Hominid enamel thickness: I. The Krapina Neandertals

Dentine morphology of Atapuerca-Sima de los Huesos lower molars: Evolutionary implications through three-dimensional geometric morphometric analysis

OBJECTIVES

This study aims to explore the affinities of the Sima de los Huesos (SH) population in relation to Homo neanderthalensis, Arago, and early and contemporary Homo sapiens. By characterizing SH intra-population variation, we test current models to explain the Neanderthal origins.

MATERIALS AND METHODS

Three-dimensional reconstructions of dentine surfaces of lower first and second molars were produced by micro-computed tomography. Landmarks and sliding semilandmarks were subjected to generalized Procrustes analysis and principal components analysis.

RESULTS

SH is often similar in shape to Neanderthals, and both groups are generally discernible from Homo sapiens. For example, the crown height of SH and Neanderthals is lower than for modern humans. Differences in the presence of a mid-trigonid crest are also observed, with contemporary Homo sapiens usually lacking this feature. Although SH and Neanderthals show strong affinities, they can be discriminated based on certain traits. SH individuals are characterized by a lower intra-population variability, and show a derived dental reduction in lower second molars compared to Neanderthals. SH also differs in morphological features from specimens that are often classified as Homo heidelbergensis, such as a lower crown height and less pronounced mid-trigonid crest in the Arago fossils.

DISCUSSION

Our results are compatible with the idea that multiple evolutionary lineages or populations coexisted in Europe during the Middle Pleistocene, with the SH paradigm phylogenetically closer to Homo neanderthalensis. Further research could support the possibility of SH as a separate taxon. Alternatively, SH could be a subspecies of Neanderthals, with the variability of this clade being remarkably higher than previously thought.

Enamel thickness variation of deciduous first and second upper molars in modern humans and Neanderthals

Enamel thickness and dental tissue proportions have been recognized as effective taxonomic discriminators between Neanderthal and modern humans teeth. However, most of the research on this topic focused on permanent teeth, and little information is available for the deciduous dentition. Moreover, although worn teeth are more frequently found than unworn teeth, published data for worn teeth are scarce and methods for the assessment of their enamel thickness need to be developed. Here, we addressed this issue by studying the 2D average enamel thickness (AET) and 2D relative enamel thickness (RET) of Neanderthal and modern humans unworn to moderately worn upper first deciduous molars (dm(1)s) and upper second deciduous molars (dm(2)s). In particular, we used 3D μCT data to investigate the mesial section for dm(1)s and both mesial and buccal sections for dm(2)s. Our results confirmed previous findings of an Neanderthal derived condition of thin enamel, and thinner enamel in dm(1)s than dm(2)s in both Neanderthal and modern humans. We demonstrated that the Neanderthal 2D RET indices are significantly lower than those of modern humans at similar wear stages in both dm(1)s and dm(2)s (p < 0.05). The discriminant analysis showed that using 2D RET from dm(1) and dm(2) sections at different wear stages up to 93% of the individuals are correctly classified. Moreover, we showed that the dm(2) buccal sections, although non-conventionally used, might have an advantage on mesial sections since they distinguish as well as mesial sections but tend to be less worn. Therefore, the 2D analysis of enamel thickness is suggested as a means for taxonomic discrimination between modern humans and Neanderthal unworn to moderately worn upper deciduous molars.

Variation in enamel thickness within the genus Homo

Recent humans and their fossil relatives are classified as having thick molar enamel, one of very few dental traits that distinguish hominins from living African apes. However, little is known about enamel thickness in the earliest members of the genus Homo, and recent studies of later Homo report considerable intra- and inter-specific variation. In order to assess taxonomic, geographic, and temporal trends in enamel thickness, we applied micro-computed tomographic imaging to 150 fossil Homo teeth spanning two million years. Early Homo postcanine teeth from Africa and Asia show highly variable average and relative enamel thickness (AET and RET) values. Three molars from South Africa exceed Homo AET and RET ranges, resembling the hyper thick Paranthropus condition. Most later Homo groups (archaic European and north African Homo, and fossil and recent Homo sapiens) possess absolutely and relatively thick enamel across the entire dentition. In contrast, Neanderthals show relatively thin enamel in their incisors, canines, premolars, and molars, although incisor AET values are similar to H. sapiens. Comparisons of recent and fossil H. sapiens reveal that dental size reduction has led to a disproportionate decrease in coronal dentine compared with enamel (although both are reduced), leading to relatively thicker enamel in recent humans. General characterizations of hominins as having 'thick enamel' thus oversimplify a surprisingly variable craniodental trait with limited taxonomic utility within a genus. Moreover, estimates of dental attrition rates employed in paleodemographic reconstruction may be biased when this variation is not considered. Additional research is necessary to reconstruct hominin dietary ecology since thick enamel is not a prerequisite for hard-object feeding, and it is present in most later Homo species despite advances in technology and food processing.

