Fine-scaled climate variation in equatorial Africa revealed by modern and fossil primate teeth.

Variability in resource availability is hypothesized to be a significant driver of primate adaptation and evolution, but most paleoclimate proxies cannot recover environmental seasonality on the scale of an individual lifespan. Oxygen isotope compositions (δ18O values) sampled at high spatial resolution in the dentitions of modern African primates (n = 2,352 near weekly measurements from 26 teeth) track concurrent seasonal precipitation, regional climatic patterns, discrete meteorological events, and niche partitioning. We leverage these data to contextualize the first δ18O values of two 17 Ma Afropithecus turkanensis individuals from Kalodirr, Kenya, from which we infer variably bimodal wet seasons, supported by rainfall reconstructions in a global Earth system model. Afropithecus' δ18O fluctuations are intermediate in magnitude between those measured at high resolution in baboons (Papio spp.) living across a gradient of aridity and modern forest-dwelling chimpanzees (Pan troglodytes verus). This large-bodied Miocene ape consumed seasonally variable food and water sources enriched in 18O compared to contemporaneous terrestrial fauna (n = 66 fossil specimens). Reliance on fallback foods during documented dry seasons potentially contributed to novel dental features long considered adaptations to hard-object feeding. Developmentally informed microsampling recovers greater ecological complexity than conventional isotope sampling; the two Miocene apes (n = 248 near weekly measurements) evince as great a range of seasonal δ18O variation as more time-averaged bulk measurements from 101 eastern African Plio-Pleistocene hominins and 42 papionins spanning 4 million y. These results reveal unprecedented environmental histories in primate teeth and suggest a framework for evaluating climate change and primate paleoecology throughout the Cenozoic.

Teeth reveal juvenile diet, health and neurotoxicant exposure retrospectively: What biological rhythms and chemical records tell us.

Integrated developmental and elemental information in teeth provide a unique framework for documenting breastfeeding histories, physiological disruptions, and neurotoxicant exposure in humans and our primate relatives, including ancient hominins. Here we detail our method for detecting the consumption of mothers' milk and exploring health history through the use of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) mapping of sectioned nonhuman primate teeth. Calcium-normalized barium and lead concentrations in tooth enamel and dentine may reflect milk and formula consumption with minimal modification during subsequent tooth mineralization, particularly in dentine. However, skeletal resorption during severe illness, and bioavailable metals in nonmilk foods, can complicate interpretations of nursing behavior. We show that explorations of the patterning of multiple elements may aid in the distinction of these important etiologies. Targeted studies of skeletal chemistry, gastrointestinal maturation, and the dietary bioavailability of metals are needed to optimize these unique records of human health and behavior.

Telemedicine for Cranial Radiosurgery Patients in a Rural U.S. Population: Patterns and Predictors of Patient Utilization.

Introduction: Telemedicine retains potential for increasing access to specialty providers in underserved and rural communities. COVID-19 accelerated adoption of telehealth beyond rural populations, serving as a primary modality of patient-provider encounters for many nonemergent diagnoses. Methods: From 2020 to 2021, telemedicine was incorporated in management of stereotactic radiosurgery patients. Retrospective data on diagnoses, demographics, distance to primary clinic, and encounter type were captured and statistically analyzed using descriptive measures and Cox proportional regression modeling. Graphical representation of service areas was created using geo-mapping software. Results: Patients (n = 208) completed 331 telemedicine encounters over 12 months. Metastases and meningiomas comprised 60% of diagnoses. Median age was 62 years with median household income and residential population of $44,752 and 7,634 people. The one-way mean and median travel distances were 74.6 and 66.3 miles. The total potential road mileage for all patients was 44,596 miles. A total of 118 (57%) patients completed video visits during the first encounter, whereas 90 (43%) opted for telephone encounters. At 12 months, 138 patients (66%) utilized video visits and 70 (34%) used telephone visits. Predictors of video visit use were video-enabled visit during the first encounter (hazard ratio [HR] 2.806, p < 0.001), total potential distance traveled (HR 1.681, p < 0.05), and the need for more than one visit per year (HR 2.903, p < 0.001). Discussion: Telemedicine can be effective in radiosurgery practice with predictors of video-enabled use being pre-existing patient comfort levels with videoconferencing, total annual travel distance, and number of visits per year. Age, rural population status, and household income did not impact telemedicine use in our patient cohort.

Biological clocks and incremental growth line formation in dentine.

