Paper #: 06-02-004
There is a variety of questions one might like to answer about the origin of animal bodyplans: When did these bodyplans arise? What was the rate of developmental and morphological innovation associated with these events? How reliably does the fossil record reflect the pattern of metazoan divergences and the timing of origin of bodyplans? How do these events relate to environmental and ecological changes? And more broadly, what, if anything, does this evolutionary episode tell us about the nature of the evolutionary process? Each of these questions has been the subject of learned discourse and even summarizing the history of these discussions would exhaust both the available space and the reader’s attention (for an excellent recent and comprehensive review see Valentine, 2004). Here I will focus largely on the developmental aspects of the origin of animal bodyplans, particularly as revealed over the past decade or so by studies of recent organisms. Contrary to all expectation, such comparative studies have revealed remarkable conservation of regulatory elements across considerable phylogenetic distance. Placed in a phylogenetic framework, these studies have permitted inferences about the nature of many nodes on the phylogenetic tree, and from this we can develop and evaluate models of the processes of developmental evolution. As will become evident, my own view is that evidence of conservation of sequence and even regulatory relationships are not guarantees of functional conservation. Consequently, inferring the morphologic attributes of early metazoa is much more problematic than some have argued (see also Erwin and Davidson, 2002). Understanding these developmental innovations is important for another reason: identifying the complexity of various nodes during the early history of animals is critical to constraining the dates of these nodes and, more importantly, distinguishing between alternative forcing functions for the radiation of the bilaterian metazoans. If early animals, and in particular the last common ancestor of all bilaterians, already possessed high developmental complexity, then developmental innovations alone would seem to be an unlikely cause of the metazoan radiation (see Valentine and Erwin, 1987). Alternatively, it could be that we can identify a suite of developmental innovations both necessary and sufficient for some or all of the new bodyplans that appear during the Ediacaran-Cambrian metazoan radiation. If, however, we find that the necessary genetic and developmental toolkit for building the panoply of bodyplans pre-dates the metazoan radiation, this is strong evidence that we must search instead for either changes in the physical environment or in the dynamics of ecological interactions. These latter two issues are considered in more detail in Erwin (2005).