Table of Contents  
REVIEW ARTICLE
Year : 2017  |  Volume : 9  |  Issue : 2  |  Page : 75-79

A brief description about the evolution of the masticatory complex, its causes and future effects: A review


1 Masters of Public Health, University of Sheffield, School of Science and Health, United Kingdom
2 Department of Oral Pathology, KIDS, KIIT University, Bhubaneswar, India

Date of Web Publication8-Jan-2018

Correspondence Address:
Dr. Sujatha Ramachandra
Department of Oral Pathology, KIDS, KIIT University, Campus 5, Bhubaneswar, 751024
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jofs.jofs_125_16

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  Abstract 


The diversity of mammalian teeth remains a major attraction both scientifically and aesthetically. Bizarre-shaped teeth are a good reminder of both evolutionary flexibility and the precision of the developmental control mechanism. With time and adaptation to changing environments, several changes have occurred over a span of million years. These adaptations resulted in both positive and negative changes. The review article was written after an initial thorough search of both online and offline databases regarding articles related to evolution and craniofacial evolution. All available information related to the field of dentistry was compiled together. Ethical clearance was not necessary due to the nature of the study. This article provides a brief review on the changes in the teeth and jaw over the course of evolution and the factors that triggered it. A question arises whether, keeping the past in mind and seeing the present, we can predict the future changes that might occur in the human dentition.

Keywords: Dentition, evolution, homo sapiens, mandible, masticatory muscles, maxilla, speech, temporomandibular joint


How to cite this article:
Pal I, Ghosh B, Ramachandra S. A brief description about the evolution of the masticatory complex, its causes and future effects: A review. J Orofac Sci 2017;9:75-9

How to cite this URL:
Pal I, Ghosh B, Ramachandra S. A brief description about the evolution of the masticatory complex, its causes and future effects: A review. J Orofac Sci [serial online] 2017 [cited 2018 Jan 18];9:75-9. Available from: http://www.jofs.in/text.asp?2017/9/2/75/222387




  Introduction Top


The characteristics of the craniofacial complex have been derived from a number of developmental processes that are affected by both genetic and epigenetic stimuli.[1],[2],[3],[4],[5],[6],[7],[8],[9],[10] The diversity of mammalian craniofacial form including its teeth is one of the main scientific and aesthetic attractions. Teeth that are abnormally shaped remind us of evolutionary flexibility and also the precision of developmental control mechanism.[11] The knowledge of human evolution has mostly been derived from fossil records. Modern day genetical studies have revealed information about human evolution like never before. Thus, fossil records and genetic discoveries have together shed light on the long-term hidden sphere of human evolution.

The human masticatory system, which consists of the maxilla, mandible, teeth, temporomandibular joints (TMJs) and masticatory muscles, is involved both is feeding and speech. The masticatory system also evolved through the history of man just like all other anatomical systems.[12] There are many factors that have shaped the evolving craniofacial complex. Among these environmental factors, masticatory loading, which is the response to variations in hardness, toughness and the particle size of food, is thought to be particularly important.[13],[14],[15]

Evolution, according to Oxford dictionary, is defined as ‘the process by which different kinds of living organisms are believed to have developed from earlier forms during the history of earth’.[16] Evolution proceeds by the selection of phenotypic variation and, therefore, indirectly by the genotype that produces them.[11]

Human evolution is a lengthy process by which we originated from ape-like ancestors. The course of human evolution has been divided into many stages. During the Miocene epoch, the family Hominoidea diverged into two sub-families: the Pongidae (apes) and the Hominidae (humans). They were succeeded by the Dryopithecus and Ramapithecus.[17] The early hominids walked upright with massive mandibles and robust cranial features. Fossil records show that the first hominids, for example Kenyanthopines and the Australopithecines, first appeared in Africa approximately four million years ago.[18] The Australopithecus were then succeeded by the Homo genus. The first species in our line is believed to be Homo habilis. They appeared in Africa approximately two million years ago. There is evidence that this species used stone tools for the first time.[19],[20],[21] As our line evolved from Homo ergaster to Homo erectus, we saw the spread of hominids, for the first time, out of Africa and into Eurasia. Homo sapiens neanderthalensis or Homo neanderthalensis were found to exist between about 200,000 and 30,000 years ago.[22] Homo sapiens sapiens, or modern man, first appeared approximately 100,000 years ago in Africa.[23]

To understand the process of evolution, we need to know not only what changes have in fact taken place, but also why these changes were of advantage to the functional life of animals. Through millions of years, we have evolved from being primates to becoming the most intelligent creatures. We have indeed studied the aspects of human evolution in great detail, but we have significantly neglected the study of the evolution of the human head and the neck regions, most importantly the jaws and the teeth and its importance in the field of dentistry. The pattern of vertebrate dentition is very important for the survival of individual organisms.