Dentine- and enamel-forming cells secrete matrix in consistent rhythmic phases, resulting in the formation of successive microscopic growth lines inside tooth crowns and roots. Experimental studies of various mammals have proven that these lines are laid down in subdaily, daily (circadian), and multidaily rhythms, but it is less clear how these rhythms are initiated and maintained. In 2001, researchers reported that lesioning the so-called master biological clock, the suprachiasmatic nucleus (SCN), halted daily line formation in rat dentine, whereas subdaily lines persisted. More recently, a key clock gene (Bmal1) expressed in the SCN in a circadian manner was also found to be active in dentine- and enamel- secretory cells. To probe these potential neurological and local mechanisms for the production of rhythmic lines in teeth, we reexamined the role of the SCN in growth line formation in Wistar rats and investigated the presence of daily lines in Bmal1 knockout mice (Bmal1-/- ). In contrast to the results of the 2001 study, we found that both daily and subdaily growth lines persisted in rat dentine after complete or partial SCN lesion in the majority of individuals. In mice, after transfer into constant darkness, daily rhythms continued to manifest as incremental lines in the dentine of each Bmal1 genotype (wild-type, Bmal+/- , and Bmal1-/- ). These results affirm that the manifestation of biological rhythms in teeth is a robust phenomenon, imply a more autonomous role of local biological clocks in tooth growth than previously suggested, and underscore the need further to elucidate tissue-specific circadian biology and its role in incremental line formation. Investigations of this nature will strengthen an invaluable system for determining growth rates and calendar ages from mammalian hard tissues, as well as documenting the early lives of fossil hominins and other primates.

Age at first molar emergence in Pan troglodytes verus and variation in the timing of molar emergence among free-living chimpanzees.

Age at lower first molar (M1) emergence is a commonly used proxy for inferring life-history scheduling in fossil primates, but its utility is dependent on knowing to what extent extant populations vary in this datum and how this variation correlates with the scheduling of life-history variables. Here, we address the first of these issues among extant chimpanzees. While age at M1 emergence has been documented in several live individuals from the Kanyawara population of Pan troglodytes schweinfurthii in Uganda, it has been estimated for only one individual of Pan troglodytes verus, based on a deceased animal from the Taï Forest in Côte d'Ivoire. To further explore interpopulation variation in this variable in chimpanzees, and using dental histology, we calculated ages at death for two wild-shot individuals of P. t. verus with erupting M1, both collected in Liberia during the mid-1950s, and estimated ages at M1 emergence from the ages at death. The overall range for these two individuals is ∼4.2-4.6 yr, compared with an age of ∼3.7 yr for the individual from the Taï Forest, and <2.5-3.3 yr for the several individuals of P. t. schweinfurthii. While the absolute range of ∼2 yr in these samples combined is little greater than in captive chimpanzees, the disparity between the samples of P. t. schweinfurthii and P. t. verus is striking, although it cannot be determined if this disparity represents a subspecies difference or simply population differences expressed in two different subspecies. While life-history data are unavailable for the population to which the Liberian chimpanzees belonged, the difference in M1 emergence ages between these individuals and those from Kanyawara still suggests caution when attempting even broad life-history inference in fossil apes and hominins based on age at M1 emergence.

Enamel thickness and dental development in Rudapithecus hungaricus.

The fossil record of middle and late Miocene Eurasian hominoids has expanded considerably over the past few decades, particularly with the recovery of numerous isolated teeth and jaws. Scholars have turned to assessments of internal tooth structure and growth to make sense of the evolutionary radiations of these primates as well as their affinities to the living great apes (hominids). Here we characterize full-dentition enamel thickness and dental development in several juvenile Rudapithecus hungaricus individuals using multiple imaging modalities. Relative enamel thickness (RET) values for the anterior teeth and premolars of Rudapithecus are broadly akin to those of gorillas and chimpanzees and are thinner than those of orangutans. First molar RET values are most similar to chimpanzees, while posterior molar values are closer to thicker-enameled orangutans. When compared to Miocene hominoids, Rudapithecus shows an intermediate molar RET condition that is especially similar to other dryopithecines. Long-period line periodicity values are comparable to African apes and most Miocene hominoids, and lower than living and fossil orangutans. The mean cuspal daily secretion rate is similar to that of several other Miocene hominoids but is greater than extant great apes. Cusp-specific molar crown formation times generally exceed those of chimpanzees, are lower than those of orangutans, and are broadly like those of other Miocene apes. While Rudapithecus appears to have a somewhat unique pattern of enamel thickness and dental development relative to individual great ape genera, these structural and developmental features are consistent with its designation as a hominid.

