Homologies whether it derived from a single origin
and Convergences in the Nervous System
There is constant debate as to whether the current nervous system structure was
derived from an homology evolutionary origin or if the similarities between the
current and ancestry systems are due to convergent evolution.
Homology is a term that can be used to describe common evolutionary ancestry of
corresponding structures throughout different species ultimately derived from a
last common ancestor (Wagner, 1989). Whereas, convergence is evolutionary
independent and can form analogous structures (not present in last common
ancestor of species), or know by Darwin as ‘decent with modification’. Evidence
to support this concept can be found by tracking small alterations in neural
pathways that cause changes in an organisms abilities. These abilities were
found to have evolved independently in taxa of common ancestors that lack these
traits (Nishikawa, 2002).
essay, we are going to examine the evidence found to determine the evolution of
the nervous system by differentiating and investigating the homologies and
convergences between species.
Origin of the Nervous System
It is still unknown as to the origin of the nervous system; whether it derived
from a single origin or developed independently in ctenophores (comb jellies of
invertebrates)/a common ancestor (Moroz, 2009). In
context of the animal tree, the position of ctenophores are considered to be a
large contributor to the evidence of determining the nervous systems origin.
Through phylogenetic work it was found that it was controversial for
ctenophores to be placed as a sister group; and that nervous system-bearing
animals were formally known as a monophyletic group (derived from a common
evolutionary ancestor). Therefore, it is unknown as to whether the nervous
system developed independently in ctenophores or was absent in sponges and
placozoans (Pisani et al., 2015).
Understanding the distribution of various characteristics associated with
nervous system function can inform us of the early development of these
Synapses are used to send signals that work by using a protein framework on the
postsynaptic terminal that serves as a locus for modulatory pathways (Emes et al., 2008). They depend on proteins
not specific to the nervous system, such as how neurotransmitter release is
managed by an early complex protein that also controls intra-cellular functions
such as exocytosis and transport (Kloepper,
Kienle and Fasshauer, 2007). Likewise, with similar proteins, junctions can
be formed between lymphocytes and antigen presenting cells in order to create a
immunological synapse formed by a release in lytic granules (Angus and Griffiths, 2013). Various post-synaptic
proteins are traceable to protist ancestors, however there are thousands of proteins
that have arose more recently in vertebrates that have more complex synapses (Burkhardt et al., 2011) ;(Emes and Grant, 2012); (Emes et al., 2008).
non-vertebrates have very different morphologies; synapses in particular. Vertebrates
tend to possess unidirectional synapses that have separate pre and
post-synaptic specialisation, whereas ctenophore and non-vertebrates have more
diverse morphologies that are less clear (Cobb
and Pentreath, 1978). Ctenophores have bidirectional synapses, which is beneficial
in motor nerve nets where direction of flow does not contain much information (Anderson, 1985). Additionally, sponges and
placozoans have protein families that are associated with synapse proteins such
as neuropeptides and post-synaptic density proteins, despite lacking true synapses
(Smith et al., 2014). However, it was
discovered that sponges do contain contractile behaviours based on
neurotransmitter activity (Elliott and Leys,
Neurones use potential energy generated by ion channels and pumps that regulate
ionic gradients to drive action potentials along the axon. Action potentials
within non-animal species are commonly delivered as an eruption of ions that
will affect the intracellular physiology directly (Hille,
2007). Moreover, within animals, action potentials are mainly carried by
sodium ions along axons which, unlike calcium channels, do not activate
intra-cellular pathways. As sodium channels allow constant firing without the
consequence of toxic intra-cellular calcium build up, it is considered to have
contributed to a key transition in the origin of the nervous system as shown in
figure 1 below (Hille, 1989). This
transition is believed to have occurred twice by the substitution of convergent
amino acid substitutions (Liebeskind, Hillis and
Figure 1- Animal tree showing expansions
of gene families in association with synapses at different periods in time
during early evolution (Liebeskind et al.,
potassium channels control the duration and frequency of an action potential,
which contains neural code information. Potassium channels are similarly
believed to have undergone convergent changes, which suggests changes in code
complexity in lineages due to large gene family expansions independently
occurring in ctenophores (Martinson et al.,
2014); (Li et al., 2015). Within the same branches as potassium channel
expansion, ion channels that facilitate synapse signalling have also undergone
expansions, as shown in figure 1 (Liebeskind,
Hillis and Zakon, 2015).
Early Evolution of the Nervous System
A consensus view of
early nervous system evolution is valuable in determining the origin and
development of the modern nervous system. Very
early animals contained various genes that are currently prompting modern neurons.
Whether these early structures constituted as a nervous system depends on
whether a nervous system is considered as chemical signalling coupled with electrical
impulses, or if it consists of a complex electrical code maintained by specialised
axons and dendrites. If it is the latter then the nervous system may have
originated thereafter and possibly altered more than twice. It was found that non-nervous
system bearing animals, in comparison to nervous system bearing animals, have
more complex behaviours (Liebeskind et al.,
It was discovered that several small stature animals have lost essential
features of neurones such as sodium ion generated action potentials (Nordstrom et al., 2003). After the 5 major
lineages (ctenophores, sponges, placozoans, cnidarians and bilaterians) have
diverged; there were expansions in the gene families that are associated with
synaptic and electrical complexity. This caused a large biophysical change
which arose convergently in the stem of these lineages (Emes and Grant, 2012); (Liebeskind, Hillis and Zakon, 2015); (Martinson et al., 2014).
