NATURAL HISTORY OF THE HUMAN MASTICATORY SYSTEM
THE IMPORTANCE OF MASTICATION IN EVOLUTION
Masticatory systems led the way in evolution. Improvements in mastication provided new sources of nutrition that made possible whole new lines of species.
"Feeding is a function of such paramount importance that natural selection seldom acts with such decisive vigour as when a process of such fundamental nature is involved. Thus it is postulated that even the speed which the primitive vertebrate predator gained by the development of a flexible backbone and large paired fins was largely complementary to the improvement in the functional efficiency of the jaws in procuring food."1
“Without the predatory powers of jaws and teeth and the possibility of swift and accurate pursuit of prey there would have been no evolution of the sense organs of smell, sight and hearing, of elaborate muscular coordination, of prevision of how to get from here to there and the possible consequences of the transit - in short, there would have been no centralization of the nervous system such as ultimately produced the brain, and the earth would never have known the phenomenon of consciousness, at least of an order superior to that of the lobster, scorpion, or butterfly."2
Mouths became important in evolution 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 masticatory systems of fishes, reptiles, and amphibians could grasp, rip, and tear; but they could not really chew. The simple, slightly recurved, identical, conical teeth were continuously blunted by abrasion and thereby needed continuous replacement. 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 evolutionary developments, because vast nutritional sources were stored there in nuts and seeds locked up inside cell walls and still unavailable to the digestive process. To enable the increase in metabolic activity needed to hold the body out of water and transport it around on land would require access to these nutritional sources. Breaking the food down in the beginning of the gut could greatly increase the surfaces accessible to the digestive juices, and consuming smaller quantities of more thoroughly masticated food provided more nutrients with less energy spent procuring it.
THE TEMPOROMANDIBULAR JOINT
The breakthrough that made real chewing possible was the development of a temporomandibular joint (TMJ) about 70 million years ago when the back ends of the mandible retruded far enough to make direct contact with the temporal bone of the skull and a joint formed where a synovial bursa appeared between two layers of rubbed periosteum and an intercepted muscle tendon. This TMJ allowed much better processing of food than was possible previously, and the resulting improvements in chewing efficiency enabled the development of species that could maintain a constant high body temperature. In such manner, the TMJ ushered in the age of mammals.
"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 TMJ was an entirely new creation and not an adaptation of a previously existing structure. 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 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.
A key feature of the new TMJ was that it allowed transverse mandibular movements. Generally the joint was divided into two compartments – an upper compartment that allowed sliding of the condyle to produce a horizontal range of movement of the mandible and a lower compartment that allowed rotation of the condyle for opening and closing the mandible from a great variety of condylar positions. The two compartments were separated by a tough articular disk held in place by a surrounding ligamentous sleeve. The sliding in the upper compartment produced a mandibular range of motion that had an oval shape as seen frontally. The oval was very wide in some mammals and very narrow - almost a teardrop - in others. However, its consistent lateral component, even if small, distinguished it from the truly straight vertical jaw movements of reptiles and amphibians.
THE MAMMALIAN DENTITION
The freedom of movement in the TMJs was accompanied by freedom of movement in the dentition. The transverse movements of the mandible produced contact glides between the teeth, which produced attrition, which transformed the occlusal surfaces into effective chewing and cutting tools that stayed sharp in spite of abrasion. The attrition produced facets surrounded by edges that stayed sharp. Food could be crushed between the facets and cut at the facet edges. Teeth no longer needed continuous replacement.
As permanent structures, mammalian teeth acquired highly differentiated shapes which were individualized to fit their locations. Generally the front teeth became incisors, the back teeth became molars, and the teeth between became canines and premolars. Their construction blended enamel, dentin, and cementum (which have different wear rates) in patterns that responded to occlusal wear by producing effective chewing surfaces.
