A critique of the «increasing population density effect»

The «increasing population density effect» (IPDE) proposed by Macchiarelli and Bondioli (1986) represents one of the most recent attempts to account for dental reduction in modern human populations. Under the IPDE, the marked reduction in tooth size observed in post-Pleistocene human groups is seen as merely a side-effect of a more general reduction in body size, which resulted from an increase in population density. The model is dependent upon a strong correlation between tooth size and body size, which after numerous attempts has yet to be convincingly demonstrated in humans. This paper argues that the IPDE neglects the negative consequences on individual fitness of teeth which are too large to fit into diminishing jaws or are more susceptible to pathology, and that the worldwide reduction of tooth size is the result of selection for smaller teeth due to shifts to a softer and/or more cariogenic diet. Although increased population density may have intensified this selective effect by decreasing general fitness by lowering resistance to oral infections, it was not the primary cause of dental reduction. All proposed mechanisms of dental reduction, however, are in need of additional testing, and possible directions are offered for future studies of the phenomenon.     

Patterns of size variation and correlation in the dentition of the red colobus monkey (Colobus badius)

There have been numerous studies on variability and correlation in dental crown size, but the significance of the resulting patterns remains unclear. Regions of low variation and high correlation have been hypothesized to represent the poles of Butler's morphological fields, to be related to absolute tooth size, or to be related to morphological complexity of the teeth and functional efficiency. Variation and correlation of tooth lengths and breadths were investigated in 138 red colobus monkeys to further assess the relations among size associations, crown morphology, and absolute tooth size. In the maxilla and mandible, the postcanine teeth are the most highly correlated and least variable, followed by the incisors, then the canines. There are also lower correlations between premolars and molars than within either group. While there appears to be a relation between degree of morphological differentiation and levels of correlation and variation, there are no notable differences in the correlation of opponents along the dental arcade, which is the most important functional consideration. This suggests that different levels of correlation and variation within upper or lower teeth are “artifacts” of tooth dimensions that contribute to different geometric designs in different tooth groups as the germs develop. This morphological effect is coupled with the influence of integration fields, indicated by higher variability and lower correlations of the third molar, the largest or most molarized tooth. It is concluded that there are wide functional tolerances in occlusion with respect to the gross dimensions of dental crowns and their interrelationships.     

Human dental reduction: natural selection or the probable mutation effect

Dental reduction has been sufficiently widespread among human populations to render the phenomenon of reduced tooth size worthy of scientific explanation. One of the most controversial models invoked to explain structural reduction in organisms is referred to as the "probable mutation effect" (PME). According to this model, structures no longer functional owing to ecological or cultural changes will experience a relaxation of selection pressure, permitting an accumulation of mutations in the population that inevitably will result in the reduction in size or the loss of the concerned structure. Although the PME continues to be offered as a viable explanation of human dental reduction, it is based upon several premises that modern dental clinical experience fails to support. Known enzyme defects resulting from mutations, factors predisposing to dental infections, and the deleterious effects of teeth that are too large or too small reveal that the PME does not logically account for the reduction of tooth size. Given such information, this paper proposes models of dental reduction based upon natural selection, which, unlike the PME, are testable in both modern and archaeological populations. The integration of clinical and skeletal data permits a more thorough understanding of dental reduction in the hominid fossil record.     

Biomechanical association of dental and temporomandibular pathology in a medieval Nubian population

An analysis of the relationship between oral pathology and degenerative change at the temporomandibular joint (TMJ) was undertaken on an archaeological sample of 122 adult crania from the Medieval site of Kulubnarti in Sudanese Nubia. The crania were sorted into 2 groups: those demonstrating clearly visible bony changes at the joint (TMJ+) and those without visible change (TMJ-). These groups were compared according to 1) age; 2) sex; 3) active dental pathologies (abscesses, caries, partial socket resorption); 4) tooth loss with complete socket resorption; and 5) dental attrition. No statistically significant association was evident between degenerative change at the TMJ and age, active dental pathologies, or dental attrition; however, sex differences and posterior tooth loss with complete socket resorption revealed a significant correspondence to degenerative TMJ changes. Both of these factors agree with the clinical literature and with biomechanical models (most notably that of Hylander) based upon modern populations. Furthermore, the results support the contention that paleopathological conditions can be analyzed from a clinical and functional biomechanical perspective.     

