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In-Depth Information
Porifera
Eumetazoa
CNS
Muscle cells
Synapses
Nerve cells
Neuromuscular cells
?
?
?
Evolution of synapses
=
Paracrine-to-electrochemical
dominance transitiony
Synaptic scaffold
Contractile cells
Paracrine prenervous integration system
Neurotransmitter systems (e.g. GABAergic system)
Voltage gated ion channels (action potentials)
Figure 5.7 Summary of the coevolution of integration and effector systems in Metazoa. The
phylogenetic tree follows, and others, with alternative scenarios presented as dashed lines.
The position of Placozoa remains a matter of debate but will be of special interest for our
understanding as soon as more details on the placozoan integration system are known. The
evolution of the ultrastructure of synapses remains unresolved, but the synapse gene repertoire
seems to be plesiomorphic to all Metazoa. Abbreviations : CNS, central nervous system;
GABA, γ-amino butyric acid.
Source : Nickel (2010) .
Bilateria gave rise to the explosive radiation of millions of species and higher taxa
that dominate the animal kingdom as we know it: 99% of extant animals belong to
Bilateria (Finnerty et al., 2004). Why does Bilateria enjoy such a huge taxonomic
superiority over its sister groups?
From the neoDarwinian view, the Cambrian explosion is a great paradox. It is
an event that cannot be predicted by the conventional biological wisdom. The most
widely accepted neoDarwinian explanations involve changes in genes, especially
those in the genetic toolkit (which ironically is highly conserved), in GRNs, and reg-
ulatory sequences (see The “Regulatory” Hypothesis of Morphological Evolution,
p. 247, and The GRN Hypothesis of Evolution, p. 248 ). Here it is necessary to
 
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