Early metazoan development: the origin of the Cambrian exuberance.

DESCRIPTION: A number of common features can be observed in the earliest developing embryos of all animal phyla. A simple extant model of morphogenesis is outlined here, with the aim of giving a model of the relatively rapid appearance of Cambrian animals, 541-515 mya. Developmental patterning, elucidated by a simple linear model with only short-range diffusion of ligands, is given as the origin of the most primitive animals. The key aspect of the model involves the interaction between the emergence of the Wnt and Hedgehog (Hh) signaling pathways. The non-canonical Wnt pathway is crucial in first establishing a sphere of cells, by way of cell-cell connection fi bers. A mutation in the Wnt pathway at the dawn of multicellular organisms is argued to have given rise to the early Hh pathway, and their interaction gives two spatially separate gene determination regions, the key goal of biological patterning.