10.1 NON-METRIC VARIATION IN TOOTH FORM

I. METRIC AND NON-METRIC VARIATION IN TEETH

Metric variations are features that are directly measured. The mesiodistal crown diameters of teeth are just one example. These features are relatively easy to measure, they distribute in a Gaussian fashion, and can be treated statistically. Metric variation has generated a larger literature than any other aspect of dental anthropology.

Non-metric features are scored visually in terms of presence, absence, degree of development, or form. Non-metric features are quite complex and their assessment requires uniform standards. This has been accomplished with the use of cast plaster plaques, a process initiated by Dahlberg in the 1940s.

Many studies have attempted to calculate relationships between different populations based on frequencies of non-metrical variants which can be compared with biological or other affinities between populations.

In the next section we review a particularly well known study of the relationship between Indians of the New World and their presumed Asian ancestors using dental, serological, and linguistic evidences.

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II. SINODONTS AND AMERINDS: TRACING NATIVE AMERICAN ORIGINS

Anthropologists for more than a century have been intrigued by the biological similarities between Siberians and North American Indians. Christy Turner II of Arizona State University has studied the changing physical characteristics of Native American teeth, especially in their crowns and roots, and has compared them with those of Old World Asian populations.

The dental features studied by Turner are more stable than most morphological traits. There is a high genetic component that minimizes the effect of environmental differences, sexual dimorphism, and age variations. Turner has studied more than 4000 individuals, ancient and modern. From this, he has developed a series of hypotheses about the first settlement of the Americas based on dental morphology.

Prehistoric Americans display many fewer variations in their dental morphology than do Eastern Asians. Turner calls these features sinodonty. It is a pattern of dental features that includes shovel-shaped incisors, single-rooted upper first premolars, triple rooted lower first molars and other attributes.

(My note: what kind of frequencies are observed? Three-rooted lower first molars are reported in 25-40% of Eskimo-Aleuts and 6% for most North American Indian groups.) The cornerstone of Turner's hypothesis is this: sinodonty only occurs in northern Asia and the Americas. Sinodonty does not occur among the neighboring Mal'ta people of Lake Baikal or in the Stone Age Ukraine.

Turner's earliest evidence for sinodonty comes from northern China in about 18,000 B.C. He believes, however, that it emerged much earlier, perhaps as early as 40,000 years ago. According to his calculations, the Sinodonts migrated into Mongolia about 20,000 years ago and across the Bering Strait about 14,000 years ago.

European Upper Paleolithic peoples do not display sinodonty. It is Turner's belief that the Sinodont northern Chinese may have evolved from a primeval Southeast Asian Homo sapiens population.

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Turner's work with teeth gains support from both genetic and linguistic research. A team of American geneticists has collaborated on a study of the variants (called Gm allotypes) of one particular protein found in the serum portion of the blood. All proteins drift, or produce variants over the generations, and members of an interbreeding population will share a set of such variants. Thus by comparing the Gm allotypes of the two different populations , one can work out their genetic distance which itself can be calibrated to give an indication of the length of time since the populations last interbred.

The research team found that more than 14,000 Central and South American Indians in their samples share the same Gm allotypes. These allotypes are believed to have resulted from a single migration of hunter-gatherers across the Bering Straits 14,000 years ago. Two later migrations brought the present-day Athabaskans and Eskimo-aleuts to North America. Such Gm allotype data as it comes from Asia appears to link all three groups with northeastern Siberia.

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The other correlative evidence is from linguistics and is based on the work of Stanford linguist Joseph Greenberg. His argument is that the 300 or so languages in North America at the time of contact are divisible into three language groups: the Aleut-Eskimo on the northern fringe of North America, the Athabascan of Western Canada (with fragments in the American Southwest), and all of the rest are the so-called Amerind family of languages. His belief is that there were three separate migrations into the New World and these language groups are linguistic remnants of these events. It is only fair to mention that not everyone agrees with Greenberg.

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Is there supporting archaeological data? At present there is no solid evidence for Siberian sites dating to earlier than 18,000 years ago. Unfortunately, sophisticated microblade sites which could offer critical evidence are more recent than 18,000 years ago. Microblades are a widely shared and successful material culture that appeared at the end of the last glaciation and flourished well into post-glacial Holocene times.

