Journal Article

Artificial neural networks reconstruct missing perikymata in worn teeth

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Dental evolutionary studies in hominins are key to understanding how our ancestors and close fossil relatives grew from the early stages of embryogenesis into adults. In a sense, teeth are like an airplane’s ‘black box’ as they record important variables for assessing developmental timing, enabling comparisons within and between populations, species, and genera. The ability to discern this type of nuanced information is embedded in the nature of how tooth enamel and dentin form: incrementally and over years. This incremental growth leaves chronological indicators in the histological structure of enamel, visible on the crown surface as perikymata. These structures are used in the process of reconstructing the rate and timing of tooth formation. Unfortunately, the developmentally earliest growth lines in lateral enamel are quickly lost to wear once the tooth crown erupts. We developed a method to reconstruct these earliest, missing perilymata from worn teeth through knowledge of the later-developed, visible perikymata for all tooth types (incisors, canines, premolars, and molars) using a modern human dataset. Building on our previous research using polynomial regressions, here we describe an artificial neural networks (ANN) method. This new ANN method mostly predicts within 2 counts the number of perikymata present in each of the first three deciles of the crown height for all tooth types. Our ANN method for estimating perikymata lost through wear has two immediate benefits: more accurate values can be produced and worn teeth can be included in dental research. This tool is available on the open-source platform R within the package teethR released under GPL v3.0 license, enabling other researchers the opportunity to expand their datasets for studies of periodicity in histological growth, dental development, and evolution.

Dental remains of the Middle Pleistocene hominins from the Sima de los Huesos site (Sierra de Atapuerca, Spain): Mandibular dentition

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The Middle Pleistocene site of the Sima de los Huesos (Sierra de Atapuerca, northern Spain) has yielded a considerable number of human fossils during the period 1984–2020. Among them, up to 314 mandibular teeth have been identified. In this second paper dedicated to the dentition we present the description of the eight dental classes of the mandible following the Arizona State University Dental Anthropology System (ASUDAS) classification. In addition, we show the mean mesiodistal and buccolingual diameters obtained in these teeth compared to those of Neanderthals and a modern human sample. The morphology of both the anterior and posterior teeth suggests a close relationship of the Sima de los Huesos hominins with the populations of the second half of the Middle Pleistocene of Europe and the Near East, as well as with the so-called classic Neanderthals of Europe. The combination of dental traits in these populations is characteristic and diagnostic and suggests grouping the Sima de los Huesos hominins with the other paleodemes in a Neanderthal clade. The dental evidence of the Sima de los Huesos hominins is key to propose a complex model for the settlement of Europe during the Middle Pleistocene. In this period, different migrations of human groups probably coming from Southwest Asia, replacements, prolonged isolations, as well as hybridization and introgression processes would have contributed to the diversity of hominins in Europe.

Dental remains of the Middle Pleistocene hominins from the Sima de los Huesos site (Sierra de Atapuerca, Spain): Maxillary dentition

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The Middle Pleistocene site of the Sima de los Huesos (Sierra de Atapuerca, northern Spain) has yielded a considerable number of human fossils during the period 1984–2020. Among them, up to 253 maxillary teeth have been recovered. In this article, we present the description of the eight dental classes of the maxilla following the Arizona State University Dental Anthropology System classification. In addition, we present the mean mesiodistal and buccolingual diameters of these teeth compared to those of Neanderthals and a modern human sample. The morphology of both the anterior and posterior teeth suggests a close relationship of the Sima de los Huesos hominins with the populations of the second half of the Middle Pleistocene of Europe and the Near East, as well as with the so-called classic Neanderthals of Europe. Features with a recognizable taxonomic signal allow grouping the Sima de los Huesos hominins with different paleodemes into a Neanderthal clade. The dental evidence of the Sima de los Huesos hominins is key to suggest a complex model for the settlement of Europe during the Middle Pleistocene. During this period, different migrations of human groups probably coming from Southwest Asia, replacements, prolonged isolations, as well as hybridization and introgression processes would have contributed to the diversity of hominins in Europe.

