. "3 The Theory of Facilitated Variation--JOHN GERHART and MARC KIRSCHNER." In the Light of Evolution: Volume 1. Adaptation and Complex Design. Washington, DC: The National Academies Press, 2007.
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In the Light of Evolution, Volume I: Adaptation and Complex Design
though animal anatomy and physiology have changed. These conserved functional components comprise that which is regulated in the animal; regulation of them has changed in animal evolution.
To indicate their diverse indispensable contributions to the phenotype, we enumerate core processes in Table 3.1, associating each with one of four major episodes of pre-Cambrian functional innovation (mostly protein evolution). These biochemical, molecular genetic, cell biological, physiological, and developmental components (which fill the textbooks of these fields) were carried forward, unchanged, in all bilateral animals. This, we argue, was such a powerful and versatile toolkit that post-Cambrian animals could largely omit further functional innovation at the gene product level (protein and functional RNA evolution) and instead exploit regulatory innovation to diversify anatomy, physiology, and development. What is remarkable about the processes, as a large set, is that they can be
TABLE 3.1 The Metazoan Toolkit of Conserved Functional Components and Processes: When Did They First Arise in Evolution?
First Arose in Evolution
Conserved Functional Components and Processes
Three billion years ago, in early prokaryotic organisms
Components of energy metabolism, biosynthesis of the 60 building blocks, DNA replication, DNA transcription to RNA, translation of RNA to protein, lipid membrane synthesis, transmembrane transport
Two billion years ago, in early eukaryotic cells
Components of the formation of microfilament and microtubule cytoskeletons, motor proteins moving materials along the cytoskeletons, contractility processes, movement of the cell by cilia and ruffling membrane action, shuttling of materials between intracellular organelles, phagocytosis, secretion, chromosome dynamics, a complex cell cycle driven by protein kinases and protein degradation, sexual reproduction with meiosis and cell fusion
One billion years ago, in early multicellular animal life forms
Components of 15–20 cell–cell signaling pathways, cell adhesion processes, apical basal polarization of cells, junction formation, epithelium formation, specialization of cells toward physiological ends, some developmental processes of the single-celled egg to the multicellular adult
Near pre-Cambrian, in animals with early body axes
Components of complex developmental patterning, such as anteroposterior axis formation (Wnt/Wnt antagonist gradients) and dorsoventral axis formation (Bmp/antagonist gradients), inductions, complex cell competence, additional specialized cell types, formation of the body plan’s map of selector gene compartments (both transcription factors and signaling proteins), various regulatory processes
