Chapter 1

 NATURAL HISTORY OF THE HUMAN JAW SYSTEM

EARLY MOUTHS
Jaw systems became important as soon as organisms developed an internal tube through which portions of the external environment could pass. The mouths of early unicellular protozoans were just gashes in their sides. The mouths of jellyfish became surrounded by tentacle-like folds to help engulf food. The mouths of early fish were used to suck food off the ocean floor.  One of the first big steps in the evolution of the masticatory system took place roughly 300,000,000 years ago when the space between the lips and the throat widened to accomodate moveable jaws. 

“Plankton gathering is neither the quickest nor easiest way of obtaining food and, therefore, it is not surprising that in the succeeding vertebrates a number of evolutionary experiments occurred, aimed at changing from a microphagous to a macrophagous habit. Such experiments included, for example, the development of a horny-toothed oral sucker and tongue for rasping away the flesh of other animals which still survives today in the lampreys. However, the one really successful experiment was the modification of the skeleton supporting the two anterior gill arches to form opposable jaws."³

The mandible was the first bone to be attached to the body by a flexible joint, and the mechanism created for that joint paved the way for attaching arms, legs, and all other appendages. "It seems that other joints in the fishes' body are never as highly developed as the jaw joint... When jaws were devised, they were the first speedy, wide swinging, vigorous appendages to be attached to the body. The true diarthrodial joint was first formed here in fishes, and its basic structure remained essentially unchanged when it was appropriated by the limbs of land animals... Thus, the jaws led the way in all joint evolution."⁴

The property which made moveable jaws especially valuable was their ability to bear teeth which could be used as tools to crush or incise food. Teeth arose almost simultaneously with moveable jaws by the simple modification of the dermal denticles surrounding the mouth. The first teeth were epithelial outgrowths in the skin or mouth lining. Soon afterwards teeth became attached to the underlying bones - appearing on all the jaw bones, several palatal bones, and the rod-like tongue in many primitive fish. The teeth were specialized for different food sources.  Sharp teeth for cutting were found in sharks, and flat teeth for crushing were found in rays and skates.

These early jaw systems of fishes, reptiles, and amphibians could grasp, rip, and tear; but they could not really chew. Their teeth had to be continually replaced, because they were continuously blunted by abrasion. During function they acted as grasping or restraining devices rather than food processing devices. The mandible was moved straight up and down and used in combination with the neck muscles to tear off food and swallow it.

Life on land provided impetus for major new jaw system developments, because vast nutritional sources were stored there in nuts and seeds locked up inside cell walls and still unavailable to the digestive process. Breaking down those cell walls provided the potential increase in metabolic activity needed to hold the body out of water and transport it around on land, but that required chewing.

THE MAMMALIAN TEMPOROMANDIBULAR JOINT
About 70 million years ago, a temporomandibular joint (TMJ) developed where the condyles at the back ends of the mandible retruded back far enough to make direct contact with the temporal bone, and a joint formed where a synovial bursa appeared between two layers of rubbed periosteum and an intercepted muscle tendon. This was not an adaptation of a previous structure but an entirely new creation. The reptilian jaw joint had supported both the hearing and chewing mechanisms, and a stapes bone that was large enough to support vigorous chewing limited the sensitivity of the auditory mechanism. In the mammal-like reptiles, the lower jawbone increased in size, developing a coronoid process for muscle insertions and a temporal fossa for muscle origins; but the quadrato-articulate joint continued to operate the sound conducting mechanism. In the true mammals, support for the lower jawbone became entirely the responsibility of the new TMJ, and the old reptilian articular-quadrate joint was completely removed from any role in jaw support. The ear bones separated from the lower jawbone, and the tympanic ring became attached to the braincase. The quadrate and articular bones became incorporated into the middle ear where they joined a much reduced stapes to produce a chain of three tiny ear ossicles. Thus a new mammalian jaw joint developed right beside the old reptilian jaw joint, completely separating the jaw and auditory systems and thereby giving mammals the advantage of being able to chew and hear simultaneously.

The feature that made the new TMJ so valuable was that it enabled the mandible to move horizontally, which enabled rubbing the teeth together, which produced wear facets with crushing surfaes bordered by sharp edges, which enabled real chewing. While the reptilian jaw movement pattern was simply straight opening and closing, the mammalian jaw movement pattern had an oval shape as seen frontally. Whether the oval was very wide or very narrow, at its top end (where the teeth contact) the ability to shift the mandible horizontally even small amounts enabled the application of shearing forces between the teeth, which improved the processing of food enough to provide the energy required to maintain a constant high body temperature. 