Similarities and differences in the dental tissue proportions of the deciduous and permanent canines of Early and Middle Pleistocene human populations

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The two- and three-dimensional assessment of dental tissues has become routine in human taxonomic studies throughout the years. Nonetheless, most of our knowledge of the variability of the enamel and dentine dimensions of the human evolutionary lineage comes from the study of permanent dentition, and particularly from molars. This leads to a biased view of the variability of these features. Due to their early formation and rapid development, the deciduous teeth allow more simplified inferences regarding the processes involved in the dental tissue development of each group. Therefore, their study could be very valuable in dental palaeohistology. In this research, we have explored the dental tissue proportions of the deciduous canines belonging to some human samples of the Early and Middle Pleistocene. The purpose of this was to discuss the meaning of the similarities and differences observed in their histological pattern, as well as to evaluate the degree of covariance with that observed in the permanent dentition of these populations. Our results show that, although there are some similarities in the dental tissue proportions between the deciduous and permanent canines of the study samples, the two dental classes do not provide a similar or comparable pictures of the dental tissue pattern present in the dentition of fossil hominins. Future works on the dental tissue patterns of the anterior and posterior dentition, including deciduous teeth, of fossil samples, may help to shed light on this hypothesis.

The Ratón Pérez collection: Modern deciduous human teeth at the Centro Nacional de Investigación sobre la Evolución Humana (Burgos, Spain)

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Objectives The aim of this report is to present the large deciduous tooth collection of identified children that is housed at the National Research Center on Human Evolution (CENIEH) in Burgos, Spain. Methods Yearly, members of the Dental Anthropology Group of the CENIEH are in charge of collecting the teeth and registering all the relevant information from the donors at the time of collection. In compliance with Spanish Law 14/2007 of July 3, 2007, on Biomedical Research (BOE-A-2007-12945), all individuals are guaranteed anonymity and confidentiality. When the donor hands in the tooth, they fill out a Donor Information Form and sign the Informed Consent Form. At the same time, another person completes the data label for the transparent polyethylene zip lock bag where the tooth is temporarily stored. All teeth are then transferred to the CENIEH Restoration lab, where the specialists apply the same protocol as for the fossil remains. Results Although the sample is still growing, from the first collection campaign in 2014 to date it comprises 2977 teeth of children whose ages of tooth loss are between 2 and 15 years. Each tooth is associated with basic information of the individuals and their parents and grandparents (sex, date, and place of birth, ancestry, country of residence), as well as important data about early life history (pregnancy duration, breastfeeding, bottle-feeding) and other relevant information provided by the donors (such as if they are twins, dental loss, or dental extraction). Conclusions Due to the scarcity of deciduous dental samples available, the Ratón Pérez collection represents a highly valuable sample for a wide range of disciplines such as forensic, dental, and anthropological fields among others.

Sexual dimorphism in the enamel-dentine junction (EDJ) of permanent canines of European modern humans

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Objectives Dental anthropological investigations into sexual dimorphism have conventionally concentrated on evaluating the dimensions and configuration of the enamel cap of canines. However, the morphology of the crown dentine surface can be closely linked to that of the enamel surface. This link can facilitate examination of crown morphology even when the enamel surface is slightly worn. Here, we determine if the morphology of the enamel-dentine junction (EDJ) differs within (maxillary vs. mandibular) and between a sample of male (n = 26) and female (n = 21) contemporary human permanent canines from Europe. Methods The morphological data of the EDJ were gathered employing a template comprising 96 landmarks and sliding semilandmarks. Subsequently, the data underwent analysis through form space principal component analysis following Procrustes registration, utilizing standard 3D geometric morphometric techniques. Results Significant differences in the morphology of the EDJ were observed between the sexes, particularly concerning the overall shape of the crown, the symmetry of the mesial and distal edges, and the development of the distal accessory ridge. Conclusions Sex differences in the morphology of the EDJ could relate in part to retention of the canine-premolar honing complex in males. Our results indicate that analyses of the permanent canine EDJ may potentially provide a novel method for estimating the sex of adult and nonadult skeletons.