There is a lack of information and insufficient research conducted in the past regarding the sphere of the evolution of the head and neck regions and its significance in dentistry in India. To understand the different changes and patterns that persist in our dentition today, we have to know the patterns of our ancestors.


  The Causes for the Evolution of Human Dentition Top


A number of causes may have contributed to the evolution of the craniofacial structures. Among the many environmental factors that may have influenced the evolution of the highly derived human face, masticatory loading in response to variations in hardness, toughness and particle size in diet is thought to be particularly important.[13],[24],[25],[26],[27],[28] Our ancestors, who were hunter-gatherers feeding on grains and tough raw meat, had larger and stronger teeth, for they needed it to tear or crush the food. The development of stone tools helped humans eliminate the need for many carnivorous adaptations, as the tools replaced the sharp teeth, large canines and strong jaws, which is a typical feature of carnivores.

When humans began cooking their food in large earth ovens (about 200,000 years ago), it made the meat and other foods more tender and easier to chew, again reducing the need for a large, robust dentition. The uses of fire and pottery were also additional factors that caused changes in the food texture and consistency, which led to the adaptive changes in our body. Speech is also said to be one of the important causes for the evolution of the head and neck regions. The repetitive contraction of the tongue and the perioral musculature because of the originating of speech is said to be one of the causes for the formation of the chin protuberance.

Apart from these, bipedalism, increase in the brain size, genetics and environmental factors have also played major role in the evolution of human.


  Evolutionary Changes in the Head and Neck Regions Top


A number of differences can be noted between the craniofacial features among the pre-historic human and the modern human.

Endocranial volume

Fossil records have shown an increase in endocranial volume as we evolved. To start with, the primates had endocranial volume as low as 700 cc, which increased to 1400 cc in modern man. We find Homo sapiens neanderthalensis with the largest cranial capacity among any hominid species.[23]

Cranial morphology

The overall structure of the skull changed from a pear-like shape due to the constriction just behind the orbits to being ovoid in modern man due to the inflated appearance of the frontal area and gracile orbits.[17] The front and side views of adult chimpanzee and human skulls highlight differences in the patterns of facial growth. In both species, the faces grow wider and taller; however, chimpanzee faces grow out and up to become more projecting, while the smaller human face remains tucked under the cranium.[29]

A reduction of the facial part of the skull, also called as debrachycephalisation,[30] has been reported. Large orbits and large nasal openings are the characteristic features of our ancestors. The posterior most part of the skull, also called as the occipital bun, was very much pronounced in our ancestors, and it eventually reduced in size.

If we view the skull from behind, we see that the widest part of an ancient skull is the base of it, whereas the widest part of the human skull is the parietal bulge.

Foramen magnum

The foramen magnum is the largest foramen of the head and neck regions.[31] As we changed from quadripedalism to bipedalism, the position of the foramen also changed. The foramen magnum in our ancestors was placed more posteriorly because of a more drooping posture. However, as we became bipedal beings, we started having an erect posture to support the weight of the head; therefore, the foramen magnum occupied a more central position.

The maxilla

If we have a gross view of the maxilla, it is undoubtedly evident that our ancestors had robust jaws with bigger teeth.

On viewing the palate occlusally, our ancestors had a rectangular form, with the canines taking the corner most position of the jaw. The posterior teeth were arranged somewhat parallel to the opposite side. The jaw appeared to be constricted narrow but protruded. On the contrary, our maxilla is narrow and gracile with smaller teeth.

Our palate occlusally gives a more ovoid form. The posterior teeth are arranged not parallel but in an axis away from the midline, or in other words, it may be called as posteriorly divergent. The jaw appears to be wider but more retruded.

The mandible

Our ancestors did not know how to talk; they were just able to communicate with the help of sounds that escaped their voice box. As we started to talk, our voice box modified and a lot of changes happened in the larynx, as well as the oral cavity. One of the most distinct features resulting because of the development of speech was the appearance of the chin. Ancient man had a retrognathic appearance due to the absence of mental eminence or the chin. With evolution, the chin became prominent.

A very important feature of the primates and ancient man was the lingual projection or the simian shelf; it was a more posterior projection of the lingual part of the mandible. This simian shelf disappeared with time and is completely absent in modern man.

