THE CAMBRIAN EXPLOSION: THE CONSTRUCTION OF ANIMAL BIODIVERSITY (Invited Presentation)

The extraordinary diversification of clades during the late Ediacaran and Cambrian represents a major evolutionary transition in the history of life, encompassing changes in the physical environment, particularly the oxygenation of shallow shelves, the establishment of new ecological interactions, and the growth of new developmental regulatory networks. Although many discussions of Ediacaran-Cambrian events have focused on just one segment of this macroevolutionary triad, we argue for the importance of understanding the network of interactions among and within changes to the physical environment, ecology and development. Recent molecular clock evidence (Erwin, et al. 2011, Science) favors a divergence of sponge and cnidarian clade ancestors c. 780 Ma, followed by the origin of major bilaterian clades during the Ediacaran, and the origin of bilaterian crown groups close to the Cambrian boundary, consistent with evidence from the fossil record. These results suggest that the primary cause for the bilaterian diversification at the base of the Cambrian was either environmental or ecologic, building on a wealth of developmental tools that had evolved by the late Cryogenian or early Ediacaran. Theories for the cause of the Cambrian explosion differ strongly in the extent to which they follow currently operating mechanisms vs. invoking non-uniformitarian explanations, a dichotomy particularly apparent among geochemical workers, but subtly present among hypotheses involving ecology and development as well.