Homology of the
nervous system across phyla, would suggest many taxa experiencing evolved alterations,
such as loss or reduction of the neural organisation derived from an ancestor,
with regression of the nervous system being maintained by signals and cascades (Karaiskou et al., 2014).
These evolved modifications could have happened on multiple occurrences throughout
the evolution of the nervous system due to ontogeny of a species, as shown
though the development of atrophy in the eyes and the optic tecta of the cavefish,
where it conserved17% of the resting metabolism of the brain (Niven, 2015).
In order to understand the evolution of the nervous system, it is necessary to
consider examples of origin of sensory organs such as the eye. In a study by Randel and Jékely, the possibility of an evolutionary
transition of a visual sensory system from one that is not, supports the theory
of the evolution of image-forming eyes through selection pressures such as the
evolution of photosensitive cells and their underlying circuits (Randel and Jékely, 2015).
Furthermore, in the 1990s it was argued, amongst most phyla, whether the circuit
and functional correspondences of olfactory systems were universal (Hildebrand and Shepherd, 1997). Moreover, the discovery
of nerve cells, not as part of syncytium, but as distinct elements helped distinguish
bipolarity and monopolarity in the nerve cells of vertebrates and invertebrates
(Edwards and Huntford, 1998).
Early animals who were able to respond and thus move to stimuli, suggested they
were equipped with sensory-motor circuits. Brunet and Arendt hypothesise that
these organisations originated in unicellular eukaryotes where the development
of sensory cilia could be acted upon by action potentials causing an
appropriate response. Similarly, multicellular organisms would show formation
of muscles and neurones due to appearance of mechanoreceptors (Brunet and Arendt, 2015).
It is also
suggested by Eisthen and Theis that microbes were heavily involved in evolution
of sensory systems and cellular communication throughout metazoan evolution.
They found that the physiology of the nervous system can be effected by
environmental and symbiotic microbes, thus these relations may prove how microbes
have contributed to the interaction between epithelial and microbes which
resulted in the development of proto-neurones by internalising specialised
conducting cells (Eisthen and Theis, 2015).
Work by Wenger et al, suggests that, throughout
evolution, proto-neuronal functions within ancestral epithelial cells of basal
metazoans differentiate progressively into specialized cells (Wenger, Buzgariu and Galliot, 2015). This was
further suggested by Angelika Stollewerk who demonstrated divergence by showing
variation in neurogenesis activity and management of neural genes in arthropods,
which may have encouraged divergent evolution of neurogenesis (Stollewerk, 2015).
Furthermore, Moroz and Kohn focused on the nervous systems of ctenophores, as
it was uniquely distinct from that of cnidaria and bilaterians. Ctenophores
were found to have a great number of peptide signalling, however lacking a transmitter
characterized in other nervous systems within metazoans;
suggesting that ctenophores nervous systems were independently evolved. The
theory of neurones evolving multiple times independently suggests that neurones
are not homologous across phyla, but that their synaptic organisation may have
evolved multiple times (Moroz and Kohn, 2015).
There are many
theories for the origin of the nervous system in a vertebrate from an invertebrate
predecessor. William Bateson theorises that during development, a species known
as ‘Balanoglossus’, had a condensed
nervous system (Bateson, 1884). This
taxon is currently considered key evidence in the investigation of the vertebrate
nervous system ancestry, as their nervous system expressed several genes involved
in brain and spinal cord patterning of the chordate known as proneural genes (Lowe et al., 2003).
There have been multiple studies considering the similarities of the brain and
nervous system between vertebrates and arthropods that are argued to be
homologous. This was further investigated by Wolff and Strausfeld who
identified many similar genetic, molecular and structural characteristics
shared between arthropod bodies and vertebrates hippocampus. Such similarities
found included a neuronal pattern that outlines the structure of the forebrain,
as well as similar common ancestry relationships regarding the olfactory
system. Additionally, within mice and flies, proteins have been found that play
a key role in memory as they define the distinct brains of some acoela (class
of simple, bilaterian invertebrates) which brings to question whether these
systems originated in early bilaterian evolution (Wolff
and Strausfeld, 2015).
To gain more knowledge on the diversity of nervous system evolution, less
familiar taxa must be studied. Studies of the penis worms lava nervous system
by Hejnol at al, resulted in identifying how ventral nerves with a causal
ganglion were created via condensation of neurones made by proteins and
neuropeptides used in early developmental stages (Martín-Durán
et al., 2015).
It was shown by Paul Katz that evolutionary alterations of homologous circuits
can cause divergent evolution of behaviours, or a common leitmotif. It was also
shown that while large changes of ancestral circuits can result in
corresponding changes in behaviours, there have been cases where divergence of circuits
have occurred lacking any observable changes in behaviour. Katz work suggested
that rhythmic behaviours have evolved convergently, however neural circuits are
different, therefore behavioural evolution cannot be assumed (Katz, 2015).
of the development of the electrical and synaptic complexity of the nervous
system provides clear evidence for the modern nervous system having been
convergently derived. Studies by Randel
and Jékely further support the theory of
convergent evolution as they provide evidence of independent of selective
pressure involved in the development image-forming eyes from non-visual sensory
eyes (Randel and Jékely, 2015). However,
there are still uncertainties as many aspects of the nervous system is complex
thus hard to determine where/when many structures appeared or developed and
how. Additionally, there is lacking information from non-nervous system bearing
organisms and ctenophores to make an accurate assumption (Pennell et al., 2014). With further research
into ctenophores and more uncommon organisms it will become easier to
deconstruct phenotypes and the diversity of the nervous system can be fully
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