Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 282

antientropic demon ( sensu Maxwell) that also made possible four indispensable
functions of the control system in the development and reproduction of animals:
● Spatial restriction of gene expression in the animal body.
● Manipulative expression of genes (see page 197, Making Environmental Signals
Intelligible to Genes ).
● Selective recruitment of genes.
● Epigenetic marking of genes (see page 71, Neural Origin of Epigenetic Information
in Epigenetic Structures ; page 206, Where Does the Epigenetic Information for DNA
Methylation Come from? ; and page 202, The Source of Information for Selecting Sites of
Histone Modification ).
The Biological Demon 1—Precise Spatial Restriction of Gene Expression
During development, the developing human embryo produces billions to trillions
of cells of more than 400 different types. The type to which a cell is differentiated
at this stage depends on the genes it expresses and the patterns of gene expression.
Since the different cell types are intertwined in complex patterns in tissues, the
organism must possess a highly precise mechanism, at the microscopic level, capa-
ble of providing individual cells the right morphogen. Needless to say, the text-
book mechanism of gradients of concentrations of morphogens does not come into
account. No plausible genetic mechanism or hypothesis exists to explain how the
organism can determine the intricate arrangement of cell type patterns of cells in
animal structures. Indeed, even simpler biological tasks, such as the localized action
of circulating hormones on specific cells, tissues, and organs, while preventing their
action throughout the rest of the body, is beyond the explanatory power of modern
genetics.
The spatial restriction of the expression of genes has been a crucial requirement, a
sine qua non , for cell differentiation, histogenesis, and organogenesis, for the evolu-
tion of the animal world from the beginning of metazoan life.
How can we learn about this mechanism of restricted gene expression employed
by Cambrian biota more than a half a billion years ago? The principle of phyloge-
netic actualism again tells us that we may extrapolate mechanisms of the restricted
spatial expression of genes in extant animals to the Cambrian biota. In view of
their indispensability, these mechanisms have been operative from the onset of the
Cambrian period.
We know the neuroendocrine system, via various hormonal cascades and GRNs,
controls expression of numerous genes. But we do not know how the organism
restricts the action of thousands of factors that circulate throughout the animal body
to target cells, tissues, or organs only.
A considerable number of examples of local innervation are involved both
in the local expression of specific genes and in restricting the action of circulat-
ing hormones to specific regions of the body that are known as targets of hormo-
nal action ( Cabej, 2004 ). Local innervation restricts the expression of hormonally
induced genes to particular regions, where needed, by inhibiting the expression of
specific receptors, which act as mediators to these hormonal actions. This is a novel