"In the later part of the Triassic epoch, some 200 million years ago, mammals came into existence. This was a gradual process of evolution from a group of reptiles called the Therapsida or mammal-like reptiles. The earliest representatives were probably unlike any modern mammal. They were very small creatures - smaller even than the tiny pigmy shrew. We do not know if they were viviparous or if they suckled their young. We do not even know if they were warm-blooded (they could probably keep their body temperature above that of their surroundings, but not maintain it constant). We do, however, know that they had developed the mammalian jaw joint between the dentary and squamosal bones - our temporo-mandibular joint, and, in zoological terms, this classifies them as mammals...  At this stage we see, also for the first time, attrition of upper and lower teeth." ⁵

THE MAMMALIAN DENTITION

The freedom of movement in the TMJs of mammals is accompanied by a parallel freedom of movement on the surface of the bite table, where the rubbing contacts between the teeth produce attrition facets surrounded by edges that stay sharp until the teeth have worn away completely and thereby no longer need periodic replacement. As permanent structures, mammalian teeth acquired highly differentiated shapes which were individualized to fit their locations. Incisors, canines, premolars, and molars each blended enamel, dentin, and cementum in a manner which took advantage of their differential wear rates in order to turn the teeth into longlasting food processing tools with sharp edges for cutting it and ridges and rings of protruding enamel for grating it.

WEAR OF THE BITING SURFACES

To support the stability of the bite table in spite of the wear that took place on the biting surfaces of the teeth, each tooth root is surrounded by a metabolically active periosteal layer on one side and a proliferating layer of cementoblasts with embedded Sharpey's fibers on the other side. These structures provide an eruption force that moves the teeth to fill in any spaces between them. Primate experiments show that anything that lowers the postural position of the mandible promotes additional tooth eruption. The continual eruption force has been measured at 6 to 8 grams in rodents.^{16 -17}  It is probably only about a few grams in humans, but it sufficient to replace every micron of tooth structure lost to wear by a micron of new tooth structure brought up into the bite table. As a result, the framework of bones and teeth that supports the face is able to maintain a stable height and orientation, allowing the jaw closing muscles to maintain constant resting and working lengths. With continual eruption and gingival recession in balance, the distance between the alveolar crests and the bite surfaces stays constant, the gingiva can maintain an architecture which protects that space from food trapping, and the periodontium receives the stimulation it needs to pump accessory circulation that helps flush out waste products, even as the teeth wear down to their root tips.¹⁸  

Continual eruption also helps maintain the health of the periodontal structures by allowing the old cementum that had accumulated bacterial toxins at the bottom of the sulcus to be continually replaced by new sterile cementum on the erupting root surfaces. DuBrul explains, "Cementum, like bone, ages and finally degenerates.  In bone this process leads to resorption of the old and its replacement by new bone. In the cementum such turnover is impossible.  Instead, the aging cementum is covered by the formation of an additional young layer of cementum. This continuous apposition of new cementum occurs, in all probability, in waves separated by periods of rest. Growth of cementum is evidently indispensable for the integrity of the dentition. Continued growth of the cementum, however, needs space, and space is provided by the continued active eruption of the teeth.  The latter in turn depends on continued occlusal and incisal wear. Thus attrition as the prerequisite of compensatory active eruption is itself a necessary factor for the health of the teeth."¹⁵  

Continual wear and eruption of the teeth require continual pulpal recession to keep the tooth pulps protected from exposure to the bacteria-laden oral environment; therefore, the pulps of the teeth respond to temperature changes and mechanical stimuli by receding down into the roots and leaving behind layers of secondary dentin that absorb minerals to harden and become new chewing surface. If attrition on any tooth proceeds faster than the pulp is able to recede, sensitive thermal and tactile receptors in the dentin overlying the pulp produce pain so that chewing on that tooth is avoided until secondary dentin has mineralized enough to recreate sufficient pulpal protection.

WEAR ON THE INTERPROXIMAL SURFACES

Wear also occurs where adjacent teeth rub together, on their proximal surfaces. This interproximal wear improves the fit between adjacent teeth by creating perfectly fitting interproximal facets that function much like the articular surfaces of joints connecting the teeth, interproximal joints.  A line of these interproximal joints connecting all the teeth in a dental arch allows each tooth to maintain a small but definite range of movement while preventing it from drifting out of line with the other teeth in the arch.

