The Eskimo-Aleut Dentition: Crown and Root Morphology

Abstract

Objective of work

This paper provides an overview of crown and root morphology in Eskimo-Aleut populations of the American Arctic. For context, Eskimo-Aleut dental variation is compared to closely related American Indians and distantly related Europeans.

Material and methods

The characterization of dental trait frequency variation is based on observations made on approximately 10,000 dentitions scored by the late Christy G. Turner II and the author. Sixteen crown and five root traits were scored following the conventions outlined in the Arizona State University Dental Anthropology System.

Results

Of the 21 dental traits considered, only three showed slight differences among Eskimo-Aleuts, American Indians, and Europeans (UM1 cusp 5, LM2 groove pattern, LM2 root number). For the remaining traits, there was typically a dramatic contrast between the two New World populations and Europeans. While generally similar, Eskimo-Aleuts and American Indians showed differences in UI1 winging, shoveling, and double shoveling, UM1 Carabelli trait, 2-cusped UM2, 3-rooted UM2, and especially 3-rooted LM1.

Conclusion

The differences between the three groups are likely a product of genetic drift and founder effect although recent work on the EDAR V370A allele suggests some dental variables like shoveling and lower molar cusp number may indirectly reflect natural selection operating on other variables influenced by this allele.

Key words

Tooth Crown; Tooth Root; Alaska Natives; North American Indians; European Continental Ancestry Group

Introduction

Linguistically, Eskimos and Aleuts are in the same language family (Eskaleutian), but they diverged from one another between four and five thousand years ago. Later, Eskimos diverged into two linguistic subgroups, Yupik and Inuit. Yupik speaking Eskimos are found on St. Lawrence, Nunivak, and Kodiak Islands, in the Yukon-Kuskokwim delta region of southwest Alaska, and a small area in far eastern Chukotka. Inuit speaking groups extend from the Seward Peninsula in western Alaska across the farthest reaches of northern Canada to east Greenland. Aleuts inhabit the Aleutian archipelago extending far into the Pacific from the southwest corner of Alaska (Fig 1).

Fig. 1.

Distribution of Eskimos and Aleuts across the American Arctic (Aleutian Island archipelago too extensive to be illustrated).

In circumpolar North America, Aleuts and Eskimos range from 174.2^oW (western Aleuts) to 10^oW longitude (East Greenland Eskimos). Living primarily on islands or along coastlines, subsistence adaptations are predominantly maritime. Depending on location, groups subsist on whales, walrus, diverse seal species, and anadromous fish (e.g., salmon, Arctic char). Terrestrial resources like caribou and musk oxen are utilized by groups who can exploit both inland and coastal resources. In many areas, winters are long and cold while summers are short and cool. During most of the year, the diet is high in protein and fat and deficient in carbohydrates and calcium. During the winter, the primary staple is the dried flesh of marine mammals and fish (1).

Although many people envision Eskimos living in igloos (ice houses) in a snow-covered landscape, this describes mostly groups living in central Canada. Environments inhabited by circumpolar populations are quite diverse. The Aleutian archipelago is comprised of 69 volcanic islands that extend across 1900 km. Temperatures are moderated by a cool wet maritime environment so Aleuts, like Scandinavians, have more issues with hypothermia than frostbite. Kodiak Island, the Alaska Peninsula, and southwest Alaska likewise provide relatively benign environmental settings where adjacent seas never freeze. These temperate environments are inhabited by Aleuts and Yupik Eskimos.

The lower latitude groups provide a stark contrast to their northerly neighbors who contend with frozen seas annually. The northern groups who experience severe cold and dry winters are the Inuit. These populations had to adapt their clothing and shelter to avoid frostbite and allow movement across the landscape for subsistence activities. To combat subzero temperatures and high winds and lacking any other material, central Canadian Inuit built igloos out of ice blocks. In northern Alaska and much of Greenland, some combination of turf, stone, driftwood, whale bones, and animal hides were used to construct semi-subterranean shelters with depressed Arctic entryways that limited heat loss from the primary living area (2).

