Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 268

reiterate that no correlation is shown to exist between Hox genes, considered as
major players in animal development, and the evolution of animals.
Hox genes may be traced back to the LCA of bilaterians and cnidaria ( Schierwater
and Kamm, 2010 ). The cnidarian N. vectensis , for example, has more homeodo-
mains than Drosophila ( Ryan et al., 2006 ) and the worm C. elegans has half the Hox
genes of its ancestor ( Aboobaker and Blaxter, 2010 ). There is no evidence to prove
gene duplications as “one of the primary driving forces in the evolution of genes and
genomes” ( Roth et al., 2007 ). To the contrary, the evidence shows that the Cambrian
falls in the “silent periods” of gene duplications. There is no link between these
bursts and the Cambrian explosion ( Miyata and Suga, 2001; Suga et al., 1999 ).
A bare glimpse on the Cambrian explosion would not have missed the tempo-
ral coincidence of five major transitions at that point in time: the emergence of the
centralization of the nervous system, triploblasty, bilateral symmetry, evolution of
organs and organ systems, and cephalization. Let us remember that this evolutionary
leap occurred in a sister group of cnidarians that showed none of the above charac-
ters. If one excludes miracles, the simultaneous emergence of four major transitions
requires a causal explanation.
A closer examination into the causal net between the four coinciding evolutionary
developments may help us recognize the possible causal agent(s).
My hypothesis is that the centralization of the nervous system, which repre-
sents evolution of the full-fledged ICS in animals, was the driving force behind this
momentous landmark of animal evolution.
Centralization of the nervous system is associated with the evolution of the
bilateral symmetry. This is in clear distinction to the cnidarians, where the diffuse
nervous system is generally associated with radial symmetry ( Figure 5.8 ). Bilateral
symmetry and cephalization enabled directed forward movement and increased the
efficiency of movement, which is essential for a predatory lifestyle (both for preying
on and escaping from predators) of the Cambrian biota. Directed movement implies
the existence of a centralized nervous system. In individual development, the bilate-
rian symmetry during development first appears at the neurula stage.
In the ontogeny of vertebrates, bilateral symmetry appears at gastrulation in the
stage of the primitive streak (Hensen node) and primitive streak, at the time of the
notochord process and the neural plate. Not only is bilateral symmetry established
at this time but the three fundamental axes of the body and of the future neural tube
also become evident: a longitudinal axis with rostral and caudal ends (cephaliza-
tion), a vertical axis with dorsal and ventral surfaces and a horizontal axis with
medial (proximal) and lateral (distal) positions along the axis.
Flores-Sarnat and Sarnat (2008)
This is clearly before the expression of the A-P (anterior-posterior)-related Hox
genes; hence, if ontogeny tells us anything about evolutionary history, it is that the
above suggests that the evolution of body axes in bilaterians is not related to Hox
genes and their colinear expression in extant animals.
Triploblasty seems related to (not that it is a cause of!) the development of organs
and organ systems, since most organs develop from the mesoderm, which is absent