THE MAMMALIAN JAW MUSCLES

To optimally work these bite surfaces together, the jaw muscles in mammals are selectively developed to power the chewing strokes and mandibular range of motion that produce the most effective chewing of the food in their environment. The single reptilian adductor muscle has separated into distinct units (temporals, masseters, and pterygoids), which are arranged in pairs that formed slings converging onto the mandible from widely separated origins and thereby enable fine control of the position of the mandible. The temporalis muscles, with long straight fibers, are especially useful for wide opening and fast vertical chopping; while the masseter and pterygoid muscles, with short thick transversely oriented fibers, are especially useful for horizontal movements to crush food. Each species can develop the musculature it needs. During chewing, a central pattern generator strums a constant background of rhythmically alternating jaw opening and closing muscle firings, which can be modified by neuromuscular reflexes to create a wide variety of mammalian chewing patterns with species specific variations.    

THE MAMMALIAN SKULL

The mammalian skull has a simple recurrent architectural theme, with viscerocranium, neurocranium, and the top of the vertebral column all aligned sequentially. In front is the viscerocranium, an elongated pyramid which houses the upper air and food passages and supports the sense organs. Its upper surface is formed by the nasal bones, its sides by the premaxillae and maxillae, and its underside by the premaxillae, maxillae, palatine, and pterygoid bones. Behind the viscerocranium, the neurocranium houses the brain. Reinforcing the connection between the viscerocranium and the neurocranium, the zygomatic arches form wide laterally placed braces.  At the back of the neurocranium, where the spinal column continues straight backward from the brain, a flat occipital plane faces squarely backward to connect with the neck.  The cranial base, as a forward extension of the vertebral column, determines the shape of the cranium. The brain sits on its upper surface, and the organs of the face and neck hang from its lower surface. The mandible acts as a curved shield that fits around the face and neck.⁶

The mandible, on the underside of the front of the cranium, is designed to deliver the forces needed for chewing. Typically, because the forces of power-crushing on the working side are antero-medially (forward and inwardly) directed, the upper teeth are embedded in alveolar bone that is well buttressed palatally, and the mandibular teeth are embedded in alveolar bone that is well buttressed buccally. 

The midface is designed to withstand the forces delivered by the mandible. The upper bite table, which directly receives the forces, is braced by pillars of bone extending in many directions to distribute the forces to areas dispersed widely around the cranium. At the front of the face, compressive forces are transferred directly up to the roof of the neurocranium via the triangular nasal septum as well as around the nasal cavity and the orbit, necessitating the presence of horizontal connectors. The sense organs, areas for airway passage, and any other structures not concerned with chewing are made to fit in the remaining spaces.

MANDIBULAR BRACING

The teeth are the fragile and vital components of mammalian jaw systems; therefore mandibular bracing became an important reflex for protecting them. Bracing the mandible immobilizes it by clamping it forcefully up against the underside of the cranium, at the same time the TMJs are braced in their close packed positions.

During mandibular bracing, the TMJs exhibit the resiliency typical of pressure-bearing synovial joints, with progressive resistance in the microstructure of the tissues. On the surfaces of the articulating bones, proteoglycans within the load-bearing areas respond to compressive force by expressing water molecules and deforming to spread the compressive forces onto a larger surface area.  When the compressive force moves to a different area, these proteoglycans from the previous area of compression reabsorb water molecules and return to their resting contours. Larger compressive forces are absorbed by increasingly stiff layers - the plastic lubricating film and fibrous articular covering, the calcified cartilage, the thin subchondral bone, multiple stiffer and more vertically oriented bony trabeculae in the spongiosa, and finally the cortical bone.  

During mandibular bracing, the bite tables exhibit similar resiliency by absorbing compressive force with the viscoelastic and hydrostatic properties of the tooth sockets. Each tooth is suspended in the middle of its socket by a circumferential fibrous sling embedded in a layer of collagenous ground substance with interstitial fluid and extensive vascular plexes that cushion its movements in response to bite forces. Biting pressure intrudes the roots into this structure, tugs on the surrounding fibrous sling, drives fluids into nearby vessels, bends out the walls of the socket, and thereby encounters resistance that increases steadily as the tooth moves further from the center of the socket. Release of biting pressure permits rebound of the tooth, first by elastic recoil and then by a slow hydrodynamic phase with superimposed pulsation.^{19 - 22}  At the same time, the upper and lower bite tables absorb compressive forces by rapidly shifting the first teeth to contact over small distances within their sockets in order to spread those larger compressive forces onto more teeth until large compression forces include most or all of the teeth. For example, during light closure in the central bracing area, the load falls on the teeth in the middle of the dentition – primarily the first molars and the premolars. More forceful closure compresses or otherwise shifts those teeth until the load spreads out to include the canines and the second molars.  

