of the nervous system, the nervous system
instead evolved in two separate events - that of the ctenophore and that of the
following phylums. This researching finding means that there are two entirely
separate organizations of the nervous system that have evolved entirely
independent of each other. Looking at nervous system evolution through this
light means that the origin and evolution of our own nervous system is not as
clear as we had once thought.


(second phylogeny image source: https://whyevolutionistrue.wordpress.com/2013/12/20/the-outgroup-for-animals-ctenophores/)
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