Introduction

Morphological differences between adult taxa of closely related species, including ancestors and descendants, must arise from differences in development. It follows that to understand how and why such differences arose – and how they are manifested in patterns of integrated morphology – we need to understand the shifts in the developmental processes that generate them. For example, are changes in brain shape responsible for most of the differences in overall cranial shape between modern and archaic humans (Weidenreich, 1941), or do the differences in cranial shape between these taxa reflect multiple pathways of selection on particular aspects of the face and neurocranium that may be adaptations to climate, mastication, speech, and other such factors? Recent advances in evolutionary developmental biology (for reviews, see Hall, 1999; Carroll et al., 2001), as well as studies of craniofacial integration and development (e.g., Ackermann & Krovitz, 2002; Krovitz, 2000; Ponce de León & Zollikofer, 2001) indicate that our null hypothesis should probably be the latter. Apparently most (but not necessarily all) evolutionary changes occur through shifts early in development that make use of pre-existing developmental pathways to generate novel but highly integrated morphologies.

Unraveling the complex relationship between patterns of growth and development and their underlying processes is especially interesting but challenging for studies of the origin of our own species.