The lack of archaeological data is not surprising. The first Americans were big game hunters whose cultural artifacts were perishable except for stone tools. Many of their campsites are now under water following the rise in sea after the end of the Ice Age.

Thus, the existing evidence supported by Turner's work suggests that the first Americans were Sindodont late Stone Age hunters and gatherers. They brought with them a dental and genetic components very much like that of modern American (North and South) American Indians. At some point, a few of these people, perhaps no more than a few families walked--or paddled across eastward along the Bering Strait. The date of intrusion into the New World is a continuing debate.

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III. THE CARABELLI TRAIT

This is the non-metric trait best known to dentists. This dental trait, the tuberculus anomalus, was first described in 1841 by Georg Carabelli, who was court dentist to the Austrian Emperor Franz.

The trait when present is located on the mesiolingual corner of upper first permanent molars and second deciduous molars. The trait is found infrequently in upper second permanent molars.

When present as a cusp, that feature is variable in size. The trait may express itself as a pit. The cusp version may rival the main cusps in size. In its pit version, it may appear as a small ridge, pit, or furrow. Sometimes the trait is a sort of lingual cingulum; a similar feature is found amongst the gibbon, chimpanzee, gorilla, and orangutan.

Scoring the trait in a consistent manner was difficult until cast plaster plaques were devised. Before them, it was difficult to compare the work of different researchers. Dahlberg was the first to provide plaques (in the 1940s). Others since have created a series of cast plaster plaques for standardized study. (My note: see Hillson for an excellent discussion on this topic)

The determinants of the Carabelli trait appear early in embryogenesis; some speculate that it is quite 'old' phylogenetically.

There is a demonstrated relationship between the Carabelli trait and distolingual cusp size in upper molars and also between the Carabelli trait and the protostylid of lower molars.

The Carabelli trait has a high incidence of expression in Caucasoid populations with a low level of frequency of shovel-shaped incisors. Conversely, the Carabelli trait is seldom fully expressed in Mongoloid populations, which possess a high frequency of shovel-shaped incisors.

Thus this non-metric dental trait along with others has been used to extimate times of evolutionary divergence between local races, their ancestry, and migration patterns of populations. This was demonstrated in the sinodont narrative earlier in this article.

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IV. OTHER COMMONLY RECORDED VARIANTS

(1) Shovel Shaped Incisors

In incisors, the marginal ridges may be especially prominent and enclose a deep fossa in the lingual surface. There are a variety of systems for grading this trait. Dahlberg and others subsequently have devised cast plaster plaques for standardized study.

The shoveling is most common in permanent and deciduous upper incisors, but can at times appear in lower incisors also. At times, the shoveling can create a pit on the lingual surface of central incisors.

The highest frequencies (greater than 90%) are found amongst Asians and Native Americans and lowest amongst Europeans. Shovel shaped incisors appear in Homo erectus, suggesting that this is a very ancient trait.

(2) Uto-Aztecan Upper Premolar

In the permanent upper first premolar, the buccal cusp may bulge out to the buccal with a marked fossa in its distal shoulder. The form is known only in Native Americans, with its highest frequencies in Arizona. The name is taken from a regional linguistic division.

(3) Protostylid of Lower Molars

The protostylid is a feature on the buccal side of the lower molar crown ranging from a pit in the buccal grove, through a furrow to a prominent cusp. It is seen especially on the first or third permanent molars or in deciduous lower second molars. It may be present in up to 40% of a population.

(4) Taurodontism

The term was coined by Sir Arthur Keith to describe the 'bull-like' condition in cheek teeth in which the tall root trunk encloses a high pulp chamber and short roots. It is best measured in radiographs. In living people it may appear up to 5% in some populations; it is prominent amongst the Krapina Neandertal specimens.

(5) Catalog of Other Traits Useful to the Dental Anthropologist

This is a listing of other traits; the reader is referred to Hillson and others for more details. Other traits you should know about are: canine ridges, cingulum features of the anterior teeth, variation in the main cusps of the upper molars, metaconule of upper molars, lower premolar cusps, cusp number on lower molars, molar groove and fissure patterns, deflecting wrinkle and trigonid crest, enamel extensions, and root number in cheek teeth.

