Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 266

have evolved at the same time and along with neurons. This is the “synapse first”
model of Ryan and Grant (2009) .
Crucial in transmitting signals through synapses is the group of postsynaptic
proteins known as postsynaptic density (PSD) proteins, embedded in the postsyn-
aptic membrane. PSD is a discoidal structure composed of several hundred proteins
(scaffold proteins linked to the actin cytoskeleton as well as receptor molecules,
protein kinases, phosphatases, etc.) and protein complexes. Genes coding for PSD
proteins are found across the animal kingdom ( Alie and Manuel (2010); Galliot and
Quiquand, 2011; Kosik, 2009 ).
Once evolved in cnidarian-like animals, the neuron seems to have conserved not
only its synaptic composition and structure but also its functions and morphology
to an exceptional degree; it secretes almost all essential neurotransmitters and their
receptors throughout the metazoan species ( Figure 5.7 ).
A close examination of the differences between cnidarians and ctenophores on
one side, and the bilaterians on the other, shows that the only relevant difference is
that the latter evolved a higher organized CNS with a greater number of neurons.
Centralization of the nervous system in bilaterians increased its computational capa-
bilities and its ability to generate the epigenetic information required for erecting
specialized supracellular structures of different types of cells and tissues into organs,
and organs into organ systems.
The diffuse neural net in cnidarians represents a not yet full-fledged ICS. The ani-
mals of this eumetazoan group have not yet acquired the function of a central regu-
lator of homeostasis like the one we saw in bilaterians. This is why cnidarians and
ctenophores did not evolve in their respective specialized organs and larger bodies, but
still rely on diffusion that is efficient only in these thin-walled diploblastic animals.
The primitive diffuse nervous system that led to the evolution of these primitive
eumetazoans soon became the cause of their evolutionary stagnation. Further pro-
gress into the evolution of the animal world waited for a major evolutionary inno-
vation, in the form of the centralized nervous system, which brought expanded
computing capabilities, that performs the functions of a more advanced ICS.
The advent of the neuron in organisms comparable to extant cnidarians and cteno-
phores about 600 Mya marks the evolution of an early offshoot that is higher in the
tree of life than sponges, yet not evolved enough to join the Cambrian club.
Evolution of the Integrated Control System and the Cambrian
Explosion
Hard fossil evidence on the evolution of bilateria exists only for the lower Cambrian
with only one case, that of Kimberella, dated in the transition period between the
Ediacaran and Cambrian, about 555 Mya. Hence:
the most parsimonious interpretation of the Cambrian fossil record is that it repre-
sents a broadly accurate temporal picture of the origins of the bilaterian phyla.
Budd (2003) ; Budd and Telford (2009)