During mandibular bracing, the bite tables also stabilize by interproximal bracing. Adjacent teeth are normally separated by a gap of at least several microns, probably closer to 20 microns in herbivores, which permits good resting circulation. When the dental arches get compressed by biting, the teeth get squeezed downward and inward until they make full physical contact, which allows groups of neighboring teeth to absorb forces as a single structural unit, like one long tooth supported by many roots.  

MANDIBULAR CHEWING

The other functional activity that involves the three joints of the jaw system functioning in harmony is chewing. When the mandible moves in any direction away from its central bracing area, the ball (the condyle) rides up on the walls of the socket in that direction, with the teeth that are most directly in the path of the movement supporting the mandible along its pathway. For example, when the mandible moves forward, the lower front teeth ride up onto the upper front teeth. When the mandible moves forward and laterally, the lower canine and neighboring teeth of that side ride up onto the upper canine of that side. When the mandible moves laterally, the lower premolars and molars of that side ride up onto the upper premolars and first molars of that side. When the mandible moves backward, the last lower molars of both sides ride up onto the last upper molars of both sides. 

DIVERSIFICATION OF MAMMALIAN JAW SYSTEMS

The biggest advantage of the mammalian jaw system is its adaptability. With so many different types of food available, each species can alter the same basic mammalian chewing system form to best fit its functional demands. By minor alterations in tooth shape, joint contours, and musculoskeletal features; a wealth of different chewing systems can be differentiated from the same basic musculoskeletal plan. Each species can develop its own unique arrangement, angulation, and proportional development of the same basic mammalian jaw muscles in order to enhance the vectors required for chewing the food it utilizes most commonly, a skull shape which provides advantageous points of attachment for those jaw muscles, tooth shapes uniquely designed to best chew those foods, and a facial framework tailored to apply and resist the forces involved in chewing those foods. A new mammalian species can develop jaws suitable for grinding plant matter, grasping and holding live prey, gnawing bones, shredding roots, cracking nutshells, or crushing insects. Animals that need wide grinding mandibular movements develop wide skulls with large molars, animals that need chopping mandibular movements develop long skulls with pointed incisors, insect eaters develop molars suited for puncture-crushing of insect shells, and animals that nibble or gnaw food develop large incisors surrounded by extensive sensorineural structures for fine control.

In time, a few divergent developments of the same fundamental mammalian jaw system characteristics proved most successful. 

CARNIVORES
Carnivore jaw systems have vertically arranged structural components to accommodate mandibular movements that are mostly vertical. Meat is such a rich food source that it doesn't need much preparation before digestion. The jaw system just needs to be able to tear it off and swallow it.  Of prime importance is wide opening and fast snapping closure, requiring long sharp canines for grasping prey and securely locked-in temporomandibular joints for protection during violent predatory action. Since these actions require long fibers, the temporalis muscles are especially well developed. An expanded temporal fossa and an enlarged coronoid process provide horizontal space for more muscle mass at the temporalis origins and insertions, and a long narrow skull provides vertical length to accomodate long temporalis fibers. Extensive development of type 2B muscle fibers provides the rapid forceful contractions that are useful in the capture of prey.¹¹

In the dentition, the interdigitation of the lower canines in front of the upper canines with each closure serves to protect the mandible from a blow which could drive its condyles into the vital inner ear structures just behind it. Since the canines are so much taller than the other teeth, their overlap protects the mandible against a distally directed blow like a kick, even when the mouth is part way open in resting posture.

In the TMJs, the cylindrical condyles of the mandible are locked in tubular temporal bone slots of the temporal bones to protect them from displacement during the trauma that presents a real danger when animals being preyed upon have only seconds to fight for their lives. Mandibular movements consist primarily of opening and closing the mandible by rotating around its condyles. The tightly locked-in TMJs still permit shearing due to a small but important lateral shifting at the top of the long thin oval pattern of mandibular movement, about 5 mm in big cats like tigers.

The shearing actions maintain sharp functional edges on the teeth. In the molar region, the lower dental arch fit completely inside the upper dental arch so that the buccal surfaces of the lower molars face the palatal surfaces of the upper molars. These facing surfaces of the maxillary and mandibular buccal segments are convex, both antero-posteriorly and supero-inferiorly, and hence cannot be brought into contact at more than one point at a time. As the mandible shifts laterally during function, these surfaces worked together like two shears which feed each other and thereby maintain a marked mesiodistal cutting edge.  Interproximal contact areas are small, because the teeth are used for occasional crushing and tearing rather than continual rubbing.