Severe Enamel Defects in Wild Japanese Macaques

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Plane-form enamel hypoplasia (PFEH) is a severe dental defect in which large areas of the crown are devoid of enamel. This condition is rare in humans and even rarer in wild primates. The etiology of PFEH has been linked to exposure to severe disease, malnutrition, and environmental toxins and associated with systemic conditions. In this study, we examined the prevalence of enamel hypoplasia in several populations of wild Japanese macaques (Macaca fuscata) with the aim of providing context for severe defects observed in macaques from Yakushima Island. We found that 10 of 21 individuals (48%) from Yakushima Island displayed uniform and significant PFEH; all 10 specimens were from two adjacent locations in the south of the island. In contrast, macaques from other islands and from mainland Japan have a low prevalence of the more common types of enamel hypoplasia and none exhibit PFEH. In Yakushima macaques, every tooth type was affected to varying degrees except for first molars and primary teeth, and the mineral content of the remaining enamel in teeth with PFEH was normal (i.e., no hypo- or hypermineralization). The aetiology of PFEH might be linked to extreme weather events or high rates of environmental fluoride-causing enamel breakdown. However, given that the affected individuals underwent dental development during a period of substantial human-related habitat change, an anthropogenic-related etiology seems most likely. Further research on living primate populations is needed to better understand the causes of PFEH in wild primates.

Inter- and intrapopulation variability of dental tissue proportions of European and African modern human populations’ permanent canines

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Numerous studies have shown that human dentition traits vary both between and within populations. However, there is still little knowledge about how dental tissue proportions differ between modern human groups. In this study, two samples of European and African individuals were compared to assess the possible differences and similarities present in the dental tissue dimensions of their permanent canines. For this purpose, the volumes and surface areas of the coronal dentine and pulp complex and the enamel cap of 127 canines were measured by microcomputed tomography. The results show the existence of interpopulation variability in the dental tissue pattern of both samples, which is mainly due to the presence of a larger enamel component in the African population, while dentine seems to play a less critical role in the differences described between both dental samples. We also observed a similar pattern of sexual dimorphism in the dental tissue proportions of European and African canines, but in this case, the intrapopulation variability was mainly due to the presence of a greater dentine component in males. Therefore, because the dimensions of dental tissues vary at both inter- and intrapopulation levels in modern human groups, our results highlight the importance of selecting comparative samples that are geographically mixed and sex-balanced for future paleoanthropological investigations on dental tissue patterns of extinct and extant species to avoid overestimating or underestimating any possible similarities or differences.

Sexual dimorphism of deciduous canine dental tissues dimensions of modern human populations

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The dental tissue proportions of human permanent canines are one of only a few sexually dimorphic features that are present in childhood, and therefore offer the opportunity to estimate the sex of immature individuals. This work aims to evaluate for the first time the degree of sexual dimorphism ‍in the three-dimensional (3D) measurements of deciduous canine dental tissues, to assess their potential in sexual assessment. Computed microtomographic techniques have been employed to anal‍yse the maxillary and mandibular deciduous canines of 65 individuals (36 females and 29 males) of known sex and age. The teeth were scanned and the volumes and 3D surface areas of the enamel cup and the dentine–pulp complex were obtained. Our results did not show statistically significant differences in either the absolute or relative dimensions of the enamel and dentine between female and male teeth. We hence conclude that volumes and 3D surface areas of deciduous canine dental tissues do not allow for sex determination, which contrasts with what has been observed in permanent canines by other authors. The differences in the degree of sexual dimorphism in dental tissue proportions be‍tween permanent and deciduous canines seem to be due to a decrease in the intersexual variability of ‍the dentine component dimensions. Since the dentine component is a tissue capable of responding to ‍changes in sex hormone concentration levels, our results might indicate that hormones play a more important role in the development of sexual dimorphism in the permanent dentition than previously thought.