Oral Structure & Biology (eBook)

CHAPTER 3

Evolution

Summary

●Recombinations and mutations of genetic material (genotype) can lead to changes in the observable form (phenotype) of individuals. Change in the inheritable characteristics of all populations from generation to generation is known as evolution.

●Changes in phenotype are due to molecular changes at the level of the genotype. Small molecular changes—if they are compatible with viability—can result in far-reaching changes in the phenotype. This also explains abrupt changes to the phenotype during the course of evolution.

●There are various theories to explain the evolution of the form of the human tooth: the differentiation theory, the concrescence theory, and the dimer theory (which actually is also a concrescence theory). These theories contradict one another.

●The reduction in size of the dentition region as a result of walking upright is frequently described as a process within evolution. However, questions about the underlying biologic, physiologic, and embryologic factors remain unanswered.

●The reality of evolution of organisms per se is not in doubt. Nevertheless, a great deal more research is needed to combine knowledge about the embryonic development of individuals with historical and evolutionary developments. This concerns the role of genes in relation to the development of body form as well as the mechanisms and criteria of selection. Against this background, epistemologic considerations view the concepts of evolution with skepticism.

●Clinical decision-making should not allow itself to be influenced by arguments based on evolutionary biology.

Evolution of the Orofacial Region

The theory of evolution describes how species change over time. According to this theory, the species living today are a consequence of selection. Evolutionary biologic explanations are given for the development of the cranium, the dentition, and the individual teeth in humans. The generally held view is that the form of the human cranium and dentition must be seen in relation to humans walking upright and using their hands.

During the course of human evolution, the transition to an upright posture released the front extremities from the tasks of locomotion (walking and swinging by the arms). To allow stereoscopic control of the area of action of the hands, the eyes had to migrate in an anterior direction. This new coordination of hand and eye required an enlarged brain volume. As the anterior extremities, which had formerly been used only for the purpose of locomotion, came to be used with increasing success to manipulate objects, a powerful dentition was no longer necessary. Because food could be cut up and prepared manually, the dentition was eventually reduced to the modern dentition system of humans (Fig 3-1). This evolutionary scenario has been described in detail,1 but no explanation has yet been given for how this change based on observations of the phenotype took place in a biologically, physiologically, and embryologically coordinated fashion. The visible changes (ie, upright posture, hand development, eye migration, brain development, dentition reduction) appear to be very goal-oriented. However, precisely how relevant changes in the genetic material and in embryonic development with subsequent selection led to these physical modifications in a coordinated way is not known.2–4

Fig 3-1 Reshaping of the cranium with enlargement of the brain and reduction of the viscerocranium as a result of upright posture and freeing of the anterior extremities from locomotion tasks (after Leroi-Gourhan1).

Evolution of the Teeth

The teeth of human beings differ widely in shape. Anterior teeth are flat and shovel-shaped and bear a cutting edge, the canines are more conical, and the molars bear one buccal and one lingual row of cusps. Furthermore, the number of cusps varies, and there are different patterns of fissures, bulbosities, marginal ridges, and tubercles.

Because teeth are a very durable element of fossil finds, they have repeatedly become the subject of evolutionary research. Horny teeth, which represent a tough thickening of the epithelium of the oral cavity, are regarded as precursors of true teeth. They are found in turtles and tortoises, for instance. True teeth, which are made up of enamel and dentin and contain pulp, are assumed to have emerged from these thickenings of the skin. Many fish bear teeth that are mainly composed of dentin and are coated with a very thin layer of enamel. The form or shape of teeth can vary considerably. In amphibians, uniformly conical (homodont) teeth are usually arranged in a row.5–8

In evolutionary research, it is assumed that the different teeth shapes in humans originated from the basic conical shape of the reptilian tooth. According to the differentiation theory (Fig 3-2a), additional side cusps formed on this homodont conical tooth until the tricuspid tritubercular tooth arose, from which all mammalian tooth forms can in turn be derived.9–11 The Carabelli tubercle in humans, which may be present on the mesiopalatal cusp of the maxillary first molar, is often regarded as evidence of this process.12–14 This cusp is only made up of enamel; the usual underlying dentin cusp is absent.

The concrescence theory (Fig 3-2b) also uses the conical reptilian tooth as the starting point. In this case, during the course of the gradual shortening of the rows of teeth, several individual teeth are believed to fuse into a tritubercular tooth.20 This theory directly contradicts the differentiation theory.

Fig 3-2 (a) The differentiation theory.9–11 (b) The concrescence theory.15–20

In humans there are only two consecutive dentitions. Other species shed their teeth several times in succession. In sharks, a constant replacement of the dentitions is seen as the worn-out outer teeth are replaced by new teeth growing from the oral direction. Several dentitions also succeed one another in reptiles. However, the teeth of these species do not bear buccal and lingual (palatal) rows of cusps on molars. It was thus concluded that the buccal and lingual (palatal) rows of cusps in humans originated from two generations of teeth present simultaneously and fused together, which in reptiles would erupt in succession. The dimer theory16,18 formulated by Bolk hence assumes a fusion of unicuspid tooth generations that were previously successional.1 According to other models, the premolar would then be a starting point that multiplies in a posterior direction as multicuspid molars.21–24 Moving in an anterior direction, the lingual or palatal tubercles would then be reduced former cusps.20,25–29 Viewed in this light, the dimer theory is a type of concrescence theory combined with certain aspects of the differentiation theory. In contrast to most other mammals, dolphins have conical teeth that resemble those of fish or reptiles. This phenomenon is referred to as secondary reduction.8

The field theory21 assumes the different tooth forms originate under the influence of molarization fields, caninization fields, and incisivization fields (Fig 3-3). The field theory no longer describes the historical process of transformation of tooth forms but refers to tooth development in the individual. Species bearing teeth of variably complex shapes along their dental arch would have to carry these fields within themselves. These fields must have changed during the course of evolution as tooth shape altered. However, no details of the nature of such fields have been presented.30 There is evidence that the dentition is formed under the influence of pattern-forming homeobox genes31,32 that determine the number and type of teeth along the dental arch. This suggests that a substrate for the sought field would be found.33

Fig 3-3 The field theory, according to which the initially uniform tooth germs develop into the present human tooth forms under the influence of incisivization fields (i), caninization fields (c), and molarization fields (m).21

It is generally assumed that the entire human dentition has reduced over the course of evolution and will continue to reduce20,34,35 because our dietary habits no longer require a powerful dentition.36 The wealth of paleontologic discoveries (particularly tooth finds, because they are so hard and durable) show the diversity of size and form of the teeth,37–38 and comparative anatomy maps out common features and differences.8 Strictly speaking, however, evolution of the teeth still cannot be concluded in the sense of the tooth form altering plastically and continuously over long periods of time.39 This conclusion...