Mechanisms were also needed to compensate for the loss of tooth height due to occlusal wear. If shortening of the teeth from continuous wear allowed the bite platform to continually become shorter, the upper and lower jawbones would continuously move closer together, and the jaw muscles would have to keep changing their working lengths to maintain chewing efficiency. To prevent such problems, one mechanism that evolved to compensate for continuous occlusal wear was continuous eruption. By means that we don't yet fully understand, mammalian teeth (and to some extent the supporting portions of the surrounding bones) continually erupt out of their bony bases into the bite table, as if they were spring-loaded, with a force of about 6 to 8 grams, as measured in rodents. 16 17 As a result, every micron of tooth structure lost to occlusal wear was replaced by a micron of new tooth structure brought up into the bite table, and the framework of bones and teeth that supported the face was able to maintain a stable height, allowing the jaw elevator muscles to maintain constant resting and working lengths. With continual eruption and gingival recession in balance, the distance between the alveolar crests and the occlusal surfaces stayed constant, the gingiva could maintain its architecture, and the periodontium could always receive adequate stimulation to keep them healthy. 18
The combination of continual wear and continual eruption required continual pulpal recession to keep the tooth pulps protected from exposure to the bacteria-laden oral environment. Therefore, in response to temperature changes and mechanical stimuli, the pulps of the teeth responded by receding down into the roots and leaving behind layers of secondary dentin that mineralized rapidly in the mineral rich environment of pre-industrial mouths to become new chewing surface. If attrition on any tooth proceeded faster than the pulp was able to recede, sensitive thermal and tactile receptors in the dentin overlying the pulp produced pain so that chewing on that tooth was avoided until secondary dentin had mineralized enough to recreate sufficient pulpal protection.
Specialized structures were required to create a seal between the tooth roots and the gingiva to keep out microbes. Unlike a continuous sheet of skin or mucous membrane, the junction between teeth structure and their surrounding soft tissues was difficult to protect from microbial ingress. Thus the top of each socket was covered by a flexible gingival attachment that forms a narrow sulcus through which fluid can flow outward to carry away food and bacteria. In most species, a slight overhang of tooth structure just above the sulcus protected this critical area from food impaction.
THE MAMMALIAN JAW MUSCLES
The jaw muscles could be selectively developed to power a mandibular range of motion with a whatever transverse components were needed for effective mastication. The single reptilian adductor muscle had been separated into distinct units (temporals, masseters, and pterygoids), which were arranged in pairs that formed slings converging onto the mandible from widely separated origins and thereby enabled fine control of the position of the mandible. This control permitted chewing strokes which could be customized to process each of the many different kinds of food available.
The temporalis muscles, with long straight fibers, were especially useful for wide opening and vertical chopping of food. The masseter and pterygoid muscles, with short thick fibers oriented to pull from side to side, were especially useful for crushing food in a horizontal plane. The jaw opening muscles were much smaller. During chewing, a central pattern generator initiated rhythmically alternating firings to the jaw opening and closing muscles, and these firings were then modified by neuromuscular reflexes to create a characteristic mammalian chewing pattern 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 housed the upper air and food passages and supported 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 axis, 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
At the front of the cranium, the face is designed to deliver and withstand compression by the mandible in chewing, and the midface is comprised of membrane bones aligned to withstand the compressive pressures delivered by the mandible. Typically, because the forces of power-crushing on the working side are antero-medially directed, the maxillary 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.
In order to absorb masticatory forces, the maxillary bite table is braced by pillars of bone extending in many directions to areas dispersed widely around the skull. At the front of the face, compressive forces were 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 were fit in the remaining spaces.
The maxillary and mandibular dentitions are supported by bony bases that are capable of adaptive growth to maintain their alignment and thereby preserve the capacity for effective mastication. Petrovic said, “The responsiveness of condylar cartilage growth to local factors may account for the evolutionary success of the phylogenetically new mammalian joint between the skull and the lower jaw... The regulatory possibility for the mammalian lower jaw to adjust in length to the upper jaw during growth certainly favored the selection of genetic variations resulting in facial posteroanterior shortening, in molarization of post-canine teeth, and subsequently, in mastication.”7
DIVERSIFICATION OF MAMMALIAN MASTICATORY SYSTEMS
The biggest advantage of the mammalian masticatory system design was its adaptability. By minor alterations in tooth shape, joint contours, and musculoskeletal features; a wealth of different chewing systems could be differentiated from the same basic musculoskeletal plan.
With proprioception able to direct chewing function, the consistency of the food available determined the mandibular movements that would be used for chewing it. Given the same food, two different species will handle it in the same way irrespective of differences in tooth form; and a single species will show greater variation in the way it handles two different types of food than occurs between members of different species.
As a result of particular adaptations to individual masticatory patterns, each species altered the same basic mammalian masticatory system form to develop masticatory system components that best fit its functional demands. Each species developed 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 utilized most commonly, a skull shape which provided advantageous points of attachment for those jaw muscles, tooth shapes uniquely designed to best masticate those foods, and a facial framework tailored perfectly to resist the forces normally applied while chewing those foods. A new mammalian species could develop a mouth suitable for grinding plant matter, grasping and holding live prey, gnawing bones, shredding roots, cracking nutshells, or crushing insects. Animals that needed wide grinding mandibular movements developed wide skulls with large molars, animals that needed chopping mandibular movements developed long skulls with pointed incisors, insect eaters developed molars suited for puncture-crushing of insect shells, and animals that nibbled food developed large incisors surrounded by extensive sensorineural structures for fine control.
