5.2 VARIATION OF TOOTH SIZE AND SHAPE

I. INTRODUCTION

 

The study of the size and shape of teeth has generated more literature than any other aspect of dental anthropology. One reason for this interest is that tooth crowns are formed full-size in childhood before eruption into the oral cavity. This permits an interesting dimension of study not accessible with skeletal material: Their morphology can be studied in mixed collections of individuals of varying ages, whereas studies of skeletal morphology are based on fully grown adults to eliminate the factor of growth. Furthermore, you can study living subjects quite easily by taking a dental impression of their mouths.

In the modern world, with convenient and rapid travel, human populations are becoming more diffuse. Yet, in many parts of the world, boundaries are vigorously maintained by a variety of social, cultural, and economic reasons. The prudent reader should always pay close attention to what is being studied and what population the sample is supposed to represent.

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Abundant dental materials for measurement are available from living subjects. Sample groups taken from living populations can be neatly categorized by age, sex or any other parameter that the contemporary investigator might select. Above all, the investigator can be reasonably confident that a well chosen sample is a fair reflection of the entire population. Sex and age can be recorded with ease.

For archaeological material, the investigator is limited to what is available. The number of specimens may be limited, which presents a statistical problem: for validity, the minimum sample size should be at least 200-400 individuals. A cemetery, although it may contain a large number of specimens, may not be a fair sampling of the community. Often the cemetery contains the very young, the elderly, and the infirm. Add to this the selective nature of preservation and it becomes apparent that the individuals in a cemetery derived population are less than a random, representative sample. Sex can be determined in mature individuals if the post-cranial skeleton is intact. In young children and infants, determination of sex from skeletal material is difficult.

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II. USE AND DETERMINATION OF OVERALL TOOTH DIMENSIONS

Usually, the maximum dimensions of the teeth are the most commonly used. This in effect describes the minimum size of an abstract box into which the tooth would fit. Three basic dimensions are available for study.

(1) Mesiodistal diameter (length) of the crown. This is taken as the greatest mesiodistal dimension taken parallel to the occlusal and facial surface. The measurement is typically taken using calipers with arms machined to a fine point. There is a difficulty with proximal wear; therefore, most researchers exclude teeth with marked proximal wear. If occlusal attrition is excessive, the accepted practice is to exclude heavily worn teeth.

(2) Buccolingual crown diameter (breadth). This measurement is the greatest distance between the facial and lingual surfaces of the crown, taken at right angles to the plane in which the mesiodistal diameter is taken. It is easiest to use calipers which have broad, flat surfaces. Buccolingual diameters are unaffected by approximal wear, but can become unusable when occlusal wear is excessive.

(3) Crown height (height). This is defined as the distance from the tip of the highest cusp to the cervical line on the buccal side. Any occlusal wear at all renders this measurement unreliable; therefore, it is seldom employed.

For one population and a single sex, the mesiodistal and buccolingual diameters of each tooth type have normal (Gaussian) distributions. This is usual for the dimensions of anatomical structures in adults, and is true of skeletal measurements as well. Archaeological and museum collections may well deviate from the ideal of normality, but this is to be expected with small numbers of individuals and their uncertain derivation.

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In modern humans, there is a moderate correlation between mesiodistal and buccolingual diameters. Those correlations are slightly greater in females than males, in upper than lower teeth, and in cheek teeth than anterior teeth.

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Correlations for diameters between different teeth in the same jaw are moderate for both permanent and deciduous teeth.

If a tooth from one part of a jaw is large, then teeth from other parts tend to be large also, but when the anterior teeth are treated as a group, their crown diameters are inversely related to the cheek teeth treated as a group. Thus, individuals with larger than normal anterior tooth crowns have correspondingly smaller than normal cheek teeth and vice versa.

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III. ASYMMETRY IN CROWN DIAMETERS

Dental traits ordinarily exhibit a high degree of symmetry. When asymmetry appears has long intrigued researchers for what they might reveal about the underlying genetics and developmental biology of teeth.

