Evolution of Jaws

Jaws are an example of an extreme evolutionary advantage. The evolution of jaws allowed early gnathostomes (jawed fishes) to grasp objects firmly and, in conjuncture with teeth, cut and grind food into small pieces. Early gnathostomes were able to exploit new food resources that were otherwise not available to their jawless relatives (Pough, Janis, & Heiser, 2009).



 The importance of jaws not only rests on the ability to use new resources but, according to The Jaw Hypothesis, that adaptive immune system evolved in jawed vertebrates due to the evolution of jaws (Matsunaga & Rahman, 1998). But how and why did jaws evolve? Most scientists believe that jaws evolved from the first brachial arch (mandibular arch) of an ancestral gnathostome with the second brachial arch (hyoid arch) providing jaw support (Mallatt, 1996). The issue of why jaws evolved is a question asked by few, because by today's standards having a jaw is superior over lacking one. This rational is largely due to the misconception that jaws and teeth go "hand in hand". However, when jaws first evolved, early gnathostomes lacked teeth (Mallatt, 19996). One theory proposes the idea that jaws were initially important for ventilation rather than prey processing and capture. By enlarging the mandibular arch into "protojaws", it allowed more forceful movements of water across their gills (Mallatt, 1996), which incresed their oxygen intake. It was once thought that agnathas and gnathostomes were two separate lineages based on brachial arch position and gill shape, however it is now thought that gnathostomes evolved out of early agnathas (Pough, Janis, & Heiser, 2009). Geneticists have discovered that similar genes are expressed in the mandibular segment in both lampreys and gnathostomes, indicating that they are likely homologous (Kurantani, 2004).