In time, a few divergent developments of the same fundamental mammalian masticatory system characteristics proved most successful. The mandibular articulations (TMJs and dentitions) were vertically arranged in carnivores, laterally arranged in herbivores, and antero-posteriorly arranged in rodents. Omnivores developed mandibular articulations with components in all these directions.
Carnivore masticatory systems had vertically arranged structural components to accommodate mandibular movements that were almost straight up and down. Meat was such a rich food source that it didn't need much preparation before digestion. Of prime importance was 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 required long fibers, the temporalis muscles became especially well developed. An expanded temporal fossa and an enlarged coronoid process provided horizontal space for more muscle mass at the temporalis origins and insertions, and a long narrow skull provided vertical length to accomodate long temporalis fibers. Extensive development of type 2B muscle fibers provided the rapid forceful contractions that were useful in the capture of prey.
In the TMJs, the cylindrical condyles were locked in tubular temporal bone slots that protected 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 consisted primarily of opening and closing the mandible by rotating around its condyles. The tightly locked-in TMJs allowed only slight lateral shifting at the top of the long thin oval pattern of mandibular movement, probably only about 5 mm in big cats like tigers.
In the dentition, the interdigitation of the lower canines in front of the upper canines with each closure served to protect the mandible from a blow which could drive its condyles into the vital inner ear structures just behind it. Since the canines were so much taller than the other teeth, their overlap protected the mandible against a distally directed blow like a kick, even when the mouth was part way open at rest.
The small amount of transverse mandibular movement maintained 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 faced the palatal surfaces of the upper molars. These facing surfaces of the maxillary and mandibular buccal segments were convex, both anteroposteriorly and superoinferiorly, and hence could not be brought into contact at more than one point at a time. As the mandible shifted laterally during function, these surfaces worked together like two shears which fed each other and thereby maintained a marked mesiodistal cutting edge.
Brodie likens carnivore mastication to a pair of scissors, explaining, "Scissors have two blades with faintly concave surfaces facing each other but only one edge of these surfaces can be brought into contact with the other. The blades, when viewed from their edges, are also faintly concave in their length dimension so that only one point of the edge can be in contact with the other at one time. Thus, when the scissors are closed, the edges of their blades are in contact only at their very ends. As the scissors are opened, this point travels backward until, at full opening, it can be seen that the cutting edges cross each other. Upon closing, the contacting point travels forward and it is only at this precise point that work is done. Since the cutting edges of the blades are beveled and their facing surfaces are concave, the scissors sharpen themselves while they work."8
In almost direct contrast to carnivores, herbivores developed masticatory systems with structural components that were horizontally oriented to fit their wide lateral chewing movements. Herbivore chewing was characterized by leisurely prehension followed by prolonged forceful milling of relatively hard resistant plant substance, and therefore herbivorous masticatory systems were suited for extended periods of slow powerful grinding rather than short bursts of quick chopping. Maximum gape was limited, and lateral movements were wide and free.
To power such function, the pterygoid and masseter muscles rather than the temporal muscles became well developed. The zygomatic arches and pterygoid plates where they originated were set far apart in short wide skulls so the pterygomasseteric slings converging onto the mandible could pull the mandible strongly from side to side. Fatigue resistant type 1 muscle fibers predominated. There was only a short, slim, coronoid process and small temporal fossa for the attachment of the much diminished temporalis muscle. The lateral pterygoid muscles were well developed to assist in grinding. The contours of the condyles and the glenoid fossae were relatively flat.
For the dentition to accommodate such a wide range of movement, the occlusal table was also flat and wide. The incisors and canines were diminutive or absent where no vigorous prehension was required. The molars were 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 occlusal wear, the teeth of large herbivores such as horses continued to erupt at the rate of about 3 mm per year.
The molars were 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 left ridges of enamel that acted as grinding flutes after the cementum wore away. In most cases, the grooves on the bite surfaces were oriented antero-posteriorly so they would grind efficiently when the mandible moved in laterally directed power strokes. In a few herbivores, like elephants, the grooves were oriented buccolingually, and the mandible moved mesiodistally.
During masticatory tasks, the mandible rocked 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 was swung to the working side. Attrition produced large horizontally oriented facets that were effective for crushing large masses of vegetable matter.
Rodents developed masticatory systems with structural components that were arranged antero-posteriorly to fit a long antero-posteriorly oriented mandibular range of motion. The mandible could function in an anterior position for gnawing with the front teeth, or it could function in a posterior position for power-crushing resistant vegetation, including nuts and seeds, with the molars.
