As additional supporting evidence, we consider the time of
appearance of regulatory genes during embryo development. From
available experimental data, we show the earliest appearance time of
regulatory genes, defined as the time when a given gene is expressed
and starts to regulate the expression of other genes. In Fig. 5, we can
see that the kernel genes generally express earlier in endomesoderm
than other regulatory genes, and most plug-ins genes appear earlier
than I/Os genes. The Karl Ernst von Baer's law states that “General
characteristics of the group to which an embryo belongs develop
before special characteristics. General structural relations are likewise
formed before the most specific appear.” That is: differentiation
proceeds from the general to particular, with taxonomically more
general parts expressed earlier in development. In this case, we can
interpret as the kernels which expressed earliest in development are
more related to the higher hierarchical level of taxon such as phylumand
superphylum-level body plan, while others are more likely
related to lower hierarchical level body plan. Genes which are
expressed earlier in development are, mostly likely, older and more
likely to be conserved during evolution, because mutations of proteins
expressed earlier in embryo development are more likely to have
larger, more pervasive, and more deleterious effects on subsequent
development (Wimsatt and Schank, 2004).