The Primary Nose and Palate in Evolution

Abstract

The aim of this chapter is to understand how the primary nose and palate formed when vertebrates emerged from water on to land, passing from the fi sh stage through that of the amphibians to the reptiles (from which evolved the fi nal vertebrate groups, the birds and mammals). 2.1 Emergence of Olfaction in the Animal Kingdom Life appeared in water, and the earliest traces of living things are more than 3.6 bil-lion years old. Garstang's hypothesis is one possible explanation of how chordates originated from an echinoderm (invertebrate) ancestor more than 500 million years ago (Fig. 2.1). Echinoderms are spiny-skinned invertebrates that live on the ocean fl oor. They give rise to the hemichordates and then urochordates. In Garstang's hypothe-sis, urochordates play a central role in the evolutionary steps giving rise to the cepha-lochordates and vertebrates (Cameron et al. 2000 ; Stach and Turbeville 2004) . As adults, urochordates are sessile and morphologically similar to sea sponges. Although adult urochordates live a sedentary life attached to substrata such as rocks, larval urochordates are free-swimming tadpoles. While urochordate larvae exhibit all four of the chordate characteristics (notochord, dorsal nerve cord, pharyngeal slits, postanal tail), most of these features, except the pharyngeal slits, are lost dur-ing the metamorphosis into the adult form (Fig. 2.2). The pharyngeal slits function both in respiration and suspension feeding. However, the original function of the perforated pharynx (gill slits) of the chordates is thought to have been fi lter feeding with gas exchange being added much later in chordate evolution. Water and food particles are drawn into the pharynx through the incurrent siphon: water is expelled through the slits into the atrium, whereas food particles trapped by mucus are swept by cilia into the digestive tract. An excurrent siphon allows the water current, fi ltered of oxygen and food, to pass out of the body.