Two types of asymmetry are relevant in dentistry. Directional asymmetry describes traits that show a distinct right or left side bias. The maxillary left central incisor in the narwhale is one example. Another is the enlarged claw in the male fiddler crab. Fluctuating asymmetry is random and shows no consistent bias within a population.

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(1) Directional asymmetry is the tendency for one side to be consistently larger than the other within a population. A component of directional asymmetry averaging + or - 0.06 mm is common in human dentitions. It varies between teeth not only in the extent of asymmetry, but also in its direction. Thus, canines may show the left to be larger than right, while first premolars show right to be larger than the left. Different populations show different patterns within one jaw.

Often the direction of asymmetry in an upper tooth is opposite in the tooth of the same number in the opposing arch. The reason for such patterning is not clear.

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(2) Random, or fluctuating asymmetry is where the largest side varies between individuals. It appears to be slightly larger for the upper dentition than the lower, and more so for boys than girls. Within one class of teeth, the more distal members of a class are more asymmetrical than the mesial members--as suggested by Butler's Field Theory. Asymmetry is more marked in children lacking one or more of their third molars.

Odontometric fluctuating asymmetry has been associated with congenital abnormalities, genetic syndromes, and elevated levels of inbreeding, and is also thought to be a general indicator of stress caused by nutritional or disease factors. There is an abundance of experimental evidence that shows increased asymmetry can be induced by a variety of environmental stressors during dental development. Animal studies have shown that symmetry is important in mate selection, particularly amongst animals with elaborate displays such a peacock's feathers. The symmetry is assumed to be a good indicator of vigor and health.

Experimental studies with laboratory rats and mice have been shown that cold, heat, noise or protein-deficient diet increase fluctuating asymmetry. Neandertal and recent Inuit also reveal high levels of asymmetry. In a study of modern-day asymmetry, Tristan da Cunha children had the most, Boston youth exhibited the least asymmetry.

Tristan da Cunha is an isolated island in the South Atlantic. It is frequently cited in anthropology texts. It was first settled by one Scottish family. The subsequent descendants of the island come from this family plus a few shipwrecked sailor. The poor terrain and inhospitable climate contributed to high environmental stress. In 1961, the inhabitants were evacuated because of an impending volcanic eruption.

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IV. INHERITANCE OF CROWN DIAMETERS

Crown diameter is the result of genetic and environmental factors. Phenotypic characteristics such as crown diameters can have any value within a given range. Those measurements form a graded series and can be treated with statistics.

Three factors must be considered in the generation of any single crown diameter: genotype, environmental factors within a family, and environmental factors that uniquely impact a single individual. They are difficult to disentangle, but must be kept in mind during any study.

Most studies estimate the relative importance of the genotypic and environmental factors by calculating a statistic known as heritability (h2). This is the slope of a regression line relating the measurements of fathers with those of sons or daughters, which indicates the extent to which children follow their parent.

There tends to be high heritability for crown diameters; however, different teeth may have different heritabilities. These results are supported by the evidence of twin studies.

Monozygotic twins share the same genotype and environment. Dizygotic twins have a different genotype but share the same environment. What do those studies show?

Dizygotic twins show a much greater variance in crown diameters than monozygotic twins.

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The pattern of correlations between parents and siblings suggest that the mode of inheritance was multifactorial, additive, and without dominance--many genes working together, each with a small but mutually enhancing effect.

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Correlations in sister/sister pairs were the highest, followed by brother/brother, and then sister/brother. This suggests that at least some of the genes controlling crown diameter are on the X chromosome. Overall it is clear that the size of the dentition is part of a complex of features related to a variety of genes located on several chromosomes.

Some studies suggest that genes controlling crown diameters are on both X and Y chromosomes. Keep in mind that the Y chromosome is very small and any gene control from the Y chromosome is probably limited.

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There appears to be a clear relationship between a child's crown diameters and the mother's health during pregnancy, implying that their heritablity included shared environmental as well as genetic factors.

