GOOD MORNING

EVOLUTION OF JAWS AND TMJ

Presenting byB. Nitin kumar PG student

DEPARTMENT OF ORTHODONTICS AND

DENTOFACIAL ORTHOPEDICS

CONTENTSINTRODUCTION

HISTORY

EVOLUTION OF JAWS

EVOLUTION OF MAXILLA

EVOLUTION OF MANDIBLE

EVOLUTION OF TMJ

CONCLUSION

REFERENCE

INTRODUCTION1,2,6

•Human masticatory, system, which consists of maxilla, mandible, teeth, temporomandibular joint, and the masticatory muscles, is functionally involved in not only feeding, but also speech.

•Just like all other anatomical features of our species, the masticatory system has also evolved during the history of men.

•Knowledge of the evolutionary development of the skull, face, and jaws is helpful in understanding the complex events involved in cephalogenesis (formation of the head).

•Evolutionary changes in the midface, mandible and temporomandibular joint can be best studied from various group of mammals.

•The primates are an extraordinarily diverse group of mammals. Over 50 extant genera and 200 species are recognized.

•Maximum life spans extend from 8.8 years in the dwarf lemur to 120 years or so in humans.

•Ecological diversity is also well recognized: some primates inhabit very narrow niches, whereas others, most notably the human, survive in virtually every climate and habitat on earth.

•Scientists have traditionally used physical characteristics that reflect shared and adaptive histories in classifying primates placing them into various families, genera and species.

•Humans and their immediate ancestors have traditionally been placed in their own family Homonidea( based on similarities in their anatomy)

•Genetic codes has revealed the specific genetic links between living primate species.

•These data indicates that humans and the African ape are more closely related than either group is to the orangutans.

•In recognition to their relationship orangutans, chimpanzees and gorillas as well as humans now placed into family hominidae.

•Subfamily ponginae used to just refer to orangutans, homininae include gorillas, chimpanzees and humans

•Humans and their ancestors are then placed into their own tribe hominini (hominin)

HISTORY3

• Two individuals strongly influenced by the scientific revolution were Charles Robert Darwin and Alfred Russel Wallace.

• Through their careful observations and identification of a plausible

mechanism for evolutionary change, they transformed perspectives of the origin of species.

•Darwin and Wallace independently developed an explanation of why this variation occurs and the basic mechanism of evolution. This mechanism is known as natural selection.

•Natural selection is defined as genetic change in a population resulting from differential reproductive success.

•Beginning in 1831, Darwin travelled for five years on a British ship, the HMS Beagle, on a voyage around the world.

•He collected numerous plant and animal species from many different environments

•In the 1840s and 1850s, Wallace observed different species of plants and animals during an expedition to the Amazon and later continued his observations in Southeast.

•Darwin and Wallace reasoned that certain individuals in a species may be born with particular characteristics or traits that make them better able to survive

•For example, certain seeds in a plant species may naturally produce more seeds than others, or some frogs in a single population may have colouring that blends in with the environment better than others, making them less likely to be eaten by predators

•With these advantageous characteristics, certain species are more likely to reproduce and, subsequently, pass on these traits to their offspring.

•Darwin called this process natural selection because nature, or the demands of the environment, actually determines which individuals (or which traits) survive.

EVOLUTION OF JAWS6,7

•Analysis of gene expression patterns in the jaw primordia of mouse and bird embryos at times before overt cellular differentiation shows that most, if not all, genes are similarly expressed in the two species

•Although there are specific genetic pathways involved in tooth and jaw development, tooth morphogenesis shares many key genes with jaw skeletal morphogenesis.

•Disruptions that affect dental patterning also produces abnormal skeletal development of the jaws.

•The jaws of humans are smaller than today’s great apes. Investigations on fossils have also shown the evidence of a decrease in the size of the masticatory system in the hominins which are accepted to be the ancestors of Homo Sapiens.

•Researchers have stated that this decrease was mostly due to the changes in the dietary habits of the species.

•The protruding chin is one of the evolutionary features which separate Homo sapiens from our ancestors. A protruding chin was absent in archaic humans.

•Many studies have been performed on the function and biomechanical basis on the formation of the chin. Some authors have claimed that the chin provided resistance to bending forces on the mandible.

•Some others stated that the chin had no functional importance

•Masticatory system related biomechanical forces were believed to play a role on the formation of the human chin.

EVOLUTION OF MAXILLA2

•The maxilla houses the upper dentition, but also provides an important foundation of the midface.