Eskimo-Aleuts are not recent arrivals to the New World as once thought. During the latter stages of the Upper Pleistocene (date range: 128,000 to 12,000 BP), populations in northeast Siberia and western Alaska were gradually shifting their range eastward toward the Americas when movement was thwarted by coastal glaciers and massive ice sheets that covered most of Canada. The ancestors of Eskimo-Aleuts were one element of a large population system that was distributed across greater Beringia from roughly 30,000 to 15,000 years before present. This so-called Beringian Standstill population included the ancestors of both American Indians and Eskimo-Aleuts (3). Around 15,000 years ago, the ancestors of American Indians broke out of the standstill and made their way down the coast into the Pacific Northwest. Once they reached this glacier-free landscape, they dispersed across the Americas from the Pacific to the Atlantic and from the northwest coast of North America to Tierra del Fuego, the southernmost point of South America.

While the ancestral-descendent groups of American Indians ultimately inhabited most of North America and the entirety of South America, Eskimo-Aleuts carved out their place in the Americas by settling the coasts and islands of subarctic and Arctic North America (4). Although their ancestors were part of the larger Native American standstill population during the late Pleistocene, their geographic placement remains in proximity to Northeast Asian populations. Through some combination of gene flow and adaptations to similar environmental settings, Eskimo-Aleuts show more ties to Northeast Asians than do American Indians. For example, Northeast Asians have a high frequency of blood group allele B (20-30%), a gene that is lacking in American Indians. Eskimo-Aleuts have the allele B but in much lower frequency than in Northeast Asians (ca. 5%). Another Asian trait, the epicanthic eye-fold, is more common in Eskimo-Aleuts than in American Indians. Many other genetic and morphological traits exhibit the same pattern, including some dental traits that are noted later.

Physically, Eskimos are often used as a textbook illustration of ecogeographical rules. They have relatively short limbs relative to trunk length (high sitting height ratio), illustrative of Bergmann’s Rule. Limb length is also reduced in conformance with Allen’s Rule. In other words, they have a physique that maintains body heat in an environment where ambient temperatures are often well below zero degrees (5, 6). Eskimo-Aleuts also have some of the world’s largest cranial capacities, a characteristic also interpreted in terms of climatic adaptation. Regarding the distribution of cranial capacity relative to stature, the highest values mirror almost exactly the distribution of Eskimo-Aleut populations in the New World (7).

Waugh (8, 9) measured bite force between the upper and lower first molars of Eskimo males and females and arrived at average values of 280 psi for males and 240 psi for females. Europeans, by contrast, generate bite forces between 90 and 120 psi. In 1977, W.L. Hylander (10) wrote an article entitled “The adaptive significance of Eskimo craniofacial morphology.” He detailed many unique characteristics that are associated with the production and dissipation of pronounced vertical occlusal forces. Changes involved in generating large bite forces include reduced prognathism where the face is situated more directly under the frontal bone. Hypertrophied temporal muscles are reflected in high and pronounced temporal lines on the parietals along with distinct sagittal keeling. Large masseter muscles are indicated by distinct gonial eversion and broad ascending rami. Characters that reflect the dissipation of forces include pinched nasal bones and thickened tympanic plates. Perhaps tied to this in some way, Eskimo-Aleuts have the world’s highest frequencies of palatine and mandibular tori (11, 12).

Directly affecting teeth, pronounced bite force can stress dental enamel beyond its breaking point. Although individuals in all populations chip their teeth, the extent of dental chipping in Eskimos is unparalleled. Turner and Cadien (13) noted that dental chipping was far more common in Eskimos than in Aleuts, reflecting a difference in jaw mechanics and dietary behavior. Far more than Aleuts, Eskimos consumed tough, frozen foods that included grit added by accident during the drying process. Scott and Winn (14) note that when Europeans show dental chipping, it is largely confined to the anterior teeth. St. Lawrence Island Eskimos, by contrast, show high levels of chipping on both the anterior and posterior teeth.

Early Arctic explorers noted how the Eskimo used their teeth as tools, like a third hand. The anterior teeth were often used by females to soften frozen boots. Males used their anterior teeth for a variety of tasks, producing a rounded form of wear on the incisors. One by-product of this wear was shortening the roots of the upper incisors (10, 20). I have observed dentitions of middle-aged Eskimos where the length of the root was shorter than the height of the crown. This often led to premature tooth loss, sometimes interpreted as intentional tooth removal, or ritual ablation (14). Given the value of the anterior teeth in Eskimo life, intentional removal would have been behaviorally counter-productive (15), so premature loss of anterior teeth was likely tied to tooth-tool use and shortened roots.