New hominoid fossils from Moroto II, Uganda and their bearing on the taxonomic and adaptive status of Morotopithecus bishopi.

The early Miocene site of Moroto II, Uganda has yielded some of the oldest known hominoid fossils. A new partial mandible (UMP MORII 03'551) is notable for its long tooth row and large, narrow M2 with well-developed cristids - a morphological combination previously unknown for large bodied catarrhines of the Early Miocene and suggesting folivory. The tooth proportions are compatible with belonging to the same taxon as the maxilla UMP 62-11, the holotype of Morotopithecus bishopi; likewise, the long tooth row and vertical planum of UMP MORII 03'551 suggest that it may represent the same taxon as mandible(s) UMP 66-01 and UMP 62-10. Canine size strongly suggests UMP MORII 03'551 is a female. Comparisons of the tooth crown morphology and tooth row proportions, relative enamel thickness, enamel-dentine junction morphology, long-period line periodicity, and dental wear patterns support significant morphological, developmental, and inferred dietary differentiation, and therefore generic-level distinctiveness, among Afropithecus, Morotopithecus and the Proconsul clade. An isolated M1 (UMP MORII 03'559) is morphologically dissimilar, and much smaller than the actual or inferred size of molars in UMP MORII 03'551, UMP 66-01 and UMP 62-10, supporting the presence of two hominoid taxa at Moroto II, M. bishopi and a smaller bodied proconsulid. Given the high level of body mass dimorphism inferred for Morotopithecus and other early Miocene catarrhines, the known postcrania from Moroto II could be attributable to either taxon. However, UMP MORII 03'551 and the femora UMP MORII 94'80 derive from the same stratigraphic interval, while the isolated M1 was deposited later, increasing the likelihood that the mandible and femora are from the same individual. These new fossils expand our understanding of the taxonomic and adaptive diversity of early Miocene catarrhines.

Barium distributions in teeth reveal early-life dietary transitions in primates.

Early-life dietary transitions reflect fundamental aspects of primate evolution and are important determinants of health in contemporary human populations. Weaning is critical to developmental and reproductive rates; early weaning can have detrimental health effects but enables shorter inter-birth intervals, which influences population growth. Uncovering early-life dietary history in fossils is hampered by the absence of prospectively validated biomarkers that are not modified during fossilization. Here we show that large dietary shifts in early life manifest as compositional variations in dental tissues. Teeth from human children and captive macaques, with prospectively recorded diet histories, demonstrate that barium (Ba) distributions accurately reflect dietary transitions from the introduction of mother's milk through the weaning process. We also document dietary transitions in a Middle Palaeolithic juvenile Neanderthal, which shows a pattern of exclusive breastfeeding for seven months, followed by seven months of supplementation. After this point, Ba levels in enamel returned to baseline prenatal levels, indicating an abrupt cessation of breastfeeding at 1.2 years of age. Integration of Ba spatial distributions and histological mapping of tooth formation enables novel studies of the evolution of human life history, dietary ontogeny in wild primates, and human health investigations through accurate reconstructions of breastfeeding history.

Dental development in living and fossil orangutans.

Numerous studies have investigated molar development in extant and fossil hominoids, yet relatively little is known about orangutans, the only great ape with an extensive fossil record. This study characterizes aspects of dental development, including cuspal enamel daily secretion rate, long-period line periodicities, cusp-specific molar crown formation times and extension rates, and initiation and completion ages in living and fossil orangutan postcanine teeth. Daily secretion rate and periodicities in living orangutans are similar to previous reports, while crown formation times often exceed published values, although direct comparisons are limited. One wild Bornean individual died at 4.5 years of age with fully erupted first molars (M1s), while a captive individual and a wild Sumatran individual likely erupted their M1s around five or six years of age. These data underscore the need for additional samples of orangutans of known sex, species, and developmental environment to explore potential sources of variation in molar emergence and their relationship to life history variables. Fossil orangutans possess larger crowns than living orangutans, show similarities in periodicities, and have faster daily secretion rate, longer crown formation times, and slower extension rates. Molar crown formation times exceed reported values for other fossil apes, including Gigantopithecus blacki. When compared to African apes, both living and fossil orangutans show greater cuspal enamel thickness values and periodicities, resulting in longer crown formation times and slower extension rates. Several of these variables are similar to modern humans, representing examples of convergent evolution. Molar crown formation does not appear to be equivalent among extant great apes or consistent within living and fossil members of Pongo or Homo.