•It serves as a buttress that resists mechanical forces, and it responds morphologically to those forces.

•In nonhuman primates, the maxilla also includes the paired premaxilla.

•The size, shape, and growth patterns of the teeth and dental arch also contribute to maxillary shape and hence to midfacial morphology.

•The premaxilla, a small wedge of bone forming the anterior-most extension of the hard palate, provides the foundation for the upper central and lateral incisors.

•Of all the structures of the primate cranium, the bone has perhaps the most storied history in part because of its highly variable nature—both developmental and morphologic—among the primates.

•In primates, the anatomy of the premaxilla correlates closely with the form and function of the anterior dentition. Primates having very small incisors—for example, the lemurs—also have a small premaxilla.

•Conversely, many mammalian species with large protruding incisors have a large premaxilla with prominent vertical arms (the nasomaxillary processes) that extend superiorly between the maxillae and nasal bones to the frontal bones.

•These processes, erroneously designated nasomaxillary bones in some studies, reach the frontal bones only rarely in primates.

• Differential growth of the premaxilla, with persistence of the premaxillary-maxillary suture, appears to be responsible for theappearance of the diastema (expansion between lateral incisor and canine) in some primates.

•This space is of considerable functional significance, enabling carnivorous animals to grip and tear meat.

•Extant monkeys and apes manifest a premaxilla that differs from humans in having three morphological components: a body (which supports the incisors), a palatine process, and a nasomaxillary process.

•The premaxilla is present in early human life, but congenitally absent (with upper incisors) in infants with median cleft lip and palate (also known as premaxillary agenesis or premaxillary dysgenesis).

•The premaxilla does not have the prominent anterior orientation of nonhuman primates, perhaps due, at least in part, to the reduced prognathism of humans.

•The premaxilla essentially disappears with obliteration of the premaxillary-maxillary suture early in life and is not identifiable as a separate bone in adult skulls.

•Enlow (1966) demonstrated this in comparing the growth of animals that possess snouts and those that do not.

•Growth of the maxillary arch is completely depository in the macaque, resulting in pronounced forward growth of the snout.

•In humans, the forward portion of the maxillary arch undergoes resorptive growth, which results in downward rather than forward growth, and a flattened face.

•The influence of the cartilaginous nasal septum upon maxillary growth has been postulated under conditions of normal development.

•The vertically hypoplastic maxilla of the chimpanzee is said to have diverged from the general mammalian pattern of prominent midface and snout.

•The infant chimpanzee has a flat face resembling that of modern humans. With growth, it becomes more prognathic, but is still small when compared to other mammals.

•These facial differences are probably related to variations in diet, in that species with higher faces eat increased quantities of hard foods such as seeds or grains.

•The forces of chewing are essentially resisted by the maxilla, a process that is manifest in both extinct and extant species.

•In A.africanus, for example, blunt ridges of bone extending along the nasal apertures to the alveolar process of the maxillae are known as anterior pillars; these and the clivus form a morphological unit called the nasoalveolar triangular frame

•In robust forms, a unique feature is the zygomaticomaxillary steps, which are prominent ridges that coincide with the zygomaticomaxillary sutures

•Early humans experienced a reduction in the size of the masticatory apparatus, and with it, a decrease in maxillary buttressing that persists among modern humans.

•Homo habilis had a maxilla that was smaller than the australopithecines, but within the range of H. erectus and H. sapiens.

•The Neanderthals diverged from this major evolutionary trend; with increased use of the anterior dentition, a significant midfacial prognathism evolved

•The size of the maxillary dental arches has been ascertained for old world monkeys and differs according to diet

•Maxillary arch dimensions have also been shown to discriminate between monkeys, apes, fossil hominids and modern humans.

•Diet can have an effect on the size of the dental arcade for example maxillary arch breadth decreases with chronic ingestion of soft food

•The hard palate is formed by the palatine processes of the maxillae and horizontal plate of the palatine bones; the premaxilla, when present, forms the anterior-most portion of the hard palate.

•The configuration and size of the hard palate are influenced by body, skull, and face size, genetic factors, growth and size of the dentition,tooth movement, as well as the development, shape, and movement of the tongue, other oral and dietary habits.

•The formation of the secondary palate is a distinguishing feature of mammals, but comparatively little is known about the structure in fossil hominids.

•The palate has a distinctive shape in the great apes. The presence of adiastema gives the palate its rectangular shape; in fossil apes—forexample, the dryopithecines—the palate is narrowed anteriorly, taking on a V shape.