Some researchers have speculated on the role of crown and root morphology as possible adaptations to the environment and dietary behavior of Eskimo-Aleuts. For example, shovel-shaped incisors provide added crown area that could strengthen and prolong the useful life of the anterior teeth. Three-rooted lower first molars, which find their highest frequencies in Eskimo-Aleuts, provide an anchor to the lower first molar that could prolong the functional life of that important tooth. To determine if there is any veracity to these suggestions, I turn to the substance of this paper – Eskimo-Aleut crown and root morphology.

Eskimo-Aleuts and dental anthropology

A key dividing line in the field of dental anthropology is the 1963 volume edited by D.R. Brothwell (16) entitled Dental Anthropology (17). Prior to that date, four monographs focused on the dentitions of specific geographic populations: T.D. Campbell on Australian aboriginals (18), J.C. Middleton-Shaw on Bantus (19), P.O. Pedersen on East Greenland Eskimos (20), and C.F.A. Moorrees on Aleuts (21). Early research on Eskimo-Aleuts played a significant role in the development of dental anthropology. Another pioneer in the field, Albert A. Dahlberg, is best remembered for his work on Southwest Indians and American whites, but he was the dental researcher brought in to work on Alaskan populations as part of the Human Adaptability Project, a subdivision of the International Biological Program (1964-1974). In this context, Dahlberg collected dental casts of living populations from Kodiak Island and Wainright, Alaska (22) and later studied St. Lawrence Island Eskimo dentitions.

Influenced by the research of B.S. Kraus (23) on the genetics of dental morphological trait expression and the standard plaques for scoring tooth crown traits developed by A.A. Dahlberg (24), C.G. Turner II initiated research on tooth crown and root variation in Eskimos and Aleuts in the early 1960s. This work culminated in a doctoral dissertation entitled The Dentition of Arctic Peoples (25, 26). Turner ultimately went on to make observations on the dental morphology of ca. 24,000 skeletons from the Americas, Northeast and Southeast Asia, the Pacific, and Europe, but he never abandoned his first focus, Eskimos and Aleuts. The combined Eskimo sample he observed numbered 1317 individuals from Alaska, Siberia, Canada, and Greenland. For eastern and western Aleuts, he scored 405 individuals. In addition to Turner’s large database, I made observations on 759 Alaskan Eskimo dentitions during my tenure at the University of Alaska Fairbanks.

This summary article characterizes crown and root traits in Eskimo-Aleuts based on the combined samples of Turner and Scott totaling 2481 individuals. Frequencies without comparisons lack context so I include trait frequencies for a large combined sample of North and South American Indians (>5000 individuals) who are like their neighbors in the Arctic but show interesting differences. Since the audience of this article is most familiar with European teeth, frequencies from a large combined eastern and western European sample (>2000 individuals) provides a second point of reference. If the reader is interested in broader world comparisons involving African, Asian, and Pacific populations, these are available in Scott and Turner (27) and Scott et al. (28).

Dental Morphology

Turner et al. (29) describe 38 dental and three jaw traits as part of the Arizona State University Dental Anthropology System. That widely cited article only had four illustrations, but this limitation was rectified in recent volumes by Scott and Irish (30) and Edgar (31), which include hundreds of photos and line drawings of over three dozen crown, root, and jaw traits. Additional illustrations of the traits that make up ASUDAS can be found in Scott et al. (28, 32) and Scott and Dumančić (33).

In his many population studies that focused on Native American, Northeast and Southeast Asian, and Pacific populations, Turner developed a list of 29 key crown and root traits (no jaw traits are included in this list). After Turner’s untimely passing in 2013 (34), I assumed the task of preserving his data for posterity in what I refer to as the Christy G. Turner II Legacy Project. This involved scanning 24,000 individual data sheets and hundreds of computer printouts that show the full class frequency distributions for his 29 key traits. To see how the data are organized, the appendix of Scott and Irish (30) is comprised of 60 tables for some of Turner’s largest samples.