HERBIVORES
In contrast, herbivores jaw systems are horizontally oriented to fit their wide lateral chewing movements. Herbivore chewing is characterized by leisurely prehension followed by prolonged forceful milling of relatively hard resistant plant substance, and therefore herbivorous chewing systems are suited for extended periods of slow powerful grinding rather than short bursts of quick chopping. Maximum gape is limited, and lateral movements are wide and free.

To power such function, the pterygoid and masseter muscles rather than the temporal muscles are well developed. The zygomatic arches and pterygoid plates where they originate are set far apart in short wide skulls so the pterygomasseteric slings converging onto the mandible can pull it strongly from side to side. Fatigue resistant type 1 muscle fibers predominate. There is only a short, slim, coronoid process and small temporal fossa for the attachment of the much diminished temporalis muscle. The lateral pterygoid muscles are well developed to assist in grinding.  The contours of the condyles and the glenoid fossae are relatively flat.

For the dentition to accommodate such a wide range of movement, the bite table is also flat and wide. The incisors and canines are diminutive or absent where no vigorous prehension is required. The molars are broad and flat with extensive chewing surfaces for milling large masses of plant matter and long roots to support extensive chewing. To compensate for continual wear on the biting surfaces, the teeth of large herbivores such as horses continue to erupt at the rate of about 3 mm per year.

The molars are reinforced with vertical plates of enamel to produce and maintain effective mastication in the presence of such continuous persistent occlusal wear. The infolding of enamel interlayered with cementum leaves ridges of enamel that act as grinding flutes after the cementum wears away.  In most cases, the grooves on the bite surfaces are oriented antero-posteriorly so they can grind efficiently when the mandible moves in laterally directed power strokes. In a few herbivores, like elephants, the grooves are oriented bucco-lingually, and the mandible moves mesio-distally. 

During chewing, the mandible rocks from side to side, alternating chewing activity between right and left sides and disarticulating the condyle on the non-working side each time the mandible is swung to the working side. On the biting surfaces, attrition produces large horizontally oriented facets that are effective for crushing large masses of vegetable matter.

On the proximal surfaces, attrition produces broad concave interproximal contact areas that fit together to define a clear range of motion between the adjacent teeth. The photographs below are from cow (right) and elk. The range of motion of the adjacent proximal tooth surface can be seen in the socket-like concavities. 

   

RODENTS
Rodent masticatory systems have structural components arranged antero-posteriorly to fit a long antero-posteriorly oriented mandibular range of motion. The mandible can function in an anterior position for gnawing with the front teeth, or it can function in a posterior position for power-crushing resistant vegetation, including nuts and seeds, with the molars. Rodent TMJs contain elongated temporal bone fossae with a tubular shape that is oriented antero-posteriorly to prevent sideways dislocation and enable the mandible to move easily forward and backward between its two functional locations. To support the antero-posterior range of mandibular movement, rodent skulls are also long antero-posteriorly and narrow mediolaterally. 

The rodent dentition is also long antero-posteriorly. In front, the incisors have enamel on the labial surfaces and dentin on the lingual surfaces so that gnawing continually sharpened the incisal edges by honing them together.  Brodie explains, “rodent incisors at eruption are cone-like and covered with enamel on only their labial surfaces. Chisel sharpness is imparted to this enamel and maintained by an alternate passing of the lower tooth against the lingual and then the labial surfaces of the upper - the lower sharpening the upper during one stroke, the upper sharpening the lower during the next. The rodent must engage in this tooth sharpening activity continually to adjust for the rapid and continuous growth of these teeth.”⁹  To compensate for the extensive wear that occurs from gnawing, rodent incisors do not close off at the base after they have finished growing, like the teeth of other mammals. Instead, the base of the tooth remains wide open, allowing the tooth to continue growing throughout life at a rate of up to 4 mm per month. Rabbit incisors erupt similarly, but are fully encased in enamel on both sides and therefore function more like herbivore teeth. In the back are well rooted molars. Between the incisors and the molars, the canines and premolars are replaced by long spaces. The incisors and molars cannot both be engaged simultaneously, but the mandible can move forward to engage the incisors or backward to engage the molars.  