•The palates of the australopithecines vary, depending on the evolution of the dentition.

•Beginning in the early Pleistocene (approximately 2 million years ago), two major changes began to occur: In robust australopithecines, the molars underwent enlargement, while the premolars became more “molarized.”

•Gracile australopithecines underwent a progressive diminution in tooth size, with concomitant reduction in palatal size as well.

•Accordingly, specimens of A. afarensis show a palate that is quite shallow, with lateral margins that taper to the midline posteriorly;the upper dental arcades are long, narrow, and roughly parallel.

•The hard palate in modern Homo conforms to the parabolic dental arch.

Hard palate and the upper dentition of gorilla (left) and modern human (right)

EVOLUTION OF MANDIBLE2

•The mandible, while not part of the midface, interacts intimately with midfacial structures from a developmental, functional, and evolutionary standpoint.

•Two primary changes occurred during the early evolution of the mandible:

(1) the angular, articular, and quadrate jaw bones of reptiles evolved into the bones of the middle ear.

(2) the volume of tooth-bearing bone expanded to comprise, at least in mammals, virtually the entire mandible.

•Other modifications—for example, displacement of the temporal region of the cranium, lowering of the face,and widening of the lower jaw— occurred for reasons thought to be related to changes in posture, locomotion, and mastication.

•Among the primates, the mandible shows extraordinary phenotypic variability in both extinct and extant species.

•Mandibular form, function, and articulation are related closely to diet and consequent requirements for mastication.

•Differences in mandibular shape and condylar position and relation to the occlusal plane among the australopithecines and early Homo are thought to be the result of changes in diet.

•This shift from a soft, chiefly frugivorous diet to a diet characterized by harder foodstuffs began in the middle Miocene, and was manifest by thickened molar enamel, enlarged incisors, and dramatically enlarged jaws.

•A hard diet is also thought to promote vertical growth of the mandible and anterior translocation of the maxilla,and in this way influence growth of the entire craniofacial skeleton.

•Fusion of the mandibular symphysis is an important evolutionary feature, though not limited to primates,and has been linked in some studies to increased stresses of mastication.

•However, among the Adapidae (small North American and European Eocene primates), increased levels of symphyseal fusion are associated with robust jaws and larger body size, rather than dietary effect

•The mandible of great apes is larger than that of humans . The rami are taller, and because of the apes’ prognathic condition, the mandibular bodies are longer.

Fossil mandibular fragment of gigantopithecus. Note the extraordinary robusticity of the mandibular body and molars

Superior view of chimpanzee (left) and human (right) mandibles. Difference in size an shape are evident. Note the characteristic parallel dental arch in the chimpanzee and the parabolic arch in humans.

•The mandible evinces parallel dental arches that are narrower than those of humans Thus, the floor of the mouth is also narrower, requiring less muscular support.

•The ape mandible has no chin but rather a simian shelf (inferior transverse torus) extending along the interior surface of the mandible posterior to the symphysis to the region of the fourth premolar.

•The mandible is the most commonly preserved fossil among the hominoids.

•The dental rows are parallel and narrowed anteriorly in A.afarensis, and in this way, resemble those of the great apes.

•The ascending ramus is broader and lower than that in the great apes, and the inferior transverse torus is much less distinct and shelf-like than in the apes.

•The mandibles of extinct and modern species of Homo show some variability, but in general they are narrower and shorter than those of other primates

•The size of the mandible of H. habilis is intermediate between that of australopithecines and H. erectus. Like the apes and australopithecines, H. erectus has no chin but rather shows a buttress (mandibular torus) on the inner aspect of the mandible

•The modern human mandible exhibits some variability in sexual differences, but it is not pronounced

•The mandible is smaller in proportion to body size and less robust, having a rounded (parabolic arch) and pronounced chin without a lingual shelf.

•In fact, the modern human is the only primate to exhibit a true chin—a structure that seems to have acquired the role of strengthening the symphyseal arch, which is a function held by the simian shelf in nonhuman primates.

•The loss of the simian shelf has been related to the acquisition of speech, but it is more likely to be related to dietary changes and reduction of sagittal and occipital crests.

EVOLUTION OF TMJ4,5,8

•The temporomandibular joint is a unique feature of the mammalia no other vertebrates have it.

•TMJ is a unique joint in which translatory as well as rotational movements are possible, and where both the ends of bone articulate in the same plane with that of other bone.