In this overview of the Eskimo dentition, I selected 15 crown traits and six root traits from the key trait list of Turner. Note that: U = upper or maxillary, L = lower or mandibular; I = incisor, C = canine, P = premolar, M = molar; 1, 2, or 3 = number in tooth district. These are broken down into four groups: (1) anterior teeth; UI1 bilateral winging, UI1 shoveling, UI1 double shoveling, and UI2 interruption grooves; (2) maxillary traits: UM1 Carabelli trait, 3-cusped UM2, UM1 cusp 5, UM1 enamel extensions, and pegged-reduced-missing UM3; (3) mandibular traits: LP2 lingual cusp number, UP and LP odontomes, 4-cusped LM2, LM2 Y groove pattern, LM1 cusp 6, LM1 cusp 7, and LM1 deflecting wrinkle; and (4) root traits; 2-rooted UP1, 3-rooted UM2, 2-rooted LC, 3-rooted LM1, and 2-rooted LM2. The break points for each trait are shown in histograms.

Anterior traits

When I refer to anterior traits, it is directed at the upper incisors and canines. Europeans have relatively simple anterior teeth, only broken up on occasion by cingular tubercles (tuberculum dentale). Asian and Asian-derived populations provide a significant contrast. They commonly exhibit one of the best-known morphological traits of the human dentition: shovel-shaped incisors (35). The hallmark of shoveling is pronounced lingual marginal ridges. A related trait on the labial surface is referred to as double-shoveling. Another trait rare in Europeans but common in Asian populations is bilateral winging of the upper central incisors. The final trait, interruption grooves of the upper incisors, either crosscut marginal ridges or extend from the crown to the root, constituting corono-radicular grooves. These traits are illustrated in Fig. 2.

Fig. 2.

Anterior traits (A: 1. UI1 bilateral winging; 2. UI2 talon cusp; B: 1. UI1 double shoveling, 2. UI1 shoveling, 3. UI2 interruption groove, 4. supernumerary UI2 with talon cusp).

In Fig. 3, the major differences between Native Americans and Europeans in the anterior teeth are indicated. Europeans exhibit low frequencies of UI1 winging, shoveling, and double shoveling, a characterization that stands in marked contrast to the two Native American groups. American Indians have the world’s highest frequencies of these three traits. In all three instances, the frequencies are significantly lower in Eskimo-Aleuts. Only for UI2 interruption grooves do Eskimo-Aleut frequencies exceed those of American Indians, with both being slightly higher than Europeans. Interruption grooves are the only trait where European frequencies approximate those of Native Americans.

Fig. 3.

Frequency variation of anterior traits.

Maxillary crown traits

Upper premolars exhibit morphological variation in terms of accessory ridges and tubercles, but these do no show clearly patterned variation among modern human groups. The most interesting variation is expressed on the upper molars. Three of the five traits are expressed on UM1: Carabelli trait (a cingular tubercle on the lingual surface of the mesiolingual cusp), cusp 5 (an accessory cuspule on the distal marginal ridge between the metacone and hypocone), and enamel extensions (cervical enamel line extends toward the interradicular projection between the two buccal roots of upper molars). Cusp number, dictated primarily by the presence and expression of the hypocone, is scored on UM2 because its expression is largely invariant on UM1. Finally, Turner combined UM3 congenital absence with size reduction, which is considered a single trait noted as pegged-reduced-missing UM3. These traits are shown in Fig. 4.

Fig. 4.

Maxillary traits (A: 1. UP1 odontome, 2. intermediate grade of UM1 Carabelli trait; B: 1. absence of hypocone on UM2, 2. cusp 5 on UM2; C: enamel extensions on UM1 and UM2; D: reduced UM3).

For the most part, Europeans have a relatively simplified dentition, referred to as the “Eurodont pattern” by Scott et al. (36). One trait that is more common and often quite pronounced in Europeans is the Carabelli trait. The histogram shows the frequency of all but the lowest grade of Carabelli expression (Fig. 5). Following this or any other breakpoint, European frequencies far exceed those of Native Americans. For the two groups of Native Americans, American Indians have a higher frequency than Eskimo-Aleuts. All Native Americans have a very low frequency of pronounced cusp or tubercle expressions (37), with Eskimo-Aleuts exhibiting the lowest frequencies in the world for all forms of Carabelli expression.

Fig. 5.

Frequency variation of maxillary traits.