Rodent jaw muscles are designed to provide powerful closing forces in both anterior and posterior mandibular positions. The lateral pterygoid muscles are well developed in order to power the extreme anterior movements which placed the incisors in an edge-to-edge position for gnawing. The masseters are extended antero-posteriorly to permit application of large closing forces when the mandible is held forward for gnawing or pulled back for power-crushing.

OMNIVORES
Although these highly specialized carnivore, herbivore, and rodent masticatory systems are very effective at dealing with the types of foods for which they were designed; eventually environments shifted and overspecialization became a disadvantage. Species that had become too dependent on the continued presence of a very specific type of environmental condition or food source were replaced by more adaptable designs. When one particular food source became no longer available, these more adaptable organisms were able to switch to a different one.

Mammals such as pigs, bears, badgers are able to chew a wide variety of foods. Their skulls have become shorter and rounder, and their jaws have become relocated deeper in the face where they can exert more power. Locating the working portion of the mandible closer to the jaw closing muscles and relocating the mandibular condyles well above the level of the rest of the mandible allows functional movements that are better controlled and more easily tailored to fit different food sources.

Most omnivores have jaw systems that blend carnivorous, herbivorous, and rodent designs. The horizontal grinding muscles (medial pterygoids and masseters) are balanced with the vertical chopping muscles (the temporals), and the lateral pterygoids are well developed for unilateral grinding or protruding the mandible for incising. The TMJs incorporate both sliding and hinging movements that allow a range of antero-posterior and lateral movements. The teeth are all-purpose chewing utensils which combine features of previous mammalian teeth. In bears, a basically carnivorous system lost the carnissal blades and enlarged the distal cheek teeth to form flattened crushing structures.

PRIMATES
In primates, the mandible has become still shorter, and the facial muscles have differentiated to allow communication through facial expression. The arms and hands have acquired a greater role in food preparation and thereby diminish the burden on incisors for ripping and tearing just to get food into the mouth. In apes, the entire dental arcade has retruded on its osseous base to a location closer to the center of mass of the jaw closing muscles, the bite table has flattened to allow a wider range of mandibular movements, and a shelf of bone (the simian shelf) has developed to reinforce the front of the mandible. The only direction in which the range of mandibular motion is still limited by the bite is posteriorly. The canines are shorter than those of carnivores, but their overlap still protects the TMJs from backwardly directed forces on the mandible, as can be seen in the frontal view of an ape dentition pictured below:

The teeth can shear. “The upper occlusal plane of most primates is arranged in a long anteroposterior convexity, while the lower is arranged along a concave curve. In this arrangement, as the jaw begins to close, the first parts to occlude are the medial posterior cusp of the lower third molar and the posterior border of the upper third molar. As the jaws close, interlocking proceeds forward like the teeth in a cog wheel. At the same time a lateral component becomes conspicuous causing the lateral elevations of the lower teeth to shear across the lingual surfaces of the outer cusps of the upper molars.”^10. The teeth can also grind. The mandible can pivot around canine contacts to drive the mandibular molars laterally for crushing tough foods.

HOMINIDS

Hominids maximize adaptability by combining portions of all these previous masticatory system designs for a range of jaw movements that is versatile. The TMJs have articular eminentia inclined about 45 degrees to the plane of the bite table - roughly intermediate between the steep vertical temporal bone walls of carnivores and the flat temporal bone surfaces of herbivores. The jaw closing muscles can deliver power-crushing forces in a wide variety of locations and directions, and each stroke can be altered to fit the mechanical requirements of the particular chewing task at hand. Bringing the dentition directly beneath the origins of the jaw muscles, which converge onto the lower jawbone from origins widely dispersed around the skull, enables chewing with maximal power and control. Withdrawing the canines into the plane of the rest of the bite table enables the dentition to accommodate a functional range of mandibular movements in any direction needed for effective chewing. As the condyles slide around the inclined articular eminence slopes during function, they combine the rotation of carnivore condyles with the lateral shifting of herbivore condyles and the antero-posterior sliding of rodent condyles.  

      RODENT  TMJ                                   HERBIVORE TMJ                           CARNIVORE TMJ                      HOMINID TMJ         

UPRIGHT POSTURE 

The most remarkable feature of the hominid jaw system is undoubtedly its location on the top of the spine.  The rebalancing of the body with the head perched on the top of a vertically upright spinal column profoundly alters many of the features of the jaw system. DuBrul  commented, "To understand the teeth, you need to look at the feet." 