Dental maturational sequence and dental tissue proportions in the early Upper Paleolithic child from Abrigo do Lagar Velho, Portugal

Neandertals differ from recent and terminal Pleistocene human populations in their patterns of dental development, endostructural (internal structure) organization, and relative tissue proportions. Although significant changes in craniofacial and postcranial morphology have been found between the Middle Paleolithic and earlier Upper Paleolithic modern humans of western Eurasia and the terminal Pleistocene and Holocene inhabitants of the same region, most studies of dental maturation and structural morphology have compared Neandertals only to later Holocene humans. To assess whether earlier modern humans contrasted with later modern populations and possibly approached the Neandertal pattern, we used high-resolution microtomography to analyze the remarkably complete mixed dentition of the early Upper Paleolithic (Gravettian) child from Abrigo do Lagar Velho, Portugal, and compared it to a Neandertal sample, the late Upper Paleolithic (Magdalenian) child of La Madeleine, and a worldwide extant human sample. Some aspects of the dental maturational pattern and tooth endostructural organization of Lagar Velho 1 are absent from extant populations and the Magdalenian specimen and are currently documented only among Neandertals. Therefore, a simple Neandertal versus modern human dichotomy is inadequate to accommodate the morphostructural and developmental variation represented by Middle Paleolithic and earlier Upper Paleolithic populations. These data reinforce the complex nature of Neandertal-modern human similarities and differences, and document ongoing human evolution after the global establishment of modern human morphology.

Brief communication: dental development and enamel thickness in the Lakonis Neanderthal molar

Developmental and structural affinities between modern human and Neanderthal dental remains continue to be a subject of debate as well as their utility for informing assessments of life history and taxonomy. Excavation of the Middle Paleolithic cave site Lakonis in southern Greece has yielded a lower third molar (LKH 1). Here, we detail the crown development and enamel thickness of the distal cusps of the LKH 1 specimen, which has been classified as a Neanderthal based on the presence of an anterior fovea and mid-trigonid crest. Crown formation was determined using standard histological techniques, and enamel thickness was measured from a virtual plane of section. Developmental differences include thinner cuspal enamel and a lower periodicity than modern humans. Crown formation in the LKH 1 hypoconid is estimated to be 2.6-2.7 years, which is shorter than modern human times. The LKH 1 hypoconid also shows a more rapid overall crown extension rate than modern humans. Relative enamel thickness was approximately half that of a modern human sample mean; enamel on the distal cusps of modern human third molars is extremely thick in absolute and relative terms. These findings are consistent with recent studies that demonstrate differences in crown development, tissue proportions, and enamel thickness between Neanderthals and modern humans. Although overlap in some developmental variables may be found, the results of this and other studies suggest that Neanderthal molars formed in shorter periods of time than modern humans, due in part to thinner enamel and faster crown extension rates.

What molars contribute to an emerging understanding of lateral enamel formation in Neandertals vs. modern humans

Two hypotheses, based on previous work on Neandertal anterior and premolar teeth, are investigated here: (1) that estimated molar lateral enamel formation times in Neandertals are likely to fall within the range of modern human population variation, and (2) that perikymata (lateral enamel growth increments) are distributed across cervical and occlusal halves of the crown differently in Neandertals than they are in modern humans. To investigate these hypotheses, total perikymata numbers and the distribution of perikymata across deciles of crown height were compared for Neandertal, northern European, and southern African upper molar mesiobuccal (mb) cusps, lower molar mesiobuccal cusps, and the lower first molar distobuccal (db) cusp. Sample sizes range from five (Neandertal M(1)db) to 29 (southern African M(1)mb). Neandertal mean perikymata numbers were found to differ significantly from those of both modern human samples (with the Neandertal mean higher) only for the M(2)mb. Regression analysis suggests that, with the exception of the M(2)mb, the hypothesis of equivalence between Neandertal and modern human lateral enamel formation time cannot be rejected. For the M(2)mb, regression analysis strongly suggests that this cusp took longer to form in the Neandertal sample than it did in the southern African sample. Plots of perikymata numbers across deciles of crown height demonstrate that Neandertal perikymata are distributed more evenly across the cervical and occlusal halves of molar crowns than they are in the modern human samples. These results are integrated into a discussion of Neandertal and modern human lateral enamel formation across the dentition, with reference to issues of life history and enamel growth processes.