Living and fossil hominoids and early fossil hominins invariably exhibit four cusped upper molars. In modern humans, the cusps of the trigonid (paracone [MB], metacone [DB], protocone [ML]) are typically retained although the metacone is sometimes reduced in size. The key cusp is the hypocone [DL], the last major cusp of the upper molars added to the tribosphenic crown (i.e., trigon) of early primate maxillary molars (38). Following the general principle ‘last cusp on, first cusp off,’ the hypocone is often absent on the upper second molar of modern humans, resulting in a 3-cusped tooth. Fig. 5 shows that about one-third of European UM2s are 3-cusped. For American Indians, this frequency is not half as high (12%). Eskimo-Aleuts, despite their overall greater dental morphological complexity, almost match Europeans with 3-cusped UM2s around 30%.

Of the remaining maxillary traits (Fig. 5), UM1 cusp 5 is basically equal among the three groups (ca. 15%), which is on the low end of world variation for this trait (28). UM1 enamel extensions, by contrast, show a major difference between Native Americans who often exhibit this trait (40-45%) and Europeans who rarely do (3%). For pegged-reduced-missing UM3, American Indians and Europeans have almost identical frequencies of around 16%. Eskimo-Aleuts, with very large jaws and no paucity of space for third molars, exhibit the highest frequency at 22%.

Mandibular crown traits

Two premolar variants that are part of Turner’s key trait list are multiple lingual cusps of LP2 and odontomes, or tuberculated premolars, of all upper and lower premolars. Key crown traits of the lower first molar include cusp 6, cusp 7, and the deflecting wrinkle. The nature of the contact between the major cusps at the central occlusal fossa (groove pattern) and the absence of the hypoconulid on LM2 that results in a 4-cusped lower second molar, are the two final key traits of the lower molars (Fig. 6).

Fig. 6.

Mandibular traits (A: 1. one lingual cusp on LP2, 2. LM2 X pattern, 3. 4-cusped LM2 (lacking hypoconulid); B: arrows point at cusp 6 on all three lower molars; C: LM1 deflecting wrinkle; D: LM1 cusp 7).

Native Americans have the lowest frequencies of LP2 multiple lingual cusps (30-40%) in the world (Fig. 7). All other populations have frequencies over 50%, including Europeans at about 60%. While extra lingual cusps are common, odontomes are rare. These conical occlusal tubercles are expressed in the sagittal sulcus between the buccal and lingual cusps of both upper and lower premolars. This trait is rare in European populations (1.6%) but attains the world’s highest frequency of about 5% in Native Americans. The highest frequency ever reported was 17% in a St. Lawrence Island Eskimo sample (39). This trait is present in Asian and Pacific populations but is rarely expressed on African and European premolars (28).

Fig. 7.

Frequency variation of mandibular traits.

Lower molar cusp number and groove pattern were two of the first polymorphic traits described for the human dentition. Fossil hominoids and early hominins exhibit five cusps on all three lower molars and contact between cusps 2 (metaconid; ML) and 3 (hypoconid; DB) at the central occlusal fossa, forming the so-called Y pattern (40). These traits were described together as the Dryopithecus Y-5 pattern, which characterized the lower molars of the European Miocene ape of the same name (41). Most modern humans still exhibit the Y-5 pattern on LM1 but LM2 exhibits derived features in terms of changes in cusp number and contact. Regarding cusp number, the distal cusp (hypoconulid) is often lost on LM2, producing a 4-cusped tooth. For LM2, it is also more common to find cusp contact between cusps 1 (protoconid; MB) and 4 (entoconid; DL), referred to as an X pattern.

LM2 cusp number is radically different between Native Americans and Europeans (Fig. 7). In line with loss of the hypocone on UM2, the hypoconulid is frequently absent on LM2 in Europeans (70-80%). Even though Eskimo-Aleuts parallel Europeans for hypocone loss and 3-cusped UM2, this is not evident for 4-cusped LM2. It is rare for either American Indians or Eskimo-Aleuts to show hypoconulid loss and 4-cusped LM2 (<10%). In contrast to LM2 cusp number, the Y groove pattern on LM2 is expressed in similar frequencies between Europeans and Native American groups (25-30%).

Beyond cusp number and groove pattern, the lower first molars exhibit three other polymorphic traits that vary widely on a world scale. Cusp 6, or tuberculum sextum, is a supernumerary cusp expressed between the entoconid and hypoconulid. Cusp 7, or tuberculum intermedium, is a wedge-shaped cusp that occurs between the metaconid and entoconid. The deflecting wrinkle is an occlusal polymorphism that involves the essential ridge of the metaconid. Typically, the essential ridge runs a straight course from the cusp tip to the central occlusal fossa. In some instances, the ridge bends or deflects about halfway along its course, producing the so-called deflecting wrinkle (29, 30).

