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

Artificial neural networks reconstruct missing perikymata in worn teeth

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.

Distinguishing primate taxa with enamel incremental variables

Enamel has long been of interest for its functional and phylogenetic significance among fossil hominins and other primates. Previous studies demonstrated that enamel incremental features distinguish among hominin fossil taxa, suggesting utility for highlighting taxonomy. However, not all features appear to be useful in mixed samples of fossils, living humans, and apes. Here we tested enamel incremental data from closely related primate taxa to determine which features, if any, distinguish among them. Enamel incremental variables were measured from the M₂ of 40 living primate taxa, and we tested our variables using discriminant function analysis at the taxonomic ranks of parvorder, family, tribe, and genus. We then included enamel incremental data from Australopithecus afarensis, Australopithecus africanus, Paranthropus aethiopicus, Paranthropus boisei, and Paranthropus robustus to determine if these features distinguished fossil taxa from living humans and apes. Our initial results show that enamel incremental variables distinguish among primate taxa, but with low classification rates. Further testing with jackknifing methods shows overlap between groups at all taxonomic ranks, suggesting enamel incremental variables are unreliable for taxonomy. The addition of many common enamel incremental growth variables also resulted in multicollinearity in our multivariate analysis. As the dentition and isolated teeth remain a significant portion of the hominin fossil record, verifying enamel incremental features as a useful taxonomic tool is fundamentally important for hominin paleobiology.

Reconstructing tooth crown heights and enamel caps: A comparative test of three existing methods with recommendations for their use

Studies of enamel growth and thickness, whether in paleoanthropology, bioarchaeology, or primatology, require measurements of crown height (CH), cuspal enamel thickness (CET), average (AET), and/or regional enamel thickness (RegAET) on complete, unworn crowns. Yet because fully unworn crowns are uncommon, three methods to bolster sample sizes by reconstructing slightly worn teeth have been developed: Profile, Polynomial, and Pen Tool. Although these methods have been tested for accuracy, no study has yet directly compared the three methods to assess their performance across CH, CET, AET, and RegAET measurements. Moreover, it is currently unclear how accurate the methods are when reconstructing crowns with varying degrees of wear. The present study addresses this gap in our understanding of how these methods perform on four key dental measurements, evaluates the degree of wear for which accurate crown reconstructions can be completed, and offers recommendations for applying these methods. Here, the methods are compared on Paranthropus robustus mandibular molars, a sample chosen because it exhibits variable morphology, presenting a challenge for reconstruction methods. For minimally worn teeth, Profile, Polynomial, and Pen Tool methods can be employed (in that order) for all measurements except CET, which cannot be reliably measured on reconstructions. For teeth with wear that obliterates the nadir of the occlusal basin or dentin horns, CH and AET can be measured using Profile and Polynomial reconstructions; however, no other measurements or methods were reliable. Recommendations provided here will make it possible to increase sample sizes and replicability, enhancing studies of enamel thickness and growth.

Molar crown formation times of fossil orangutan molars from Guangxi, China

OBJECTIVES

We aimed to investigate molar enamel development in fossil orangutans from Guangxi and shed light on the evolution of Asian great apes.

MATERIALS AND METHODS

We collected 32 fossil orangutan molars, most of which were from Guangxi apothecaries and the Guangxi Daxin Heidong cave, and prepared histological sections of each molar. We then characterized aspects of dental development, including long period line periodicity, number of Retzius lines and lateral enamel formation time, cuspal enamel thickness, and enamel formation time.

RESULTS

The long period line periodicity in fossil orangutans ranged from 9 to 10 days (mean, 9.09 days). The molar lateral enamel formation time ranged from 1.48 to 3.17 years (540-1,152 days). Cuspal enamel thickness in fossil orangutan molars ranged from 949 to 2,535 μm, and cuspal enamel formation time ranged from 0.64 to 1.87 years. Molar enamel formation time of fossil orangutans ranged from 2.47 to 4.67 years.

DISCUSSION

Long-period line periodicity of fossil orangutans from Guangxi was within the variation range of extant orangutans, and the average long period line periodicity (9.09 days) of fossil orangutans from Guangxi in this study was lower than the values for extant orangutans (9.5 days) and fossil orangutans (10.9 days) from Sumatra and Vietnam. Orangutan enamel thickness may have gradually decreased from the Middle Pleistocene to Holocene. Crown formation time of fossil orangutans was slightly longer than that of extant orangutans, and the M1 emergence age of fossil orangutans from Guangxi was about 4-6 years. These findings might indicate the regional difference or evolutionary changes in orangutans since Pleistocene. Dental development of the Guangxi fossil orangutans were more similar to that of Asian Miocene apes, suggesting the closer evolutionary relationship of orangutans to Miocene Asian fossil apes.

