The Cambrian Explosion: Developmental Potential and Ecological Opportunity  

The Ediacaran-Cambrian diversification of animal life, like most major evolutionary transitions, involved interplay between environmental possibility, ecological opportunity and developmental potential.  Understanding these events requires consideration of the role of each element this macroevolutionary triad and their interactions.  Here I focus on the role of developmental and ecological innovations. 

The discovery that the basic developmental toolkit is highly conserved across all animals, including sponges and cnidarians, and that bilaterians include expanded repertoires of many elements of this toolkit, including many gene families of transcription factors and microRNAs, provides a new perspective on the early history of animal development.  It is now clear, for example, that the burst of morphologic complexity was not accompanied by an increase in the number of genes, but rather in the degree of regulatory control, allowing more precise formation of spatial and temporal patterning within the developing animal embryo.  One implication of the early appearance of the metazoan developmental toolkit is that almost all of the Ediacaran organisms could represent cnidarian-grade organisms.  Recent studies of developmental gene regulatory networks also demonstrate how the locus of selection changed during the Ediacaran-Cambrian.  The construction of tightly linked regulatory feedback loops (kernels) shifted selection from individual genes to gene networks.  Such non-uniformitarian developmental changes challenge traditional (if implicit) uniformitarian assumptions of both microevolution and macroevolution. 

 Much of this developmental potential originated by 575 Ma, and the full bilaterian toolkit probably by 555 Ma.  Taking full advantage of this developmental potential, however, was dependent upon the construction of progressively more connected ecological networks.  Palaeontologists have often invoked ‘open ecospace’ or logistic growth models as explanations for such diversifications, but they miss the critical point, which is one of network dynamics: how to construct webs of trophic interactions.  The most important interactions in building such networks are those that involve positive spillover effects, where ecosystem engineering effectively bootstraps greater biodiversity.  During the Cambrian the initial positive spillover effect was likely provided by increased oxygen levels, and then further enhanced by bioturbation, which changed substrate redox gradients and triggered blooms of microbes.  The adaptations of Cambrian organisms can be roughly classified in terms of their potential feedback effects.