Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 271

(see page 149, Epigenetic Control of Postphylotypic Development in Animals for an
extended discussion of the crucial role of the nervous system in development).
In this context, the postulate of the phylogenetic actualism that the present mecha-
nisms of development of phenotypic traits in general are similar to those that brought
about the evolution for the first time around, syllogistically leads to the conclusion
that the CNS played a leading role in the Cambrian morphological diversification.
A second source of corroborative evidence on the role of the nervous system in
the development and evolution of Cambrian biota derives from studies in biological
regeneration. The phenomenon is observed in lower invertebrates, such as coelen-
terates (cnidarians and ctenophores) and echinoderms and up through higher verte-
brates such as amphibians, reptiles, and even mammals. In all, the studied species
regeneration depends on innervation, suggesting that the nervous system plays a
morphogenetic role in regeneration ( Kumar and Brockes, 2012 ).
Within a week after the experimental removal of its eyes, the flatworm Polycelis
nigra regenerates new eyes. But it does not do so if its brain is removed. Eyes regen-
erate if the head lysates or brains of individuals from other flatworm species are
implanted ( Kumar and Brockes, 2012 ).
Almost two centuries ago, in 1823, Tweedy John Todd (1789-1840), an English
physician, observed that salamanders could regrow amputated limbs, providing for
the first time experimental evidence that:
Nerves … may also serve to maintain the morphological and developmental integrity.
Singer and Géraudie (1991)
Regeneration of amputated limbs by amphibians also requires local innervation.
After limb amputation, the apical ectodermal cap covered by an ectodermal layer
develops on the stump. The remaining cell types in the neighborhood dedifferentiate
to form a cluster of pluripotent cells, the regeneration blastema, from which the new
limb develops. In the absence of local innervation in the early larvae of Xenopus , the
blastema does not form and the limb does not regenerate. Experimental deviation of
the sciatic nerve to wound sites in certain regions around the hind leg induces the
formation of supernumerary limbs.
An interesting example of the morphogenetic role of the innervation during regen-
eration in mammals comes from experiments on the healing of the punched ear lobe
in Murphy Roths Large (MRL) mice. When a normally innervated ear is punched, it
heals completely by developing normal cartilage and epithelial structures, including
hairs, whereas in the denervated ear the regeneration is defectuous ( Figure 5.9 ).
Centralization of the Nervous System and Evolution of the
Animal Complexity in Bilaterians
We have shown that the centralization of the nervous system coincided with the
advent of bilaterality, triploblasty, and the formation of organs and cephalization in
the Cambrian biota. Now, we will briefly review the concomitant evolution of the