•It is also called as ginglymodiarthroidal type of joint wherein it has sliding movement between bony surfaces, in addition to hinge movement, common to diarthroidal joint.

•TMJ shows remarkable morphological and functional variation in different species, reflecting not only the great mammalian adaptive radiation in feeding mechanisms, but also freedom from constraints, such as bearing body weight.

•The most extreme evolutionary variants include:

(1) loss of the synovial cavity in some baleen whales

(2) loss (or possibly primitive absence) of the disk in monotremes, some marsupials, and some edentates

(3) variations in the orientation of the joint cavity from parasagittal (many rodents) to transverse (many carnivorans)

(4) reversal of the usual convex/concave relationship

•The striking anatomical differences in TMJs are clearly tied to biomechanics. The features mentioned above are either correlates of loading or movement or both.

•Loading of the TMJ is a reaction force arising from the contraction of jaw muscles; its magnitude depends strongly on the position of the bite point relative to the muscle action line.

•The evolution of the TMJ is thought to have coincided with a period of low reaction loads with higher loading having evolved repeatedly in different lineages including our own

•Rodents fall in the category of minimal TMJ loading, especially during chewing.

•In contrast, carnivorans such as dogs probably sustain TMJ loads that are higher than those in primates.

•Opening of the jaw usually involves a combination of forward sliding and rotation around a transverse axis.

•But some carnivorans have lost the ability to slide and some specialized anteaters instead use a rotation around the long axis of the curved mandible.

•Similarly, transverse movement is usually accomplished by moving one condyle forward and the other one backward, but carnivorans use a combination of lateral sliding and rotation around the long axis of the mandible.

•Tmj are anthromorphs apes are distinctively flat, the articular fossa is wide and shallow while the articular eminence is wide and low.

•A typical feature human evolution is the connection between the development of articular eminence and the combined rotational translational movements in TMJ.

AMPHIBIA

•The single joint for the double jaws. The jaws of these creatures were used only for protection. The skull became kinetic, new movable joints appeared within the cranium.

•This ingenious device was a shuttling pterygopalatine component coordinated with joints between the frontal and nasal bones.

•In the carnivorous Pelycosauria, with which we are more directly concerned since they include the ancestors of all the later mammal-like reptiles and so of the mammals, the teeth are all of a simple blade-like form.

•They have distal and mesial cutting edges, often serrated like a bread saw.

•This demonstrates that the sharpening of the teeth by direct tooth-to-tooth contact which is so important in there in mammals, did not take place in these mammal-like reptiles.

•The skull of a primitive amphibia tends to be long, wide and remarkably flat.

•The first notable change from the amphibian structure was the disappearance of the otic notch that supported the tympanum of the primitive ear.

•A complete vertebrate is a species not only with backbone but also with biting jaws.

•The first vertebrates had no biting apparatus but the process of cephalization also produced the singular structures called the jaws.

•Furthermore, complete mammals have long been distinguished from all other vertebrates and from animals very nearly mammalian largely on the basis of their jaw ear structure.

•However, there is an even greater significance to jaw joints in vertebrate evolution.

•The true diarthrodial joint was first formed in fishes, and its basic structure remained essentially unchanged when it was appropriated by the limbs of land animals.

•Diarthrognathus and Probainognathus have contact between two bones, squamosal and dentary, which form the jaw joint in mammals.

•Diarthroses are the most complex joints with a wide single articular cavity surrounded by a specialized sleeve lined with a synovial membrane.

•Schizarthrosis are joints made more mobile by several small separated splits or joint spaces interrupting their bonding tissues.

•The simplest joints are synarthroses in which the parts are made slightly movable by a binding of cartilage, fibrocartilage or connective tissue between them.

REPTILIA

•Reptiles cannot chew. In general, they use their teeth only to seize their prey, and if this cannot be swallowed whole, it is torn apart by a number of animals or by a single animal ‘worrying’ it.

•The group of reptiles which later gave rise to mammals - the ‘mammallike reptiles diverged at an early stage from the main reptilian stem, and by the Permian, some 250 million years ago, were widespread and might reasonably be regarded as the ‘Lords of Creation

•Later their place has been taken by the dinosaurs, which remained the dominant group.

•In this group of tiny mammal-like reptiles, a number of changes took place, all probably associated with the acquisition of the ability to maintain a body temperature above that of their surroundings.

•This increased their activity and their ability to avoid their enemies, but it carried the penalty that it vastly increased the food requirement

•Reptilia have loss of bony parts. In the Pelycosauria, the lower jaw is of an essentially reptilian form.