Rodent TMJs contained elongated temporal bone fossae with a tubular shape that was 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 were also long antero-posteriorly and narrow mediolaterally.
The rodent dentition was also long antero-posteriorly. In the front were large incisors with open roots that allowed them to keep growing throughout life. In the back were well rooted molars. Between the incisors and the molars, the canines and premolars were replaced by long spaces. The incisors and molars could not both be engaged simultaneously, but the mandible could move forward to engage the incisors or backward to engage the molars. Because the molars had grooves running buccolingually, occlusal wear produced sharp ridges of enamel that increased their ability to grind up tough foods like seeds.
Rodent jaw muscles were also designed for providing mandibular elevator forces in both anterior and posterior mandibular positions. The lateral pterygoid muscles were well developed in order to power the extreme anterior movements which placed the incisors in an edge-to-edge position for gnawing. The masseters were extremely well developed and expanded in area to permit application of large closing forces when the mandible was held forward for gnawing or retruded for power-crushing.
In front, the incisors had 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 occlusal wear that occurred 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.
Although these highly specialized carnivore, herbivore, and rodent masticatory systems were 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 developed masticatory systems that were able to chew a wide variety of foods. Generally, skulls became shorter and rounder, and the masticatory system components became located deeper in the face where they could 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 allowed functional movements that were better controlled and more easily tailored to fit different food sources.
Most omnivores had masticatory systems that were blends of carnivorous, herbivorous, and rodent designs. The horizontal grinding muscles (medial pterygoids and masseters) were balanced with the vertical chopping muscles (the temporals), and the lateral pterygoids were well developed for unilateral grinding or protruding the mandible for incising. The TMJs incorporated both sliding and hinging movements on articular eminentia that allowed a range of antero-posterior and lateral movements. The teeth were all-purpose chewing utensils which combined 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.
In primates, the mandible continued to shorten and the facial muscles became differentiated to allow communication through facial expression. The arms and hands acquired a greater role in food preparation and thereby diminished the burden on incisors for ripping and tearing just to get food into the mouth. In apes, the entire dental arcade retruded on its osseous base to a location closer to the center of mass of the jaw closing muscles, the bite table flattened to allow a wider range of mandibular movements, and a shelf of bone (the simian shelf) developed to reinforce the inferior border of the symphysis.
The only direction in which the range of mandibular motion was still limited by the dental occlusion was posteriorly. Generally the canines were shorter than in carnivores, but they still protected the TMJs from retrusive forces, as can be seen in the frontal view of an ape dentition seen below:
The teeth could 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 could also grind. The mandible could pivot around canine contacts to drive the molars laterally and thereby improve crushing action in the posterior part of the dentition.
Hominids combined portions of all these previous masticatory system designs to produce a more adaptable hybrid masticatory system. The TMJs had 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. As the condyles slid around the inclined articular eminence slopes during function, they combined the rotation of carnivore condyles with the lateral shifting of herbivore condyles and a little of the antero-posterior sliding of rodent condyles.
RODENT TMJ HERBIVORE TMJ CARNIVORE TMJ HOMINID TMJ
Combining the vertical mandibular movements of carnivores with the lateral mandibular movements of herbivores and antero-posterior mandibular movements of rodents provided a range of mandibular movements that was versatile. The jaw closing muscles could deliver power-crushing forces in a wide variety of locations and directions, and each stroke could 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 converged onto the mandible from origins widely dispersed around the skull, enabled mastication with maximal power and control. Withdrawing the occlusal surfaces of the canines into the plane of the occlusal table removed the protection from mandibular retrusion that was created by the simian canine interdigitation, but it also prevented the canines from limiting the mandibular range of motion. The dentition could accomodate a functional range of mandibular movements in any direction needed for effective mastication.
The acquisition of upright posture in homo erectus fundamentally changed the masticatory system by involving many of its components in the mechanics needed to keep the tower erect. DuBrul frequently commented, "To understand the teeth, you need to look at the feet."
Upright posture provided significant survival advantages. Perched on top of a double condyle articulation with the vertebral column, the head could 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 could come close together in the midline and develop stereoscopic vision without losing a wide field of view.
However upright posture also required a skeletal system restructuring that incorporated the mandible and the jaw muscles into a dynamic upright postural maintenance mechanism. In quadrupeds, the spinal vertebrae had been arranged in a simple linear sequence forming a beam parallel to the ground and supported by four widely spaced vertical struts. Functional components simply hung from this beam. The head hung 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 passed directly through the occipital foramen. The forward ends of the food and air channels, hanging in sequence, were easily kept separate and scarcely affected by movements of the head. The role of the cervical muscles in head posture just involved 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. In contrast, bipedalism (below right) enabled upright posture 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.