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V. SEXUAL DIMORPHISM IN CROWN DIAMETERS AND CERTAIN NON METRIC TRAITS

The sexual dimorphism of permanent teeth is a well established attribute of primates. The size and distribution of that dimorphism is different in various species.

The highest levels of dental dimorphism in primate permanent teeth (70% or higher) is seen in baboon canines.

In living people today, body size dimorphism averages 10%. Human dental dimorphism is on the order of 2-6%. In humans, it centers on the canines. In most human living populations, lower canines show the greatest dimorphism (up to 7.3%), followed by the upper canine. Dimorphism in the permanent dentition is variable and it appears to have a substantial inherited component.

Females have a higher frequency of missing teeth and a loser frequency of supernumerary teeth than do males. Females show more shoveling of upper incisors than do males.

Tooth sexual dimorphism is often related to body size. In many animals, large canines are considered to be visual sexual signs of dominance and rank.

A related idea is an old issue in physical anthropology: allometry. It is the study of proportional changes correlated with variation in size of either the total organism or the part under consideration. To grasp the idea intuitively, consider that large animals have different proportions than do smaller ones. Only a few trends in dental allometry will be noted here.

The larger the canine becomes, the smaller the posterior teeth tend to be. A related trend has been noted elsewhere and is well illustrated in robust Paranthropus: a reduction is the size of canines and incisors will be accompanied by a disproportionate enlargement of premolar and molar size. Tooth size and body size seem to be independently determined. Mbuti Pygmies tend to have larger teeth than you would first expect in such tiny people. These statements anticipate the next section, tooth size and body size.

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VI. TOOTH SIZE AND BODY SIZE

Among the primates as a whole, there is a high positive correlation between body size and crown size, at least in males. Within living humans; however, the correlation between body size and crown diameters is low.

Taller people have larger hands and feet. Why not teeth? Manufacturers of porcelain prosthetic teeth for denture have long based their size recommendations on that assumption. Is it correct? Apparently not. There are positive correlations, but those correlations are low, on the order of 0.2 at most. Remarkably, crown size bears little relationship to caloric intake on a worldwide basis--the relationship is actually inverse!

Except for third molars, crown size is established by age 3 ½ years. Is there a better correlation at that age if you use body size as then attained? What about birth weight? I am unaware of such studies.

The individual published record for crown size in living people comes from the US Marine Corps. The largest teeth on average today are those of the Aboriginal Australians and the smallest crowns are generally found amongst Europeans and Asians. The largest permanent molars are found in Australian Aborigines and the smallest molars are found in Lapps.

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VII. CROWN DIAMETERS AND HOMINID DENTITION

There is a decrease in cheek tooth diameters from Homo habilis, to Homo erectus, to Neandertals and finally modern Homo sapiens. The Australopithecines display a high level of dental sexual dimorphism based on crown diameter measurements. In anatomically modern Homo sapiens, if male and female canine mesiodistal diameters are plotted together they show a single, asymmetrical mode, but the distribution for Australopithecines is bimodal.

We noted in unit 4.3 that crown reduction occurred in the last 35,000 years and that it has been accompanied by a reduction in body size and robustness. A reduction in sexual dimorphism has also been confirmed.

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One interesting theory is worth mentioning here as we conclude this unit: dental disease may have been a factor in the selection for smaller teeth and a shorter total dental arch length. Smaller teeth may have fewer crevices, pits, and fissures subject to decay.

We take today's dental treatments and antibiotics for granted. Serious infections that are a nuisance today were fatal a few decades ago. Modern day anthropologists frequently don't appreciate the contribution of dental health to individual fitness.

What were those dental infections that could have lethal consequences? In general, they include diseases such as cavernous sinus thrombosis, meningitis, bacterial endocarditis, pneumonia, and blockage of the airway with attendant suffocation. Many of these diseases and complications were more common and more frequently fatal in the past than they are today.