•The tooth bearing bone or dentary has a strictly limited extension distal to the tooth row; the other bones of the jaw are well-developed and comprise the posterior third of the jaw.

MAMMALIAN ARCHITECTURAL THEME4

Viscerocranium- provides roofs and walls to the upper food way

Neurocranium- houses the brain

•whereas the mandible is a long, straight, level bar with its fulcrum, the jaw joint at the backend.

•This is the basic characteristic of mammalian skull, from it, two adaptive feeding modifications labeled carnivores as opposed to herbivores exist.

•The early American forms are grouped as the Pelycosauria, comprising carnivorous, piscivorous and herbivorous forms.

•The specialization of the teeth and masticatory apparatus is such in herbivorous forms that their capacity for further evolution is limited.

•Major evolutionary changes are always initiated by carnivorous or insectivorous forms.

Austrolopithecusafricanus skull:•The jaws protrude sharply housing a short cheek teeth. •The upper jaw has outstanding canine buttresses whereas the lower jaw is fairly shallow. •The zygomatic arches are quite slender. •Teeth are broad , spatulate, bulky

Austrolopithecusboisei skull:•The skull dorsum is deeply vaulted. •The supraorbital ridges are bulky. •Jaws are severely retruded, no canine buttresses, and massive zygomatic buttresses give the face a “dished” in frontal plane.•The teeth are smaller and occlusal surfaces are more than three times greater than in modern man.

•Animals, such as pig, bear, and humans can masticate a variety offoods to a satisfactory degree.

•It is readily accepted that muscle attachments in the humans depend upon the functional activity of the individual for their development and that muscle development can reflect the exercise and activity that each muscle experiences.

• Modifications of the muscular elements of the apparatus were evident in the specialization of lateral pterygoid muscle and a decrease in the bulk of temporalis muscle.

•Bilateral contraction of the lateral pterygoid muscles permitted amodification of condylar movement within the glenoid fossa toinclude a protrusive excursion.

•It has been suggested that primitive man’s protrusive movement has merely been a bilateral sequel to the individual forward movement of each blanching condyle during lateral excursions of the mandible

•Changes in the masticatory apparatus of primitive man werenot only the result of his omnivorous diet, but also of the adaptationof the upright posture that necessitated a change in the position ofthe head relative to the spine.

•The inclusion of a protrusive mandibular movement and modification of the associated musculature are characteristics of the hominoid masticatory apparatus

MASTICATORY APPARATUS2

•The muscles of mastication consist of four major muscles that attach both to the cranium and the mandible.•Three of these muscles are elevators that bring the mandible up until the teeth occlude; they are the masseter, temporalis, and medial pterygoid muscles.•The fourth masticatory muscle is the lateral pterygoid whose function is to initially open the mouth—that is, depress the mandible.

•These muscles act collectively to perform the various and complex movements of the mandible not only during mastication but any time the mandible changes position.

•The muscles of mastication are similarly disposed in all hominoids, but there are differences in some of their bony attachments.

•Large, powerful temporalis muscle is attached to the sagittal crest, and occasionally the nuchal crest, in adult male gorillas and orangutans, but rarely in the females.

•The development of the sagittal and nuchal crests and the supraorbital torus in anthropoid apes enhances the ability of their crania to withstand the power of their cranial and facial muscles that is far in excess of that in humans.

•The neurocranial portion of the modern human skull has enlarged paripassu with reduction of the facial skeleton to obviate the development of such superstructures.

•H. erectus deviated from this trend in having an occipital crest or torus to which large nuchal muscles attached

CONCLUSION•A great deal is known of the ways in which craniofacial structures develop and evolve.

•However a great deal also remains unknown. Evolution is indeed a vast mosaic and thus it is unclear if a single or true primate form has evolved.

•The evolution of human masticatory complex is strongly related to diet, the use of tools and fire and finally speech has a more important part in evolution of mankind than the dentists know.

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2 Joseph R. Siebert, Daris R. Swindler. Evolutionary changes in the midface and

mandible: establishing the primate form. In Mark P. Mooney and Michael I. Siegel, editor. Understanding Craniofacial Anomalies. 1st edition. USA: Wiley-Liss Publishers; 2002. P. 345-78.

3 Melvin R. Ember, Carol R. Ember, Peter N. Pergrine. Human Evolution. Human

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Temporomandibular Joint Disorders. Journal of Indian Academy of Oral Medicine and Radiology 2010;22(4):S13-18.

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