中国古人类牙齿尺寸演化特点及东亚直立人的系统地位

对中国境内不同时代人类牙齿测量数据的发析显示,中国古人类牙齿尺寸的总体演化趋势与世界其它地区人类一致,呈缩小变化,其中一个表现特点是中国直立人与早期智人在牙齿尺寸上不差别不大,但作者根据对中国古人类化石形态特征,生存年代等方面的综合分析认为中国直立人在化石形态,生存年代等方面均与智人有明显的不同,取消直立人,将其并入智人意见的证据还是不够充分的,目前仍宜将直立人与智人作为人属内两个不同的种来看待,     

Patterning the size and number of tooth and its cusps

Mice and rats, two species of rodents, show some dental similarities such as tooth number and cusp number, and differences such as tooth size and cusp size. In this study, the tooth size, tooth number, cusp size and cusp number, which are four major factors of the tooth patterning, were investigated by the heterospecific recombinations of tissues from the molar tooth germs of mice and rats. Our results suggest that the dental epithelium and mesenchyme determine the cusp size and tooth size respectively and the cusp number is co-regulated by the tooth size and cusp size. It is also suggested that the mesenchymal cell number regulates not the tooth size but the tooth number. The relationships among these factors in tooth patterning including micropatterning (cusp size and cusp number) and macropatterning (tooth size and tooth number) were analyzed in a reaction diffusion mechanism. Key molecules determining the patterning of teeth remains to be elucidated for controlling the tooth size and cusp size of bioengineered tooth.     

The relationship of dental crowding to tooth size, dental arch width, and arch depth

This study was undertaken to answer the question, "Are there any differences in the mesiodistal tooth diameters, dental arch width and dental arch depth between the crowded and noncrowded dentitions?" The materials submitted for study consisted of the Orthoscan intraoral occlusograms of 89 subjects with well-aligned dentition and those of 74 subjects with gross dental crowding. The tooth size and arch dimensions were measured from these intraoral occlusograms. The data were subjected to statistical treatment for comparison of noncrowded and crowded groups. The findings in this study lead to the following conclusions: The mesiodistal tooth diameters of the crowded group were significantly larger than those of the noncrowded group. The dental arch widths of the crowded group were significantly smaller than those of the noncrowded group. No substantial or significant difference in dental arch depth between the crowded and noncrowded subjects could be noted. From these findings it seems that tooth size and dental arch width are determining factors in the formation of dental crowding. Clinically, these results suggest that in the treatment of dental crowding, extraction of teeth and/or expansion of dental arches should be considered depending on what is the main causative factor.     

Tooth wear and dental pathology of the Bronze-Iron Age people in Xinjiang, Northwest China: Implications for their diet and lifestyle

Tooth wear and dental pathology related to diet and lifestyle were investigated in the human dental remains unearthed from three archaeological sites of Bronze Age and Iron Age in Xinjiang of northwest China, and in comparative samples from two Neolithic sites in Henan and Shanxi in central China along the Yellow River.Our results indicate that the average tooth wear on most tooth types in the three Xinjiang sites was close to those of the Neolithic samples from central China. The variation within the Xinjiang samples was also explored. Some special wear patterns such as severe wear on the first molar and relatively heavy wear of anterior teeth were observed on the specimens from the Xinjiang sites. Obvious differences in caries and antemortem tooth loss were found between Xinjiang and comparative samples with higher frequencies of caries observed in samples from central China and higher antemortem tooth loss in samples from the Xinjiang sites. Strongly developed exostoses (tori) were also identified on mandibles and maxillae of the specimens from Xinjiang.The authors believe that the differences in tooth wear and dental pathology between Xinjiang and central China were caused by differences in diet and lifestyle. Food of a harder texture was consumed by the people who lived in Xinjiang than by the people in Henan and Shanxi of central China. The higher occurrence of heavily worn anterior teeth and some other special wear patterns, antemortem tooth loss and presence of exostoses on jaw bones in Xinjiang suggest that the people in Xinjiang lived in a relatively harsh environment, frequently gnawing hard objects, or using teeth as some kind of tools. All these activities put masticatory organs under a heavy load. The differences in caries frequencies between the frontier and central areas of China indicate that food richer in carbohydrates was consumed by the people in the central areas.It is proposed that about 3000-2000 years BP in many areas of frontier Xinjiang, people mainly relied on the type of hunter-gatherer economy with agriculture playing a smaller role in their lifestyle.     