Short and long period growth markers of enamel formation distinguish European Pleistocene hominins

Characterizing dental development in fossil hominins is important for distinguishing between them and for establishing where and when the slow overall growth and development of modern humans appeared. Dental development of australopiths and early Homo was faster than modern humans. The Atapuerca fossils (Spain) fill a barely known gap in human evolution, spanning ~1.2 to ~0.4 million years (Ma), during which H. sapiens and Neandertal dental growth characteristics may have developed. We report here perikymata counts, perikymata distributions and periodicities of all teeth belonging to the TE9 level of Sima del Elefante, level TD6.2 of Gran Dolina (H. antecessor) and Sima de los Huesos. We found some components of dental growth in the Atapuerca fossils resembled more recent H. sapiens. Mosaic evolution of perikymata counts and distribution generate three distinct clusters: H. antecessor, Sima de los Huesos and H. sapiens.

An analysis of dental development in Pleistocene Homo using skeletal growth and chronological age

OBJECTIVES

This study takes a new approach to interpreting dental development in Pleistocene Homo in comparison with recent modern humans. As rates of dental development and skeletal growth are correlated given age in modern humans, using age and skeletal growth in tandem yields more accurate dental development estimates. Here, I apply these models to fossil Homo to obtain more individualized predictions and interpretations of their dental development relative to recent modern humans.

MATERIALS AND METHODS

Proportional odds logistic regression models based on three recent modern human samples (N = 181) were used to predict permanent mandibular tooth development scores in five Pleistocene subadults: Homo erectus/ergaster, Neanderthals, and anatomically modern humans (AMHs). Explanatory variables include a skeletal growth indicator (i.e., diaphyseal femoral length), and chronological age.

RESULTS

AMHs Lagar Velho 1 and Qafzeh 10 share delayed incisor development, but exhibit considerable idiosyncratic variation within and across tooth types, relative to each other and to the reference samples. Neanderthals Dederiyeh 1 and Le Moustier 1 exhibit delayed incisor coupled with advanced molar development, but differences are reduced when femoral diaphysis length is considered. Dental development in KNM-WT 15,000 Homo erectus/ergaster, while advanced for his age, almost exactly matches the predictions once femoral length is included in the models.

DISCUSSION

This study provides a new interpretation of dental development in KNM-WT 15000 as primarily reflecting his faster rates of skeletal growth. While the two AMH specimens exhibit considerable individual variation, the Neanderthals exhibit delayed incisor development early and advanced molar development later in ontogeny.

Middle childhood and modern human origins

The evolution of modern human life history has involved substantial changes in the overall length of the subadult period, the introduction of a novel early childhood stage, and many changes in the initiation, termination, and character of the other stages. The fossil record is explored for evidence of this evolutionary process, with a special emphasis on middle childhood, which many argue is equivalent to the juvenile stage of African apes. Although the "juvenile" and "middle childhood" stages appear to be the same from a broad comparative perspective, in that they begin with the eruption of the first molar and the achievement of the majority of adult brain size and end with sexual maturity, the detailed differences in the expression of these two stages, and how they relate to the preceding and following stages, suggest that a distinction should be maintained between them to avoid blurring subtle, but important, differences.

Age-related variability in buccal dental-microwear in Middle and Upper Pleistocene human populations

Infants are thought to present a different buccal microwear pattern than adults and these, therefore, are generally analyzed separately. However, El-Zaatari & Hublin [2009] showed that occlusal texture in Neandertal and modern human juvenile populations did not differ from their elders. The microwear patterns of a sample of 193 teeth, corresponding to 61 individuals ofHomo heidelbergensis, H. neanderthalensisand anatomically modern humans (AMH), were analyzed revealing that AMH infants up to 14 years old differ from older individuals in having fewer scratch densities, whereas the Neandertals have a much more variable microwear pattern. Age-at-death and dental age since emergence showed similar though somewhat diverging results, especially in the infant and subadult samples. Differences observed between the Neandertals and modern humans could be reflecting differential wearing patterns or distinct enamel structure and resistance to hard food items consumption. Interpopulation differences in striation densities were not apparent in either subadult or adult individuals, only adult Neandertals (26-45 yrs. old) showed fewer striations than the younger age groups. The AMH sample revealed a gradual cumulative pattern of striation density with age, suggestive of a non-abrupt change in diet.