The retromolar fossa is a large area behind the third molar seen in our ancestors. This enabled them to open their mouth widely and also provided enough space for the eruption of all the teeth.

Dentition

Our ancestors had procumbent or proclined incisors at the time of eruption.[32] As the incisor size reduced, the size of the molars and premolars increased relatively.[33] In apes, the canines were larger, sexually dimorphic and were not worn down to the level of posterior teeth.[34]

The 1st molars are the largest among molars in modern human, and the tooth size is reduced. On the other hand, the 2nd molar was the largest in the earliest hominin and archaic hominin.[35] There has been reduction in the size of the alveolar process[36] along with a reduction in the number of teeth and also the mesiodistal width of teeth.[37] A decreasing rate of physiological attrition of the teeth is a well-documented evolution but has ended with the stabilisation of nutritional habits.[38] In addition, the stability of the curve of Spee has increased with time.[39]

Apart from the anatomical and physiological changes that we came across, there were also changes that occurred because of various disease processes. A lack of place in the arches led to increased prevalence of dental impactions and malocclusion.[40] Changes in our dietary pattern, especially the use of processed food, have led to an increase in the prevalence of dental caries and a decrease in dental microwear.

Temporomandibular joint

The TMJ, also called as the ginglymoarthrodial joint, is a distinctive feature found only among mammals. It is a complex sinovial joint consisting of the articular surfaces (the glenoid fossa, the articular tubercle and the head of the condyle of the mandible), articular disc, joint capsule and ligaments (the lateral ligament, stylomandibular ligament and sphenomandibular ligament).

Unique variations are notable between the structure of the joint of primates and humans, which would help us visualise the image as to how the TMJ had evolved.

In contrast with the TMJ of Homo sapiens, the glenoid fossa of the primates is shallow, and the articular eminence is poorly formed. In addition, the size of the pre-glenoid plain is larger in all primates than humans.[41]

The mediolateral positional change of the glenoid fossa has been noted. In modern man, the fossa is medially placed, whereas due to the pneumatization of the tympanic squama, the fossa was more lateral in our ancestors.[42],[43]

The structural changes indicate that the overall size of the joint might have been larger in our ancestors, because the joint was more functional in the application of higher masticatory forces.

Masticatory muscles

The muscles of mastication form an integral part of the masticatory complex. It consists of mainly four pairs of muscles (the temporalis, lateral pterygoid, medial pterygoid and masseter). These muscles work closely with the TMJ, thus, helping in the movement of the jaw and mastication.

Due to changes in the dietary habits, these muscles underwent numerous modifications. Fossil records reveal that, as we evolved, the neurocranium increased in size with a decrease in the size of the mid and lower face. The robusticity of the jaws decreased, and with this, the overall bulk and size of the muscles also decreased.

Tooth attachment complex (gomphosis)

The fibrous joint between the tooth and the alveolar bone is known as the gomphosis. A gomphosis is a specialised type of joint in which a cone-shaped process or the peg of one bone fits into a hole or socket of the opposing articulating bone. In the oral cavity, this type of joint is present both in the maxilla and the mandible, where it forms the only joint in which a bone does not articulate with another bone but articulates with the tooth. Thus, it is an amphiarthrotic type of joint, and it has very little or minimal movement.The human attachment complex consists of three main structures: the cementum, the alveolar bone and the dentine of the root. The collagenous tissue called the periodontal ligament is present between the cementum and the alveolar bone, which forms the fibrous joint or the gomphosis.

The structure of the attachment complex can be said to have evolved through the different species due to the effects of many known and unknown factors.

Among a species of fish, ‘Actinopterygion’, researchers found that there were four types of attachment complexes. Type 1 was the basic form, in which there was no special attachment, and the root and bone were ankylosed. In Type 2, a collagenous ring existed between the tooth base and the alveolar bone.

Type 3 had a larger collagenous area, and bone mineralisation was present in the anterior region. Type 4 was characterised by a larger collagenous area on the posterior side excepting the anterior bone tooth junction.[44]


  Conclusion Top


Fossil records and modern genetic methods have helped us understand the unknown and incomprehensible facts about the evolution of man. It may well be said that apart from the established factors, there are many more hidden reasons that affect our evolution. Thus, more evidences and research are necessary for the better understanding about the evolution of our species. A conclusion can be drawn that the craniofacial complex is mainly evolved due to the effect of diet, speech, bipedalism, etc., and the resulting effects have made the modern man very distinct from its archaic ancestors.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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