First molar eruption, weaning, and life history in living wild chimpanzees.

Understanding dental development in chimpanzees, our closest living relatives, is of fundamental importance for reconstructing the evolution of human development. Most early hominin species are believed to show rapid ape-like patterns of development, implying that a prolonged modern human childhood evolved quite recently. However, chimpanzee developmental standards are uncertain because they have never been based on living wild individuals. Furthermore, although it is well established that first molar tooth emergence (movement into the mouth) is correlated with the scheduling of growth and reproduction across primates broadly, its precise relation to solid food consumption, nursing behavior, or maternal life history is unknown. To address these concerns we conducted a photographic study of subadult chimpanzees (Pan troglodytes schweinfurthii) in Kanyawara, Kibale National Park, Uganda. Five healthy infants emerged their lower first molars (M1s) by or before 3.3 y of age, nearly identical to captive chimpanzee mean ages (∼3.2 y, n = 53). First molar emergence in these chimpanzees does not directly or consistently predict the introduction of solid foods, resumption of maternal estrous cycling, cessation of nursing, or maternal interbirth intervals. Kanyawara chimpanzees showed adult patterns of solid food consumption by the time M1 reached functional occlusion, spent a greater amount of time on the nipple while M1 was erupting than in the preceding year, and continued to suckle during the following year. Estimates of M1 emergence age in australopiths are remarkably similar to the Kanyawara chimpanzees, and recent reconstructions of their life histories should be reconsidered in light of these findings.

Dental eruption in East African wild chimpanzees.

Knowledge of chimpanzee development has played an essential role in our understanding of the evolution of human ontogeny. However, recent studies of wild ape dentitions have cast doubt on the use of developmental standards derived from captive individuals. Others have called into question the use of deceased wild individuals to infer normative development. We conducted a high resolution photographic study of living known-age subadults in the Kanyawara community (Kibale National Park, Uganda) to generate a comprehensive three year record of dental eruption (including tooth emergence ages). These non-invasive data allow comparisons of captive and wild chimpanzees, establish accurate developmental standards for relatively healthy wild individuals, and facilitate direct assessments of primate-wide associations between dental development and life history. Emergence ages in the Kanyawara chimpanzees are very similar to living Gombe chimpanzees, and are broadly comparable to deceased Taï Forest chimpanzees. Early-emerging teeth such as the deciduous dentition and first molar (M1) appear during a time of maternal dependence, and are almost indistinguishable from captive chimpanzee emergence ages, while later forming teeth in the Kanyawara population emerge in the latter half of captive age ranges or beyond. Five juveniles whose lower M1s emerged by or before 3.3 years of age continued to nurse for a year or more beyond M1 emergence, and their mothers showed considerable variation in reproductive rates. The third molars of two adolescent females emerged several months to several years prior to the birth of their first offspring. Given that broad primate-wide relationships between molar emergence and life history do not necessarily hold within this population of chimpanzees, particularly for variables that are reported to be coincident with molar emergence, we suggest that further study is required in order to predict life history variables in hominins or hominoids.

Developmental defects in the teeth of three wild chimpanzees from the Taï forest.

OBJECTIVES: Developmental defects in teeth (accentuated lines and hypoplasias) have played a critical role in studies of childhood disease, nutrition, weaning, environmental variation, and early mortality. While these enigmatic structures have been lauded for their potential insights into human evolution, few studies have examined defects in individuals of known histories. MATERIALS AND METHODS: Here we document defects in the molars of three wild juvenile chimpanzees from the Taï forest (Pan troglodytes verus) and compare them with behavioral, epidemiological, and environmental records. RESULTS: Accentuated lines of differing intensities were found throughout molar crown and root growth, and were most common in a juvenile who demonstrated slow skeletal growth and prolonged maternal dependence. These defects were observed in association with some but not all injuries and disease outbreaks in this community. A 10-year record of accentuated line frequency across individuals shows a significant negative correlation with rainfall, but does not correlate with fruit availability or reveal significant annual trends. Several hypoplasias formed between ∼0.6 and 5.8 years of age on molar crowns and roots of the three individuals, however, available behavioral and epidemiological records do not explain their causation. DISCUSSION: While teeth may provide precise and accurate records of illness and trauma in some cases, inferring seasonal cycles, social stress, or weaning in living or fossil primate dentitions requires additional evidence beyond the presence, absence, or degree of expression of these defects. Studies that microsample bulk and trace elements may provide a more secure context for the interpretation of environmental, physiological, and dietary changes that impact dental tissue formation.