Cavernous sinus thrombosis used to be overwhelmingly fatal. Of the cases recorded in the literature up to 1963, the mortality rate stood at 80%. Those that survived had a 75% chance of having residual and permanent deficit.

In conclusion, teeth play a major role in individual fitness. The critical evolutionary question is not whether dental health is subject to natural selection, but whether the biological factors that influence susceptibility to dental health and disease can be identified. Tooth size is important!

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VIII. HERITABILITY

Heritability is a number between 0 and 1. Heritability has an intuitive meaning. Consider a parent that differs from the population by a certain amount. If its offspring also deviate by the same amount, heritability is one. If its offspring have the same mean as the population, heritability is zero. If the offspring deviate from the mean in the same direction as their parents but to a lesser extent, heritability is between zero and one. Heritability, therefore, is the quantitative extent to which offspring resemble their parents, relative to the population mean.

The theoretical formula for heritability says virtually nothing about the actual mode of inheritance. For a thoughtful discussion on the possibilities and limits of heritability values, see Scott and Turner, pp 153-157.

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IX. TWIN STUDIES

The involvement of genes in a character is clear in Mendelian pedigree patterns. Determining the genes involved in nonmendelian characters is not so easy.

Francis Galton, who laid so much of the foundation of quantitative genetics, pointed out the value of twins for human genetics. Galton said that "Twins have a special claim upon our attention; it is that their history affords a means of distinguishing between the effects of tendencies received at birth and those that were imposed by the special circumstances of their after-lives." He also anticipated the nature of inheritance when he theorized that an individual is a sum total of genetic particles, some of which stay dormant, whilst comparatively few achieve expression.

Monozygotic twins are genetically identical clones (with a few exceptions such as antibodies & T cells, somatic mutations, and the numbers of mitochondrial DNA molecules). In contrast, dizygotic twins share half their genes, the same as for any pair of siblings.

Research on separated twins, often a captivating news story, cannot distinguish intrauterine causes from genetic causes. Separated twins have contributed relatively little to human genetic research. The most powerful way to disentangle genetic and environmental factors is with adoptions studies. The main obstacle is the lack of information about the biological family.

..... CJ '99

(My note: If you are interested in dental anthropology, the book by Hillson listed below is an outstanding text.. It is an excellent resource and is very well written. The book by Scott and Turner below is a highly recommended companion to Hillson. It covers many topics not included in the texts prepared by Hillson

..... CJ '99.

Resources

Alt, K., Rosing, F., and Teschler-Nicola, M. eds Dental Anthropoogy Fundamentals, Limits, and Prospects. New York: Springer-Verlag/Wien, 1998.

Calcagno, J. and Gibson, K. "Selective Compromise: Evolutionary Trends and Mechanisms in Hominid Tooth Size" in Advances in Dental Anthropology. New York: Wiley- Liss, Inc., 1991.

Garn, S. "The Teeth and the Rest of the Body" in Essays in Honor of Robert Moyers, Hunter, W. and Carlson, D., eds. Ann Arbor: University of Michigan Press, 1991.

Garn, S. et al "Genetic, Nutritional, and Maturational Correlates of Dental Development" J. Dent. Res. 44, 228-242 (1965)

Garn, S. et al "Genetic Control of Sexual Dimorphism in Tooth Size" J. Dent. Res. 46: 963-972 (1967)

Garn, S. et al "Relationship Between Buccolingual and Mesiodistal Tooth Diameters" J. Dent. Res. 47: 495 (1968).

Hillson, S. Dental Anthropology. New York: Cambridge University Press, 1996.

Ridley, M. Evolution, 2nd ed. Cambridge: Blackwell Science, Inc., 1996.

Hillson, S. Teeth. New York: Cambridge University Press, 1996.

Kieser, J. Human Adult Odontometrics. New York: Cambridge University Press, 1990.

Scott, G. and Turner II, C. The Anthropology of Modern Human Teeth. New York: Cambridge University Press, 1997.