Dental evidence for ontogenetic differences between modern humans and Neanderthals

Humans have an unusual life history, with an early weaning age, long childhood, late first reproduction, short interbirth intervals, and long lifespan. In contrast, great apes wean later, reproduce earlier, and have longer intervals between births. Despite 80 y of speculation, the origins of these developmental patterns in Homo sapiens remain unknown. Because they record daily growth during formation, teeth provide important insights, revealing that australopithecines and early Homo had more rapid ontogenies than recent humans. Dental development in later Homo species has been intensely debated, most notably the issue of whether Neanderthals and H. sapiens differ. Here we apply synchrotron virtual histology to a geographically and temporally diverse sample of Middle Paleolithic juveniles, including Neanderthals, to assess tooth formation and calculate age at death from dental microstructure. We find that most Neanderthal tooth crowns grew more rapidly than modern human teeth, resulting in significantly faster dental maturation. In contrast, Middle Paleolithic H. sapiens juveniles show greater similarity to recent humans. These findings are consistent with recent cranial and molecular evidence for subtle developmental differences between Neanderthals and H. sapiens. When compared with earlier hominin taxa, both Neanderthals and H. sapiens have extended the duration of dental development. This period of dental immaturity is particularly prolonged in modern humans.

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.

Evolution of the human pygmy phenotype

Small human body size, or the 'pygmy' phenotype, is characteristic of certain African, Southeast Asian and South American populations. The convergent evolution of this phenotype, and its strong association with tropical rainforests, have motivated adaptive hypotheses that stress the advantages of small size for coping with food limitation, warm, humid conditions and dense forest undergrowth. Most recently, a life-history model has been used to suggest that the human pygmy phenotype is a consequence of early growth cessation that evolved to facilitate early reproductive onset amid conditions of high adult mortality. As we discuss here, these adaptive scenarios are not mutually exclusive and should be evaluated in consort. Findings from this area of research are expected to inform interpretations of diversity in the hominin fossil record, including the purported small-bodied species Homo floresiensis.

Variation in modern human premolar enamel formation times: implications for Neandertals

A recent study demonstrated that variation in enamel cap crown formation in the anterior teeth is greater than that in the molars from two geographically distinct populations: native indigenous southern Africans and northern Europeans. Eighty southern African and 69 northern European premolars (P3 and P4) were analyzed in the present study. Cuspal, lateral, and total enamel formation times were assessed. Although cuspal enamel formation times were not consistently different between the two populations, both lateral and total enamel formation times generally were. Bonferroni-corrected t-tests showed that southern Africans had significantly shorter lateral enamel formation time for five of the six cusps, as well as significantly shorter total enamel formation time for these same cusps. An analysis of covariance performed on the lingual cusps of the upper third and fourth premolars showed that differences in enamel formation times between these populations remained when crown height was statistically controlled. A further goal of this study was to ascertain, based on perikymata counts, what Neandertal periodicities would have to be in order for their teeth to have lateral enamel formation times equivalent to either southern Africans or northern Europeans. To this end, perikymata were counted on 32 Neandertal premolars, and the counts were inserted into regression formulae relating perikymata counts to periodicity for each population and each tooth type. Neandertal enamel formation times could be equivalent to those of southern Africans or northern Europeans only if their hypothetical periodicities fall within the range of periodicities for African apes and modern humans (i.e., 6-12 days). The analysis revealed that both populations could encompass Neandertal timings, with hypothetical periodicities based on the southern African population necessitating a lower range of periodicity (6-8 days) than those based on the northern European population (8-11 days).

Incremental dental development: methods and applications in hominoid evolutionary studies

This survey of dental microstructure studies reviews recent methods used to quantify developmental variables (daily secretion rate, periodicity of long-period lines, extension rate, formation time) and applications to the study of hominoid evolution. While requisite preparative and analytical methods are time consuming, benefits include more precise identification of tooth crown initiation and completion than conventional radiographic approaches. Furthermore, incremental features facilitate highly accurate estimates of the speed and duration of crown and root formation, stress experienced during development (including birth), and age at death. These approaches have provided insight into fossil hominin and Miocene hominoid life histories, and have also been applied to ontogenetic and taxonomic studies of fossil apes and humans. It is shown here that, due to the rapidly evolving nature of dental microstructure studies, numerous methods have been applied over the past few decades to characterize the rate and duration of dental development. Yet, it is often unclear whether data derived from different methods are comparable or which methods are the most accurate. Areas for future research are identified, including the need for validation and standardization of certain methods, and new methods for integrating nondestructive structural and developmental studies are highlighted.