The Neanderthalian molar from Hunas, Germany

In this paper, we present a well-preserved isolated human molar found in 1986 in the Hunas cave ruin, south-east Bavaria. The tooth was located at the bottom of layer F2, which belongs to a long stratigraphic sequence comprising faunal remains as well as archaeological levels (Mousterian). A stalagmite from layer P at the base of the stratigraphic sequence was recently dated to 79.373+/-8.237 ka (base) and 76.872+/-9.686 ka (tip) by TIMS-U/Th (Stanford University). We identified the tooth as a right (possibly third) mandibular molar. Characteristic parameters such as crown and root morphology, fissure pattern, enamel thickness, occlusal and interproximal wear, dental dimensions and indices, and radiological features indicate that the Hunas molar represents the tooth of a Neanderthal. This is corroborated by both the palaeontological and archaeological findings (Mousterian) of layer F2.

Assessment of the accuracy of dental enamel thickness measurements using microfocal X-ray computed tomography

Tooth enamel thickness has long been an important character in studies of primate and especially hominin phylogeny, taxonomy, and adaptation. Current methods for accurately assessing enamel thickness involve the physical sectioning of teeth, because measurements of enamel thickness using some radiographic techniques are unreliable. However, because destructive methods limit sample sizes and access to important fossil specimens, it is desirable that they be replaced with nondestructive techniques. Although microfocal X-ray computed tomography (mCT) has been used recently in studies of enamel thickness, the accuracy of this technique has yet to be established. The present research compares physical sections to computer-generated mCT sections of teeth from a variety of primate and nonprimate, recent and fossil taxa to examine whether enamel thickness, tooth size, and diagenetic remineralization (fossilization) impact the ability of mCT to measure enamel thickness accurately. Results indicate that recent teeth of varying size and thickness are clearly and accurately depicted in mCT scans, with measurements from nearly identical planes in physical and mCT sections differing by 3-5%. A fossil papionin molar (ca. 2 Myr) was also accurately measured using mCT scans, although thinner enamel in much older therapsid (ca. 263-241 Myr) teeth could not be distinguished from dentine. mCT is thus an accurate technique for measuring enamel thickness in recent taxa, although heavily mineralized teeth pose an obstacle to the ability of mCT to distinguish dental tissues. Moreover, absolutely thin enamel (less than approximately 0.10 mm) is difficult to resolve adequately in raw mCT images based on pixel values alone. Therefore, caution must be exercised in the application of mCT to the study of fossilized teeth.

Three-dimensional reconstruction of enamel thickness and volume in humans and hominoids

Enamel thickness is an important diagnostic characteristic in Hominoidea. However, the sample size is extremely small, relying upon mostly fractured specimens and a few sectioned specimens, providing an estimate of enamel thickness only. What is needed to fully understand the significance of enamel thickness is a non-destructive technique that is able to obtain the thickness, density, and volume of the dental hard tissues of large samples, thereby providing an accurate means of relating thickness, area, volume, and the pattern of distribution of both enamel and dentin. Investigators have attempted to circumvent this problem by developing a variety of indexes. However, we are still left with subjective descriptions, such as 'thin', 'thick', 'intermediate thick', and 'hyperthick'. The purpose of this investigation was therefore to demonstrate the ability of high-resolution X-ray computed microtomography (HRXCT), as a non-destructive method, to produce, accurately and reliably, contiguous slices revealing the thickness and area of enamel, dentin, and pulp chamber. Using imaging software, three-dimensional reconstructions were produced, which provided volume data for enamel and dentin. Three-dimensional reconstruction of HRXCT images provide, for the first time, the capability of accurately quantifying enamel and dentin thickness, distribution and volume, thereby eliminating the necessity of destructive thin-sectional analysis.

Sexual dimorphism in modern human permanent teeth

On average, males possess larger tooth crowns than females in contemporary human populations, although the degree of dimorphism varies within different populations. In previous studies, different amounts of either enamel or dentine were implicated as the cause of this dimorphism. In this study, we attempt to determine the nature of sexual dimorphism in the crowns of permanent modern human teeth and to determine if two contrasting tooth types (permanent third molars and canines) show identical patterns of dimorphism in enamel and dentine distribution. We estimated the relative contributions of both enamel and dentine to total crown size, from buccolingual sections of teeth. Our sample consisted of a total of 144 mandibular permanent third molars and 25 permanent mandibular canines of known sex. We show that sexual dimorphism is likely due, in part, to the presence of relatively more dentine in the crowns of male teeth. However, whatever the underlying cause, dimorphism in both tooth root and tooth crown size should produce measurable dimorphism in tooth weight, though this has not been previously explored. Therefore, we provide some preliminary data that indicate the usefulness of wet tooth weight as a measure of sexual dimorphism. Both male permanent third molars and canines are significantly heavier than those of females. The weight dimorphism reported here for both classes of teeth may prove a useful finding for future forensic studies. In particular, weights of canines may be more useful as a means of sexing modern human skeletal material than linear or area measurements of teeth.