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V. ASYMMETRY AND SEXUAL DIMORPHISM IN NON-METRIC VARIANTS

Symmetry right and left is variable which creates a problem for recording the variants. One approach is to note the greatest expression of the trait, but to keep records for both sides.

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VI. NON-METRICAL VARIANTS AND CROWN DIAMETER CORRELATION

Cusp number in lower molars seems to increase with crown diameter. Evidence is contradictory whether a relationship exists between upper molar size and Carabelli trait expression.

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VII. INHERITANCE OF NON-METRIC VARIANCE

Twin studies and family studies suggest that non-metrical dental variation is inherited polygenically. There is little clear evidence for inheritance linked to the X and Y chromosomes; therefore, inheritance of non-metric variance would appear to be autosomal. Most studies show no significant difference between males and females in non-metrical variants.

One final comment: No single crown or root trait has been shown to have a simple mode of inheritance. These traits are polygenic, NOT Mendelian.

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VIII. GENETICS OF DENTAL MORPHOLOGY

The prevailing view on the genetics of tooth crown morphology has been that such traits are under strong hereditary control. Statements to that effect have been made in these articles. Most lines of evidence (twin studies, familial correlations and population variation studies) indicate that genes are a major controlling factor in crown and root development.

Trait expression, however, may be affected to some extent by environmental factors. Do you remember the term 'fluctuating asymmetry?' And do you recall that laboratory animals subjected to Selye stress during tooth development exhibited greater fluctuating asymmetry? Read this next statement carefully: these events are taken as evidence that expression of nonmetric traits is to some degree influenced by environment.

There is another aspect of dental genetics, namely the genetics of deciduous teeth. In some individuals, crown traits (such as the Carabelli trait) are expressed on deciduous teeth that are not expressed on their permanent teeth. The reverse is also true, but occurs less commonly. Deciduous teeth are discussed in the next section.

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IX. DECIDUOUS TEETH

Dental anthropologists have concentrated on permanent teeth to the near exclusion of deciduous teeth for very practical reasons. Why? The 'window of time' for deciduous teeth is a narrow one. The deciduous dentition begins at age six months, is complete by about two years and six months, is intact to only age six years and is wholly lost by age twelve years. Given these limitations, it is easy to understand that 90% of the papers written on crown and root morphology focus on permanent teeth.

It has often been observed that deciduous teeth are more primitive, or evolutionarily conservative than permanent teeth. For example. deciduous teeth have shown less crown size reduction than permanent teeth. Furthermore, deciduous teeth were also more stable in terms of numerical variations, with significantly lower frequencies of missing and extra teeth than the permanent dentition.

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X. ENDOCASTS

When I was a graduate student at the U of C, Al Dahberg handed me a tattered book and a small box of enamel remnants of fossil teeth and told me to examine them. The book was by Korenhof. His work is cited in Scott and Turner II. The story is intriguing.

What Korenhof found was this: in some soils, the dentin of teeth was lost, but the enamel 'caps' survived. Their interior surface recorded the entire enamel-dentin interface laid down during crown development. When Korenhof made endocasts of those enamel 'caps,' he was struck by this idea: enamel, especially thick human enamel, obscures the best evidence for studying evolution. That evidence was the enamel-dentin interface.

That idea was pursued in a different manner by Kraus and Jordan in The Human Dentition Before Birth. They point out the similarity between developing human teeth and some selected fossil teeth. Be warned, however: this is a thorny and complex topic. For a concise review and discussion, see Scott and Turner II, pp 87-89.

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My note to the reader: for further reading, start with Scott and Turner II, and Hillson which are listed below. Both give a wonderfully lucid and well referenced discussion of these topics.

..... CJ '98

Resources

Fagan, B. Ancient North America 2nd ed. New York: Thames and Hudson Inc., 1995.

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

Jordan, R. and Abrams, L. Kraus' Dental Anatomy and Occlusion. St. Louis: Mosby Year Book, 1992,

Krause, B. and Jordan, R. The Human Dentition Before Birth. Philadelphia: Lea and Febiger, 1965.

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

Turner II, C. "Advances in the Dental Search for Native Americans" Acta Anthropogenetica 8: 23-78.

Turner II, C. "Teeth and Prehistory in Asia" Scientific American, February, 1989 pp 88-96.