On the definition of variables in studies of primate dental allometry

Cranial and dental measurements are taken on 253 adult female primates from 32 species. Regression equations are calculated to determine allometric relationships between anterior tooth size, posterior tooth size, and body size. When cranial length or skull length is used as the measure of general size, the results of the equations differ from when body weight is the reference dimension. Similarly, using different definitions of posterior tooth size (such as mandibular second molar length and maxillary postcanine area) alters results substantially. The same occurs with different definitions of anterior tooth size. It has been common in studies of primate dental allometry to generalize from the specific variables measured to broad functional interpretations. However, highly correlated variables cannot be substituted one for another in allometric analyses without important changes in the results of the equation. Interpretation of allometric data is more highly restricted to the precise variables measured in a particular study than has been generally recognized.     

Dental reduction in post-pleistocene Nubia

Tooth size changes among Nubian archaeological populations dating from the Mesolithic through the Christian era, a period of approximately 12,000 years, are analyzed. Standard length and breadth dimensions of all permanent teeth from several cultural horizons are combined to form three large samples: Mesolithic, 10000-70000 B.C.; Agriculturalist, 3300-1100 B.C. (A-group, C-group, Pharaonic); and Intensive Agriculturalist, A.D. 0-1400 (Meroitic, X-Group, Christian). Such information not only fills a void in the knowledge of Nubian skeletal biology, but also provides a much needed African reference point for the comparison of tooth size data among anatomically modern Homo sapiens from various regions of the world. Changes in mean tooth size and associated t-tests reveal strong and significant reduction in dental size between the Mesolithic and Agriculturalist samples, followed by a continued although diminished trend of reduction for only the molar teeth between the two Agriculturalist groups. These patterns are best observed by examining tooth breadths, which are considered as the most reliable indicator of tooth size. Previously published odontometrics of the Nubian Mesolithic are briefly compared to the findings of this diachronic analysis of Nubian dental change.     

Genetic influences on dentition and dental arch dimensions: a study of monozygotic and dizygotic triplets

Previous studies of the tooth and dental arch dimensions in twins have suggested that the size and form of the dentition is more under genetic control than the size and form of the dental arch. A study of dentition and dental arch was carried out on three sets of same-sex triplets, two sets being monozygotic and one set dizygotic. No significant differences were found in the dental arch dimensions in any set of the triplets. The tooth dimensions showed no significant differenes in the monozygotic triplet sets or in the monozygotic pair from the dizygotic triplet set, but showed significant differences between the monozygotic pair and the third member of the dizygotic set. This confirms the data obtained from twin studies.     

Evolution of tooth crown shape in Mesozoic birds,and its adaptive significance with respect to diet

Both the evolution of tooth morphology and the relationship between dental features and diet in toothed birds have long been studied. Here we quantify variation in tooth crown shape in 28 key Mesozoic bird species, and examine differences in dental morphology among birds belonging to different taxonomic groupings and inferred to have had different diets. Using geometric morphometric methods (GMM) and phylogenetic comparative methods (PCM), we found few clear differences in tooth crown shape between different taxonomic and ecological categories, and our analysis provides little support for many dietary inferences drawn in previous studies. However, the Solnhofen Archaeopteryx, Jeholornis, Protopteryx, Pengornis, Longipteryx, Tianyuornis, Mengciusornis, Ichthyornis and Hesperornis all were found to possess relatively specialized tooth crown shapes, perhaps reflecting specialized diets such as insectivory, granivory, piscivory and consumption of soft-shelled arthropods. Similarity in tooth crown shape across many Mesozoic birds may indicate the lack of dietary specialization, and the association between tooth form and diet may have been weakened in any case by ‘functional replacement’ of the dentition by a horny beak and, in many cases, gastroliths.     