Enamel thickness of deciduous and permanent molars in modern Homo sapiens

This study presents data on the enamel thickness of deciduous (dm2) and permanent (M1-M3) molars for a geographically diverse sample of modern humans. Measurements were recorded from sections through the mesial cusps of unworn teeth. Enamel is significantly thinner on deciduous than on permanent molars, and there is a distinct trend for enamel to increase in relative thickness from M1 to M3. The relatively thicker enamel of M2s and especially M3s can be related to the overall reduction in size of more distal molar crowns, which has been attained through a differential loss of the dentine component. Enamel tends to be thicker on the protocone than on the paracone, and thicker on the protoconid than on the metaconid, but its distribution is not wholly concordant with models that predict increased thickness as a means by which to counter heavier attritional loss on these "functional" cusps. Indeed, the thickness of enamel tends to be more variable on cusp tips and occlusal surfaces than over the lateral aspects of cusps. The proportionately thicker enamel over the lateral aspects of the protocone and protoconid more likely serves as a means to prolong functional crown life by preventing cusp fracture, rather than being an adaptation to increase the attritional longevity of wear facets. The present data suggest that the human dentition is not predisposed to develop a helicoidal wear plane through the disposition of molar enamel thickness.

Genetics and the evolution of primate enamel thickness: A baboon model

The thickness of mammalian tooth enamel plays a prominent role in paleontology because it correlates with diet, and thicker enamel protects against tooth breakage and wear. Hominid evolutionary studies have stressed the importance of this character for over 30 years, from the identification of "Ramapithecus" as an early Miocene hominid, to the recent discovery that the earliest hominids display molar enamel intermediate in thickness between extant chimpanzees and Australopithecus. Enamel thickness remains largely unexplored for nonhominoid primate fossils, though there is significant variation across modern species. Despite the importance of enamel thickness variation to primate evolution, the mechanisms underlying variation in this trait have not yet been elucidated. We report here on the first quantitative genetic analysis of primate enamel thickness, an analysis based on 506 pedigreed baboons from a captive breeding colony. Computed tomography analysis of 44 Papio mandibular molars shows a zone of sufficiently uniform enamel thickness on the lateral surface of the protoconid. With this knowledge, we developed a caliper metric measurement protocol for use on baboon molars worn to within this zone, enabling the collection of a data set large enough for genetic analyses. Quantitative genetic analyses show that a significant portion of the phenotypic variance in enamel thickness is due to the additive effects of genes and is independent of sex and tooth size. Our models predict that enamel thickness could rapidly track dietary adaptive shifts through geological time, thus increasing the potential for homoplasy in this character. These results have implications for analyses of hominoid enamel thickness variation, and provide a foundation from which to explore the evolution of this phenotype in the papionin fossil record.

An evaluation of dental radiograph accuracy in the measurement of enamel thickness

Many studies have employed lateral radiographs to measure the thickness of tooth enamel in recent human and fossil hominid samples, but the accuracy of measurements obtained by this technique has not been assessed. In this study, 20 isolated human maxillary permanent molars were radiographed using the parallel film technique. The crowns were then sectioned longitudinally through the tips of the buccal cusps. Measurements of enamel cap area, and of linear enamel thickness in the occlusal basin and over the metacone apex, were made from the radiographs and corresponding sectioned surfaces. Comparisons of the two sets of values revealed that radiographs generally overestimated enamel thickness but there was considerable variability in the error by which measurements from radiographs either under- or overestimated the true value. Lateral radiographs may provide a rough visual impression of whether a tooth has thin or thick enamel but they do not generally provide for accurate measurement of enamel thickness. Quantitative data on enamel thickness from studies that have employed lateral radiographs should be viewed with circumspection.

A radiographic assessment of enamel thickness in human maxillary incisors

Crown sizes of human teeth are sexually dimorphic, with male larger than female. This holds for most human groups, though the extent of dimorphism varies among populations. It is not known whether size dimorphism is due to differences in enamel thickness, dentine differences, or some combination of the two. This study examined the pattern of variation in enamel thickness on the mesial and distal margins of the four maxillary permanent incisors. Standardized periapical radiographs of the incisors of 115 adolescent American whites were measured. Enamel was significantly thicker on the distal than the mesial margins of both the lateral and central incisors, with a mean difference of 0.1 mm. There was no sexual dimorphism in the maximum mesial or distal enamel thicknesses. In contrast, the widths of the dentine of the crowns were significantly greater in males, by an average of 6.5%. Sexual dimorphism in mesiodistal diameters of the incisors seems, then, to be due to the dentine component, which is the size attained at the end of the bell stage of tooth formation. Sex-specific correlations between enamel thickness and crown width of the dentine were low (and lower for males), indicating considerable independence between regulatory mechanisms of dentine and enamel development.