Upright posture offered significant survival advantages. Perched on top of a double condyle articulation with the vertebral column, the head can pivot on its base and thereby move around quite freely in relation to the rest of the body. With enlarged sternocleidomastoid muscles and elongated mastoid processes to facilitate head rotation, the eyes are close enough to the midline to develop stereoscopic vision without losing a wide field of view. Mammals like primates could use upright posture for short periods of time, but that required significant muscular expenditure, because they could not achieve a posture well balanced against the forces of gravity.

To achieve this physical balance in a habitual stance requires restructuring the skeletal system. In quadrupeds, the spinal vertebrae are arranged in a simple linear sequence forming a beam parallel to the ground and supported by four widely spaced vertical struts. Functional components simply hang from this beam.  The head hangs from the shoulders by the thick postcervical muscles attached to the prominent occipital bone, and the neurovascular connections between the brain and the rest of the body pass directly through the occipital foramen. The forward ends of the food and air channels, hanging in sequence, are easily kept separate and scarcely affected by movements of the head. The role of the cervical muscles in head posture just involves assisting in opening and closing the mouth.

Simply uprighting one of these quadrupeds would create an unstable tower, (below left), because balancing a quadruped skull on the top of an upright vertebral column would require tremendous traction forces downward at the back of the skull in order to prevent it from rolling down onto the chest.  Many of the apes were able to stand and walk upright with their powerful muscles, but they could not maintain upright posture without great effort by their muscles. Habitual upright posture requires an upright stance that was well balanced so the postural muscles can maintain a low tonus and thereby good resting circulation which keeps them ready for action. Hominid bipedalism (below right) enables a balanced upright postural stance by aligning all of the body's structural components along a single axis in which the center of mass is located generally on a plumb line through the center of the pelvic girdle and over the feet.  

In this habitual erect postural stance, the weight bearing alignment is supported by light steady tension in all the postural muscles. Acting together, the chains of skeletal muscles surrounding the vertebral column form a reciprocal tension mechanism that holds the vertebral column erect much like stays hold the mast of a sailboat erect. To distribute the forces generated from weight bearing evenly among the skeletal supporting structures, the chains of muscles on the front of the body counterbalance those on the back of the body, and those on the right side counterbalance those on the left side. During function, skeletal alignment moves smoothly away from this alignment and then back to it, guided by an almost perfectly simultaneous reciprocal inhibition of agonists and antagonists.

RESHAPING THE CRANIUM

The head had to be completely reshaped. To balance it on the top of the vertebral column, its center of mass moved closer to its pivot point on the top of the vertebral column. The long snout disappeared, improving the visual field and allowing better manipulation of close objects, while its thermoregulatory function was replaced by a nearly hairless skin containing many sweat glands and an elaborate vasomotor temperature control system.  The face moved posteriorly until it ran into the airway. The tongue has balled up and crowded back into the pharynx.                

To prevent the face from rotating upward with the rest of the skeleton and the cranium - leaving the eyes aimed uselessly at the sky, the skull is bent sharply in its midsection, thereby opening its top end and compressing its bottom end. The brain can expand upward and outward, because it no longer has to fit in between the viscerocranium and the occiput, creating a dome shaped cranial vault that provides strategic points of origin for the temporal muscles. The airway passage and food passage are compressed along with the sensory organs of the face between the steady orientation of the orbits and the forwardly rotating cervical spine. To increase the available space there, the shortened mandible has become wide and the increased angulation of the external pterygoids places greater stress on the symphysis, such as pressing the ends of a wishbone, requiring an externally located chin for reinforcement. To prevent the mandible from impinging on the pharyngeal airway during opening, its center of rotation has shifted inferiorly.  

REPURPOSING THE MANDIBLE

The mandible has become an integral component of the postural system, - a flexible link in the chain of muscles holding the cranium down on the top of the spinal column which also functions as a shield that protects the delicate sense organs in the face from the downward pull of the postural muscles on the front of the cranium. To maintain upright posture, the pull down on the front of the cranium has to be able to counterbalance the pull downward on the back of the cranium by the thick vertically aligned postcervical muscles that had been well developed to enable mammals to keep their heads from dragging on the ground. However the front of the cranium needs flexibility for functions such as swallowing, rotation of the head, coughing, vomiting, spitting and speech – each of which depends on independent movement of parts within the total framework. The mandible protects the face by transferring all of the downward traction from the anterior kinetic chain around to the sides of the head, where can be controlled by the powerful temporalis muscles, the postural muscles for the mandible.