Intra- and interspecific variation in macaque molar enamel thickness.

Enamel thickness has played an important role in studies of primate taxonomy, phylogeny, and functional morphology, although its variation among hominins is poorly understood. Macaques parallel hominins in their widespread geographic distribution, relative range of body sizes, and radiation during the last five million years. To explore enamel thickness variation, we quantified average and relative enamel thickness (AET and RET) in Macaca arctoides, Macaca fascicularis, Macaca fuscata, Macaca mulatta, Macaca nemestrina, and Macaca sylvanus. Enamel area, dentine area, and enamel-dentine junction length were measured from mesial sections of 386 molars scanned with micro-computed tomography, yielding AET and RET indices. Intraspecific sex differences were not found in AET or RET. Macaca fuscata had the highest AET and RET, M. fascicularis showed the lowest AET, and M. arctoides had the lowest RET. The latitudinal distribution of macaque species was associated with AET for these six species. Temperate macaques had thicker molar enamel than did tropical macaques, suggesting that thick enamel may be adaptive in seasonal environments. Additional research is needed to determine if thick enamel in temperate macaques is a response to intensified hard-object feeding, increased abrasion, and/or a broader diet with a greater range of food material properties. The extreme ecological flexibility of macaques may prohibit identification of consistent trends between specific diets and enamel thickness conditions. Such complications of interpretation of ecological variability, dietary diversity, and enamel thickness may similarly apply for fossil Homo species.

Brief communication: New method for measuring nitrogen isotopes in tooth dentine at high temporal resolution.

OBJECTIVES: Nitrogen isotopes (δ15N) are widely used to study human nursing and weaning ages. Conventional methods involve sampling 1-mm thick sections of tooth dentine-producing an averaging effect that integrates months of formation. We introduce a novel protocol for measuring δ15N by multicollector secondary ion mass spectrometry (SIMS). MATERIALS AND METHODS: We sampled dentine δ15N on a weekly to monthly basis along the developmental axis in two first molars of healthy children from Australia and New Zealand (n = 217 measurements). Nitrogen isotope ratios were determined from measurements of CN- secondary molecular ions in ~35 µm spots. By relating spot position to enamel formation, we identified prenatal dentine, as well as sampling ages over more than 3 years. We also created calcium-normalized barium and strontium maps with laser ablation-inductively coupled plasma-mass spectrometry. RESULTS: We found rapid postnatal δ15N increases of ~2‰-3‰, during which time the children were exclusively breastfed, followed by declines as the breastfeeding frequency decreased. After weaning, δ15N values remained stable for several months, coinciding with diets that did not include meat or cow's milk; values then varied by ~2‰ starting in the third year of life. Barium did not show an immediate postnatal increase, rising after a few months until ~1-1.5 years of age, and falling until or shortly after the cessation of suckling. Initial strontium trends varied but both individuals peaked months after weaning. DISCUSSION: Developmentally informed SIMS measurements of δ15N minimize time averaging and can be precisely related to an individual's early dietary history.

Oxygen isotopes in orangutan teeth reveal recent and ancient climate variation.

Studies of climate variation commonly rely on chemical and isotopic changes recorded in sequentially produced growth layers, such as in corals, shells, and tree rings, as well as in accretionary deposits-ice and sediment cores, and speleothems. Oxygen isotopic compositions (δ18O) of tooth enamel are a direct method of reconstructing environmental variation experienced by an individual animal. Here, we utilize long-forming orangutan dentitions (Pongo spp.) to probe recent and ancient rainfall trends on a weekly basis over ~3-11 years per individual. We first demonstrate the lack of any consistent isotopic enrichment effect during exclusive nursing, supporting the use of primate first molar teeth as environmental proxies. Comparisons of δ18O values (n=2016) in twelve molars from six modern Bornean and Sumatran orangutans reveal a high degree of overlap, with more consistent annual and bimodal rainfall patterns in the Sumatran individuals. Comparisons with fossil orangutan δ18O values (n=955 measurements from six molars) reveal similarities between modern and late Pleistocene fossil Sumatran individuals, but differences between modern and late Pleistocene/early Holocene Bornean orangutans. These suggest drier and more open environments with reduced monsoon intensity during this earlier period in northern Borneo, consistent with other Niah Caves studies and long-term speleothem δ18O records in the broader region. This approach can be extended to test hypotheses about the paleoenvironments that early humans encountered in southeast Asia.

