GENERATING DEVELOPMENTAL CAPACITY DURING AN INTERVAL OF INTENSE ENVIRONMENTAL CHANGE: THE EARLIEST HISTORY OF ANIMALS

The capacity for multicellular development and the generation of the cell types, developmental patterning and complex morphologic and behavioral attributes of animals required a suite of changes in cellular gene regulation and development associated with the origin and early evolution of animals. Comparative studies of the development of extant animals and their closest relatives (choanoflagellates, filastreans and ichthysporeans) has revealed that many components of developmental regulation originated prior to the origin of animals, possibly for the control of the various cell types required by complex life cycles. The novelties in metazoan developmental genes and processes novelties area associated with the formation of genome control mechanisms, developmental tools and developmental patterning, but do not appear to have been immediately associated with complex spatial patterning. Understanding the expansion of regulatory and developmental capacity requires consideration of their origin during an interval of intense environmental change. In particular, changes in the levels and stability of oxygen in shallow marine environments have been invoked as causal drivers. Integrating developmental, genomic and fossil data with the environmental context strongly supports a model of early metazoans of many cell types but relatively flat regulatory hierarchies and limited morphogenic patterning. Although some amount of genome expansion was involved, co-option of existing regulatory machinery for new purposes was pervasive as various eumetazoan lineages acquired the morphologic novelties needed for the larger body sizes associated with the Ediacaran-Cambrian ‘explosion’ documented by the fossil record.