Nanoindentation of lemur enamel: an ecological investigation of mechanical property variations within and between sympatric species

The common morphological metrics of size, shape, and enamel thickness of teeth are believed to reflect the functional requirements of a primate's diet. However, the mechanical and material properties of enamel also contribute to tooth function, yet are rarely studied. Substantial wear and tooth loss previously documented in Lemur catta at the Beza Mahafaly Special Reserve suggests that their dental morphology, structure, and possibly their enamel are not adapted for their current fallback food (the mechanically challenging tamarind fruit). In this study, we investigate the nanomechanical properties, mineralization, and microstructure of the enamel of three sympatric lemur species to provide insight into their dietary functional adaptations. Mechanical properties measured by nanoindentation were compared to measurements of mineral content, prism orientation, prism size, and enamel thickness using electron microscopy. Mechanical properties of all species were similar near the enamel dentin junction and variations correlated with changes in microstructure (e.g., prism size) and mineral content. Severe wear and microcracking within L. catta's enamel were associated with up to a 43% reduction in nanomechanical properties in regions of cracking versus intact enamel. The mechanical and material properties of L. catta's enamel are similar to those of sympatric folivores and suggest that they are not uniquely mechanically adapted to consume the physically challenging tamarind fruit. An understanding of the material and mechanical properties of enamel is required to fully elucidate the functional and ecological adaptations of primate teeth.     

The effect of birthweight on tooth-size variability in twins.

Studies indicate that low birthweight (LBW) children display reduced deciduous tooth size but there is little information about permanent tooth size. It has also been shown that dental fluctuating asymmetry (FA) increases in response to various environmental influences, but the relationship between birthweight and FA remains unclear. The aim of this study was to compare tooth size and asymmetry, according to birthweight, in the deciduous and permanent dentitions of a sample of Australian twins. The study sample comprised 436 twins, classified into 2 groups: normal birthweight (NBW > 2500 g) and low birthweight (LBW < or = 2500 g). For each individual it was generally possible to measure maximum mesiodistal crown diameters of both deciduous and permanent central incisors from serial dental models. Correlations were calculated between tooth-size variables and birthweight; subsequently comparisons of tooth size and FA were made between the LBW and NBW samples using Student's t tests. Small positive correlations (around .1) were noted between birthweight and tooth-size variables. There was no evidence of tooth-size reduction in the LBW male sample, but the LBW females displayed tooth-size reduction of approximately 2-3% for both deciduous and permanent incisors, compared to the NBW females. There was no evidence of increased FA in the LBW individuals of either sex. These findings indicate that developing teeth are generally well-protected from developmental disturbances during prenatal and perinatal periods. Further research is needed to clarify the biological basis of an apparently true but weak association between tooth size and birthweight.     

Development of the helicoidal plane

In this study of thebelicoidal occlusal plane the relationships between tooth wear, the transverse slopes of mandibular molars and dental arch breadths were examined in 74 pre-contemporary Australian Aboriginal skulls. With increasing age and tooth wear the orientation of the mandibular occlusal surfaces increased towards the buccal. The differential occlusal orientation from first to third molars, present at eruption, tended to increase progressively with tooth wear. These functionally induced changes, together with regional differences in relative breadths of the maxillary and mandibular dental arches, are important in the development of abelicoidal occlusal plane.     

Metamerism,morphogenesis, and the expression of carabelli and other dental traits in humans

The patterning cascade model of tooth morphogenesis has emerged as a useful tool in explaining how tooth shape develops and how tooth evolution may occur. Enamel knots, specialized areas of dental epithelium where cusps initiate, act as signaling centers that direct the growth of surrounding tissues. For a new cusp to form, an enamel knot must form beyond the inhibition fields of other enamel knots. The model predicts that the number and size of cusps depends on the spacing between enamel knots, reflected in the spacing between cusps. Recently, work by our group demonstrated that the model predicted Carabelli trait expression in human first molars. Here we test whether differences in Carabelli trait expression along the molar row can also be predicted by the model. Crown areas and intercusp distances were measured from dental casts of 316 individuals with a digital microscope. Although absolute cusp spacing is similar in first and second molars, the smaller size and more triangular shape of second molars results in larger cusp spacing relative to size and, likely, less opportunity for the Carabelli trait to form. The presence and size of the hypocone (HY) and a range of small accessory cusps in a larger sample of 340 individuals were also found to covary with the Carabelli trait in a complex way. The results of this study lend further support to the view that the dentition develops, varies, and evolves as a single functional complex. Am J Phys Anthropol, 2013. © 2013 Wiley Periodicals, Inc.     

