Origination, Variation, and Conservation of Animal Body Plan Development

Abstract

Multicellular organisms develop from fertilized eggs or asexual propagules. In the case of animals the developmental processes by which their bodies take form originated in several phases beginning between 600 and 700 million years ago, in the Ediacaran period. Genes and signaling pathways, many of which were present in unicellular ancestors, came to mediate morphogenesis and cell pattern formation by virtue of bringing into play physical effects that were newly relevant on the scale of cell aggregates. Focusing on “liquid-like” properties of cell clusters and their capacity to act as “excitable media,” this review explores how the products of ancient and some novel genes of what became the “developmental toolkit” were variously employed to mobilize well-characterized physical effects and processes (cohesivity, phase separation and disaggregation, surface and shape polarization of cells, chemical oscillation, reaction–diffusion coupling) in the cell aggregates that eventually evolved into animal bodies and organs. This interplay of physics and genetics led to the generation of morphological motifs such as the tissue layering of gastrulation, lumen formation, body elongation, triploblasty, segmentation, and patterning of endoskeletal elements. Since not all founding lineages had identical sets of toolkit genes, not all morphogenetic and patterning processes were equally present in their descendents. These “physico-genetic” factors collectively account for the conservation and diversity of body plans seen in the present-day animal phyla. Keywords: “basal” metazoans; basal lamina; convergent extension; diploblasts; liquid tissue; lumen formation; multilayering; saltational evolution; segmentation; triploblasts; tetrapod limbs