Key Transition 2: To Jaw-Jaw is Always Better
British Prime Minister Winston Churchill, the most quotable English-speaking politician of the twentieth century, once opined that “To jaw-jaw is always better than to war-war.” Churchill was using the word “jaw” to refer to the endless chatter of diplomatic negotiation, but the original function of vertebrate jaws undoubtedly had more to do with feeding than communication. Predators had no interest in negotiating with their prey in the Palaeozoic seas.
Jaws probably did not appear in the march of vertebrate evolution until sometime in the Silurian Period. Until that time all vertebrates were jawless, and several lineages of jawless fishes survived into the Devonian. Two jawless groups, the hagfish and lampreys, are still with us today. Hagfish are scavengers that tear into carrion with a protrusible tongue that bears teeth made of keratin, the protein that forms human hair and fingernails. Some lampreys do not feed as adults, but others use a similar tongue to feed on the flesh or blood of other fishes, to which they cling with a sucker-like mouth containing more keratinous teeth. Experiments on hagfish have shown that the protrusible tongue is surprisingly powerful, and effective at removing pieces of food and transporting them into and through the mouth cavity. Some fossil jawless fishes had either a fringe of small bony plates along one edge of the mouth, acting to scoop detritus from the sea floor or filter it from the water, or tooth-like mineralised structures inside the mouth or throat. A diverse range of successful feeding mechanisms has clearly existed among jawless fishes, past and present.
Fig1. The funnel-like mouth of a modern lamprey, jawless but armed with small keratinous teeth for rasping through the skin of other fishes.
However, jaws were an important innovation that opened up new functional possibilities and, in particular, must have ushered in a new age of successful predation by early gnathostomes. In jawed vertebrates the lower jaw, called the mandible, acts like a lever made of bone or cartilage. The mandible is connected by a hinge to the back end of the upper jaw, which in some fishes is itself mobile relative to the skull proper, and muscles can be used to rapidly raise or lower the mandible to close or open the mouth. Lowering the mandible quickly can draw in a sizeable volume of water along with any small prey that it may contain, the key manoeuvre in the “suction feeding” of many aquatic vertebrates, whereas raising the mandible quickly can trap prey in the mouth and forcefully drive teeth into the prey’s body.
In evolution, new structures are commonly built by modifying pre-existing ones. In the case of jaws, anatomists long ago noticed a basic similarity to the gill arches of fishes, vertical bars of bone or cartilage that separate successive gill slits and support the gills themselves. Gnathostome gill arches are made up of multiple segments, and it is tempting to interpret the upper jaw and mandible as the two surviving segments of a highly modified gill arch. In the traditional account of the origin of jaws, the gill arches were all similar to each other in the jawless ancestors of gnathostomes, and the first arch in the series was transformed into the jaws.
Fig2. The front part of a shark skeleton in side view. Each of the gill arches, structures made of cartilage that support the gills, has upper and lower portions. This is also true of the hyoid arch, which lies in front of the gill arches and helps to support the jaws. The jaws themselves look as though they might just have evolved from yet another "arch" at the very front of this series, the upper and lower jaws corresponding to the upper and lower parts of a gill arch or the hyoid arch.
However, studies of gene expression in the embryonic development of lampreys and gnathostomes fail to support this elegant scenario, suggesting instead that much of the gnathostome jaw apparatus is equivalent to structures surrounding the mouth of the lamprey rather than to the first gill arch. A major reconfiguration of the internal structure of the head clearly occurred in early gnathostomes and their close relatives among jawless fishes, but how jaws appeared as part of this reconfiguration remains a topic of extremely active research.
Whatever their exact evolutionary origin, jaws have played an important part in the march of vertebrate life ever since their first appearance. Vertebrates of different types use their jaws to crush the throats of prey animals, slice and grind their way through tough vegetation, and filter plankton out of seawater. The utility of jaws, however, goes beyond feeding. Animals may use their biting ability to fend off attacking predators or do battle with rivals of the same species, and Winston Churchill was entirely correct to draw attention to the usefulness of jaws in oral communication. Ultimately, prime ministers and diplomats owe their ability to jaw-jaw to an anatomical innovation that appeared more than 400 million years ago.
Fig3. Jaws are useful to a predator, as the osteichthyan Megamastax amblyodus of the Silurian Xiaoxiang Fauna demonstrates by biting down on a hapless galeaspid of the species Dunyu longiforus.