Inbreeding effects on dental morphometrics in Papio hamadryas

The dentition of nonhuman primates (Papio hamadryas) was utilized to investigate the hypothesis that inbreeding will affect mean tooth size and shape. More than five hundred dental casts were collected from baboons at the Institute of Experimental Pathology and Therapy in Sukhumi, USSR. In addition, inbreeding coefficients for each monkey were obtained from pedigree records, some going back seven to nine generations. Each tooth was measured and scored for the presence of discrete morphological structures. Inbred and outbred groups were differentiated and divided by sex. Statistical analysis shows that for the most part inbred monkeys exhibit larger teeth than outbred monkeys, in both male and female groups. When the tooth area of inbreds was compared to that of outbreds, the differences were significant. However, there were no significant differences in frequency of discrete traits. Therefore, it can be concluded that primate dentition is affected by inbreeding, although dental metrics is a more sensitive indicator than morphology.     

The Bronze Age burials from Cova Dels Blaus (Vall d′Uixó, Castelló, Spain): An approach to palaeodietary reconstruction through dental pathology, occlusal wear and buccal microwear patterns

This paper reports a palaeodietary investigation of the human remains found in the collective Bronze Age burial cave from Vall d′Uixó (Castelló, Spain). Dental pathology, tooth wear as well as buccal dental microwear were analysed. Percentages of dental pathologies were compared with Chalcolithic and Bronze Age sites from the same territory. Dental caries, ante-mortem tooth loss, periodontal disease and abscess frequencies indicate a diet rich in carbohydrate foods. However, dental calculus percentages and macroscopic wear patterns suggest a diet not exclusively relying on agricultural resources. In addition, buccal dental microwear density and length by orientation recorded on micrographs using a scanning electron microscope showed inter-group differences with regard to carnivorous hunter-gatherers and farming populations related to the amount of abrasives in the diet that could correspond to a different dependence on agricultural resources or food preparation technology.     

Dental pathology of prehistoric residents of Oregon

Dentitions of 208 prehistoric skeletal specimens from five geographic regions of Oregon were studied to describe their dental status. Comparisons were made of the incidence of specific pathologies among regions and between jaws. Maxillary premortem tooth losses were significantly higher than mandibular tooth loss. A relatively high caries incidence occurs in samples from the Willamette Valley and Klamath Basin, where plant foods were used aboriginally to a significant extent. A high frequency of abscesses was recorded in the Klamath Basin and the Lower Columbia River sample, which also showed the highest levels of attrition. Each of the five subsamples shows patterns of dental pathology consistent with former use of natural resources.     

Ontogenetic scaling of the morphology and biomechanics of the feeding apparatus in the Pacific hagfish Eptatretus stoutii

The form and function of the support skeleton, musculature and teeth were examined in an ontogenetic series of Pacific hagfish Eptatretus stoutii spanning about a six-fold range in total length (L(T)). Tooth area, feeding apparatus length, basal plate size, theoretical dental plate retractile force, penetration force and applied tooth stress were measured relative to body size. Morphological variables (e.g. tooth area and basal plate size) scaled with positive allometry and functional variables (e.g. retractile force and applied tooth stress) scaled isometrically with L(T). These results suggest that juveniles do not undergo ontogenetic dietary changes and consume functionally equivalent prey to adults, although adults can grasp proportionally larger portions of food. Low tooth stress in juveniles and adults imposes mechanical constraints to puncturing and tearing, which are circumvented by a preference for softer prey tissue or the inclusion of knotting behaviours for reducing tougher prey.