For two LM1 variants, there is a dramatic difference between Europeans and Native Americans but little difference between the two Native American groups (Fig. 7). Cusp 6 is very common (45-55%) in Native Americans and rare in Europeans (<10%). The deflecting wrinkle shows the same pattern with very high frequencies in Native Americans (55-60%) and a low frequency in Europeans (15%). The final variable, cusp 7, is rare in all three groups (5-10%). The frequency of this variant falls below 10% in all world populations with but one exception – Sub-Saharan Africans. African populations have cusp 7 frequencies between 30 and 40%.

Root traits

Some teeth are characterized by an almost invariant number of roots. For example, upper and lower incisors and upper canines have one root. Upper first molars exhibit three roots while lower first molars have two roots. Root number polymorphisms are evident in the lower canine, upper and lower premolars, and upper and lower second molars. Third molar root number is highly variable and environmentally labile, so they are not considered here. Briefly, lower canines sometimes exhibit an inter-radicular projection that produces two distinct roots, one buccal and one lingual. Upper premolars exhibit one, two, or three roots; for two roots, there is one buccal and one lingual root while for three rooted upper premolars, there are two buccal roots and a single lingual root. The derived condition for upper premolars is a single rooted tooth, a product of root fusion between the buccal and lingual roots. Although both upper premolars can exhibit all three root number variants, it is most common to have bifurcated roots on UP1, so this is considered the key tooth. For the upper second molar, the standard root configuration is two buccal roots and one lingual root. The variant in this instance involves root fusion producing a two rooted or single rooted UM2. Since this root fusion takes several forms, the frequency reported here is for the unfused 3-rooted UM2. The default root configuration in the lower molars is a single mesial and single distal root, forming a two rooted tooth. While LM1 almost invariably has two roots, LM2 can exhibit root fusion along the buccal margin forming a C-shaped form or both lingual and buccal margins can be fused. Again, the variant used to characterize populations is the standard 2-rooted LM2 with root fusion as the derived form. Root number for the lower canine, upper premolars, and upper and lower second molars is defined by the presence or absence of inter-radicular projections. The final root trait is different. Three-rooted lower first molars (3RM1) involve an accessory distolingual root (Fig. 8).

Fig. 8.

Root traits (A: 1. 2-rooted UP1, 2. 2-rooted LC; B: 1-rooted UM2; C: 1-rooted LM2; D: 3-rooted LM1).

Of the five root traits, Eskimo-Aleuts exhibit distinctive frequencies for three (Fig. 9). They exhibit more root fusion than the other two groups for upper first premolars and upper second molars. They almost invariably have 1-rooted UP1 along with a significantly lower frequency of 3-rooted UM2. In concert with root fusion, they exhibit by far the highest frequency for the accessory root on LM1, the 3-rooted lower first molar (3RM1). For the lower canine, it is extremely rare to find a 2-rooted variant in either Native American group. This trait, while rare in general, is a hallmark of the European dentition where it attains a frequency of 5-10% (28, 42). The root fusion in Eskimo-Aleuts that sets them apart for UP1 and UM2 is not evident in LM2. All three groups have 2-rooted LM2s in a frequency around 65-75%.

Fig. 9.

Frequency variation of root traits.

Discussion

In his extensive studies of Asian and Asian-derived populations, Turner (43, 44) observed consistent morphological differences between Northeast Asians (e.g., China, Japan, Mongolia, etc.) and Southeast Asians (e.g., Thailand, Philippines, Sumatra, etc.). While populations in the two regions exhibit the same suite of dental traits, some are consistently more common in Northeast Asians (e.g., UI1 shoveling and winging, 3-rooted LM1). With more complex and derived dentitions, Northeast Asians were placed in his Sinodont dental pattern. The more generalized and less complex dentition of Southeast Asians was described as the Sundadont pattern. Beyond Asia, he noted that Native Americans exhibit the Sinodont pattern, suggesting an ancestral-descendant relationship between Northeast Asians and New World populations. As the Pacific was settled primarily by Southeast Asians, Polynesians exhibit the Sundadont dental pattern. The dental pattern of Australo-Melanesians is reviewed in Scott et al. (28).

