Evolutionary issues in craniofacial biology.

This overview discusses evolution in the context of craniofacial development and developmental processes. It begins with a discussion of the origins of the craniofacial tissues in the dentine and bone of the dermal denticles of the Ordovician jawless vertebrates, followed by a brief discussion of the mechanisms responsible for the evolution of the jaws and the origin of vertebrate dentition. Then the unique neural crest cell origin of the craniofacial skeletogenic and odontogenic tissues is discussed with emphasis on the constancy of rostrocaudal polarization of the skeletogenic cranial neural crest. Given this constancy, the variation in the craniofacial region that occurs across the vertebrates must arise because of epigenetic interactions that evoke the differentiation of craniofacial tissues. These are discussed in the context of epigenetic cascades of epithelial-mesenchymal interactions and how such epigenetic control has itself evolved. Although several mechanisms are considered, emphasis is placed on variation of the timing of development processes (heterochrony).