Developmental and evolutionary origins of the pharyngeal apparatus

While ontogeny does not simply recapitulate phylogeny, it is undoubtedly true that ontogeny is shaped by phylogeny. Developmental processes have evolutionary histories and these can be uncovered through experimental analysis and comparative studies across a range of species. It is through these approaches that insights into how developmental processes have been assembled over evolution can be garnered. In this article, we wish to discuss the development of the pharynx and to make the case that this process has been profoundly shaped by its evolutionary history. Importantly, we are now at a point where the developmental and evolutionary studies can be brought together and we can identify steps that have emerged successively during evolution. We can uncover deeply conserved features of pharyngeal development that preceded the emergence of the vertebrates and indeed can now be seen to have evolved as early as the deuterostomes. We can also relate the remodelling of the pharyngeal region that occurs during development to evolutionary modifications that occurred within the vertebrates. We would argue that in the pharynx developmental events collectively betray our phylogenetic history.Although it is not readily apparent when considering adult anatomy, our pharyngeal apparatus has a metameric origin, arising from a series of bulges found on the lateral surface of the head of the embryo, the pharyngeal arches. These structures are first evident at about three to four weeks of human development, and it is within these that the nerves, muscles, skeletal tissues and epithelial specializations of the pharynx are subsequently laid down and fashioned. The development of these structures is, however, complex and involves interplay between a number of disparate embryonic populations (Figure？1) [1]. The ectoderm, which lies externally, will give rise to the epidermis and form localized thickenings, termed neurogenic placodes, the sensory neurons that will innervate the pharynx. Int