Behind the mandible, upright posture required a complete redesign of the pharynx, which introduced some important vulnerabilities that have recently become problemmatic. In previous mammals, the epiglottis extends up behind the soft palate to provide airway flow directly from the nasal cavity into the trachea and lungs. In hominids, the tongue base has retruded as far as possible toward the cervical spine, leaving only a narrow passage in which the airway and alimentary canal must cross. Therefore, to safeguard the exposed entrance to the pharynx in hominids; the larynx and hyoid bone have descended to a location below the sharp bend in the airway, the pharynx has elongated, a secondary palate separates the nasal passage from the mouth, and the pharyngeal muscles have expanded to enable them to separate the air and food channels while facilitating complex speech.  An oropharynx develops between the soft palate and the epiglottis as the larynx descends during infancy.   

The small flat zone in the middle of the central bracing platform, which provided the freedom to maintain bracing function in concert with postural variability, was preserved by the mandible twisting on top of the bolus during power-crushing, powered by a force couple consisting of the superior lateral pterygoid muscle on the working (chewing) side and the posterior temporalis muscle on the opposite (balancing) side.^13-14  For example, if the jaw and cervical muscles are fully relaxed, mandibular bracing occurs in the rear portion of the central bracing platform when the head is tipped back (as in drinking), and it occurs in the front portion of the central bracing platform when the head is tipped forward (alert feeding position). 

 HUMANS

In the succeeding lines of hominids, advances in the use of fire and other means of food preparation favored jaw systems that were lightweight and adaptable. Extensive neuromuscular reflexes rather than thick structural components are employed to protect vital functions, while sophisticated means for acquiring food obviated the need for massive structural components. The jaws and teeth became smaller. The protruding supraorbital region diminished. The "puffed out” lateral walls of the maxillary sinuses provided support for the teeth without impinging on space needed for respiratory and sensory functions. The muscles of facial expression became highly developed for better communication which enabled community protection, improved child care, and cooperative hunting. Eventually a species known as homo sapiens sapiens was so successful that it was able to spread out all over the surface of the earth. Humans could live in caves, deserts, or snow; and they could acquire nutrients from an enormous variety of food sources. 

 FOOTNOTES 

1 Noble, H.;Comparative functional anatomy of temporomandibular joint, chap 1 p 3 in Zarb G. and Carlsson G. (eds) Temporomandibular Joint, function and dysfunction.  C.V. Mosby.

2 Homer W. Smith, 1953, as quoted in summary by N.R. Thomas, p 37 in Sessle and Hannam, Mastication and Swallowing.

3 Poole, D. F. G.:Evolution of Mastication. in Anderson, D.J. and Matthews, B. (eds) Mastication, John Wright and Sons Limited, Bristol, 1976 p 1

4 DuBrul, E. L.: Origin and adaptations of the hominid jaw joint. in Sarnat, B. and Laskin D.: The Temporomandibular Joint: A Biological Basis for Clinical Practice 4th ed., W.B. Saunders, 1992

5  Hiimae, Mammalian Mastication, chap 23

6 Kay R. F. and Hiiemae K.M.:Jaw movement and tooth use in recent and fossil primates.  Am J Phys Anthrop. 40:227-256, 1974

7 Petrovic, Stutzmann, and Lavergne p 33.

8 Brodie A.G.:The three arcs of mandibular movement as they affect the wear of the teeth, Angle orthodontics vol 39 #4 Oct,1969.

10 Simpson C. Comparative mammalian mastication. Angle Orthod 1978;48:2.

11 Mao J., Stein R., and Osborn J.;The size and distribution of fiber types in w muscles: a review. J Craniomandib Disord Facial Oral Pain 6:192-201, 1992.

13 Miller A.;Craniomandibular muscles:their role in function and form.  CRC Press. Boca Raton, Fla. 1991 p 118.

14 Smith R.;Functions of condylar translation in human mandibular movement. Am J Orthod 88:191-202 1985.

15 DuBrul E.L.;Oral Anatomy. 8th edition Ishiyaku EuroAmerica Inc. St. Louis 1988.

16 Burn-Murdoch R.;The role of the vasculature in tooth eruption. Eur J Orthodontics 12(1990) 101-108.

17 Weinreb M., Assif D., and Michaeli Y.;Role of attrition in the physiology of the rat incisor. The relative value of different components of attrition and their effect on eruption. J Dent Res May-June 1967 p 527-53 

19 Picton D.;The effect of external forces in the periodontium. in Biology of the periodontium 363-419 Melcher and Bowen W.(eds) Academic Press. New York 1969.