While New World populations were considered to exhibit the Sinodont pattern of Northeast Asians, a reanalysis of Asian, Pacific, and New World populations reveal that Native American populations express an extreme form of the Sinodont pattern, which Scott et al. (45) note as super-Sinodont. In a cluster analysis of Asian, Pacific, and New World populations, the New World samples grouped together (Eskimo-Aleuts and Indians from the Northwest Coast, North America, Mesoamerica, and South America), while Northeast Asians clustered with Southeast Asians. To develop this magnitude of difference among Sinodonts suggests the ancestors of Native Americans were isolated from Northeast Asians for an extended period (i.e., 8-10,00 years) prior to the settlement of the Americas. In this regard, dental morphological data are consistent with classic genetic markers (46) and mtDNA (47).

What evolutionary factors were involved in the divergence of New World populations from Northeast Asians? Scott et al. (28) demonstrated that the pattern of global differentiation derived from dental morphological traits was largely consistent with the assumptions of genetic drift and founder effect. There is, however, some indication that selection played a role in this divergence. When a gene in the ectodysplasin pathway, EDAR V370A, was shown to be partly involved in the development of shovel-shaped incisors (48) and lower molar cusp number (49), this opened up the possibility that selection might somehow influence dental trait frequencies. The effect may not be direct, as in the association of sickle-cell trait and malaria, but indirect where selection acted on other key biological variables that were tied more closely to fitness. Hlusko et al. (50, p. E4426) “hypothesize that selection on EDARV370A occurred in the Beringian refugium because it increases mammary ductal branching, and thereby may amplify the transfer of critical nutrients in vitamin D-deficient conditions to infants via mothers’ milk. This hypothesized selective context for EDAR V370A was likely intertwined with selection on the fatty acid desaturase (FADS) gene cluster because it is known to modulate lipid profiles transmitted to milk from a vitamin D-rich diet high in omega-3 fatty acids.” In such a scenario, shoveling would be a genetic hitchhiker where selection acted principally on associated pleiotropic variables.

For some morphological features of the Eskimo-Aleut dentition, natural selection seems counter-intuitive. For example, why would they have the world’s highest frequency of odontomes? These occlusal tubercles are delicate structures that often fracture. If maintaining a complex dentition was a high priority, why do Eskimo-Aleuts have one of the world’s highest frequencies of pegged-reduced- missing UM3? Eskimo jaws are huge so space was not an issue; in some instances, there would be room for a fourth molar. Hypocone loss on UM2 also runs counter to the need for a more morphologically complex dentition. Although Eskimo-Aleuts are often like American Indians (LM1 cusp 6, LM2 cusp number, LM2 root number), they show less complexity in the anterior teeth with lower frequencies of UI1 shoveling, double shoveling, and winging. The only instances where traits are more common in Eskimo-Aleuts include the greater root fusion seen in UP1 and UM2 and the addition of the accessory root on LM1, or 3RM1; in some Aleut samples, 3RM1 reached a frequency of over 50%.

Conclusions

Dental morphological traits are powerful tools for assessing population origins and relationships when based on sample frequencies. These traits are also used in assessing the ancestry of single individuals (51). Despite showing some differences from American Indians, the Eskimo-Aleut suite of dental traits is closer to other Native Americans than to any other group. This similarity is due in part to the shared ancestry of the two groups who both arose from a late Pleistocene Beringian Standstill population that extended across much of Northeast Asia into Alaska. When the ancestors of American Indians finally broke free from the ice barriers responsible for the standstill, the progenitors of Eskimos and Aleuts remained in the far north. Because there was no absolute barrier to gene flow between the Old World and New World, this is the likely reason why some Eskimo-Aleut dental trait frequencies (e.g., UI1 winging, shoveling, double shoveling, and especially 3-rooted LM1) are closer to Northeast Asian populations than are American Indians. Genetic data show the same pattern (52). While natural selection may have played some role in the pattern of dental variation we observe in New World groups, chance processes through drift and founder effect were primary in establishing the pattern of dental variation we see in Asian and New World populations, including Eskimo-Aleuts with some evidence for gene flow among northern populations during the late Holocene (the past 12,000 years).