To balance the hominid head on top of the vertebral column, its center of mass had to move 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 back as far as possible, until it ran into the airway.
To prevent the face from rotating upward with the rest of the cranium - leaving the eyes aimed uselessly at the sky, the skull was bent sharply in its midsection. The top curved and the bottom buckled. 24 The curving of the top allowed the brain to expand upward and outward, because it no longer had to fit in between the viscerocranium and the occiput. Its expansion created a dome shaped cranial vault that provided strategic points of origin for the temporal muscles. The bending of the cranial base also compressed the face- squeezing the airway and the alimentary canal between the steady orientation of the orbits and the forwardly rotating cervical spine. A sharp bend was required where the oro-nasal airway met the pharyngeal airway. To accommodate this sharp bend, the tongue became balled up and crowded back into the pharynx, a secondary palate separated the nasal passage from the mouth, an elaborate epiglottis mechanism safeguarded the entrance to the pharynx, and the larynx and hyoid bone descended to a location below the sharp bend in the airway.
The mandible had to be altered to prevent simple hinge-like opening and closing movements from impinging on the airway. The condyles diverged as the back of the mandible became wider, and the lateral pterygoid muscles drew the condyles forward down a sloped articular eminence. The increased angulation of the external pterygoids placed greater stress on the symphysis, such as pressing the ends of a wishbone, requiring an externally located chin for reinforcement. 25
To maintain a habitual erect stance, the skeleton had to acquire a weight bearing alignment in which all the skeletal muscles could stay relatively relaxed. Acting together, the chains of skeletal muscles surrounding the vertebral column formed a reciprocal tension mechanism that held 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 running up the front of the body counterbalanced those running up the back of the body, and those on the right side counterbalanced those on the left side. During function, skeletal alignment moved smoothly away from this alignment and then back to it, guided by an almost perfectly simultaneous reciprocal inhibition of agonists and antagonists.
In a frontal plane, this reciprocal tension mechanism had inherent stability, because design was symmetrical, and little energy was required for maintaining a static equilibrium. The mastoid processes, shoulders, and hips extended way out to the sides to provide convenient locations for muscle attachments. Still lower, the two feet placed side by side created a stable foundation to resist sideways tipping.
In the sagittal plane, balance was far more difficult for a body so tall and flat. In the dorso-ventral direction, symmetry was lost right from the top. At the back of the head, the occiput was anchored to the vertebral column and scapulae by a thick post-cervical muscle mass that had already been well developed in quadrupeds, where it prevented the snout from dragging on the ground. However, at the front of the head, a much more elaborate mechanism was needed to maintain downward traction. The front of the neck required flexibility for functions such as swallowing, rotation of the head, coughing, vomiting, spitting and speech – each of which depended on independent movement of parts within the total framework. Speech required mobility of the larynx and elimination of the rigid support of the hyoid bone that is present in most mammals. Thus, instead of a small number of thick muscles, a large number of smaller pre-cervical muscles were attached at various angles between a series of small bones (including mandible, clavicles, and hyoid) arranged generally in parallel and stretching like links in a chain from the sternum to the mandible.
Up a little higher, at the front of the head, the postural muscles could not be directly attached to the face, because the face housed delicate and vital sense organs as well as a web of small muscles that were important for communication and would be impaired by the presence of the thick bone required to anchor skeletal muscles. Thus, to avoid impinging on the face, the long rigid mandible acted as an architectural strut that transferred all of the downward traction from the anterior kinetic chain around to the sides of the head where it could be controlled by the powerful jaw closing muscles. To enable the anterior kinetic chain to pull down on the front of the head, the powerful jaw closing muscles readily and frequently braced the mandible immovably against the underside of the skull. A free floating mandible could create a weak link in the anterior kinetic chain.
The masticatory system had become part of the postural system. The postural muscles and the jaw muscles developed coordinated firing patterns. The cranio-cervical muscles contributed to chewing, and the jaw muscles contributed to postural stability. The post-cervical muscles stabilized the head by pulling down on its back end during swallowing when the anterior kinetic chain was pulling down on its front end and by alternating firing with the mandibular elevator muscles to prevent the head from rocking during chewing.
In the succeeding lines of hominids, advances in the use of fire and other means of food preparation progressively diminished the need for more extensive chewing and larger digestive capacity. Evolution selected masticatory systems that were lightweight and adaptable. Extensive neuromuscular reflexes to protect vital functions and more 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.
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