Homologies know by Darwin as ‘descent with modification’. Evidence

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 ‘descent 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).

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Throughout this
essay, we are going to examine the evidence found to determine the origin of
the nervous system by differentiating; investigating the homologies and
convergences between species.

Origin of the Nervous System
It is still unknown as to whether it derived from a single origin or developed independently
in the comb jellies of invertebrates known as ctenophores (Moroz, 2009). In the animal tree, the position
of ctenophores are believed 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
expressed phenotypes.

Chemical Aspects
Synapses are used for cellular communication and  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).
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.,

Vertebrates and
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).

Electrical Aspects
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 modification in the evolution 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 Zakon,

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).
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).

Very early animals contained
various genes that are currently promoting modern neurons. Whether these organisations
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., 2016).

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).
To further understand the processes, 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 evolution of a visual system from a non-visual system,
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 transitioned
multiple times (Moroz and Kohn, 2015).

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).




Early studies
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 variety of the nervous system can be fully valued
and understood.




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