The origin of animal body plan is one of the central questions in
developmental biology (Arthur, 2000). A long studied subject, it
seems established that evolutionary rates of different characters and
lineages are different. Our results in Fig. 3 support Davidson and
Erwin's theory that the hierarchical structure of the gene regulatory
network has imposed constraints on the rate of further evolution of
the most basic, and earliest-evolved features. The slow speed of
evolution of the kernels that control the development of animal
phylum- and superphylum-level body plan characteristics is why no
new phylum-level body plans appeared after the pre-Cambrian
period. The number of types of classes, orders, families, and species
is increasing, and our results show that this observation is surprisingly
consistent with the increasing evolutionary speed from kernels to
plug-ins to I/Os to batteries. We propose that the slow evolution of
the top components and fast evolution of the bottom components of
the hierarchy is a universal phenomenon in evolution, not only in the
gene regulatory networks, but also in protein interaction networks,
cell signaling networks, and metabolic networks (Deem, 2007).