When an animal drinks water, two naturally occurring variants of oxygen ­ known as oxygen-18 and oxygen-16 ­ are incorporated into its growing teeth. The ratio of these variants in water changes with temperature, rainfall and other environmental conditions and therefore can provide a record of the climate during an animal's life. Teeth tend to be well preserved as fossils, which makes it possible to gain insights into this climate record even millions of years after an animal's death. Orangutans are highly endangered great apes that today live in rainforests on the islands of Borneo and Sumatra. During a period of time known as the Pleistocene (around 2.6 million years to 12,000 years ago), these apes were more widely spread across Southeast Asia. Climate records from this area in the time before human-induced climate change are somewhat limited. Therefore, fossilized orangutan teeth offer a possible way to investigate past seasonal rainfall patterns and gain insight into the kind of environments early humans would have encountered. To address this question, Smith et al. measured oxygen-18 and oxygen-16 variants in thin slices of modern-day orangutan teeth using a specialized analytical system. This established that the teeth showed seasonal patterns consistent with recent rainfall trends, and that the ratio of these oxygen variants did not appear to be impacted by milk intake in young orangutans. These findings indicated that the oxygen variants could be a useful proxy for predicting prehistoric weather patterns from orangutan teeth. Further measurements of teeth from fossilized Sumatran orangutans showed broadly similar rainfall patterns to those of teeth from modern-day orangutans. On the other hand, fossilized teeth from Borneo suggested that the environment used to be drier, with less intense wet seasons. The approach developed by Smith et al. provides an opportunity for scientists to leverage new fossil discoveries as well as existing collections to investigate past environments. This could allow future research into how climate variation may have influenced the spread of early humans through the region, as well as the evolution of orangutans and other endangered animals.

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.

Brief Communication: Elemental Models of Primate Nursing and Weaning Revisited.

Objectives: Intra-tooth patterns of trace elements barium (Ba) and strontium (Sr) have been used to infer human and nonhuman primate nursing histories, including australopithecine and Neanderthal juveniles. Here we contrast the two elemental models in first molars (M1s) of four wild baboons and explore the assumptions that underlie each. Materials and Methods: Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was employed to create comprehensive calcium-normalized barium and strontium (Ba/Ca, Sr/Ca) maps of M1 enamel and dentine at 35 micron resolution. Results: Postnatal Ba/Ca values were typically high, peaking ~0.5 years of age and then decreasing throughout M1 crown formation; all four individuals showed minimal Ba/Ca values between ~1.2-1.8 years, consistent with field reports of the cessation of suckling. Enamel Sr/Ca did not support patterns of previous LA-ICP-MS spot sampling as the enamel rarely showed discrete Sr/Ca secretory zonation. Increases in Sr/Ca appeared in coronal dentine beginning ~0.3 years, with varied peak value ages (~0.7-2.7 years) and no evidence of a predicted postweaning decline. Discussion: Inferences of baboon weaning ages from initial Ba/Ca minima are more congruent with behavioral observations than Sr/Ca maxima; this is consistent with studies of captive macaques of known weaning ages. Elemental variation is more apparent in the coronal dentine than the enamel of these baboons, which may relate to its more rapid mineralization and protection from the oral environment. Inferences of nursing histories from enamel Sr/Ca patterns alone should be reconsidered, and elevated values of Ba/Ca and Sr/Ca in teeth formed after weaning require further study.

Stress exposure histories revealed by biochemical changes along accentuated lines in teeth.

The regular incremental secretion of enamel and dentine can be interrupted during periods of stress resulting in accentuated growth lines. These accentuated lines, visible under light microscopy, provide a chronology of an individual's stress exposure. Previously, we showed that small biochemical changes along accentuated growth lines detected by Raman spectroscopy, coincided with the timing of medical history events and disruptions of weight trajectory in teeth from captive macaques. Here, we translate those techniques to study biochemical changes related to illness and prolonged medical treatment during early infancy in humans. Chemometric analysis revealed biochemical changes related to known stress-induced changes in circulating phenylalanine as well as other biomolecules. Changes in phenylalanine are also known to affect biomineralization which is reflected in changes in the wavenumbers of hydroxyapatite phosphate bands associated with stress in the crystal lattice. Raman spectroscopy mapping of teeth is an objective, minimally-destructive technique that can aid in the reconstruction of an individual's stress response history and provide important information on the mixture of circulating biochemicals associated with medical conditions, as applied in epidemiological and clinical samples.

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.

Enamel thickness in Bornean and Sumatran orangutan dentitions.

Dental enamel thickness has received considerable attention in ecological models of the adaptive significance of primate morphology. Several authors have theorized that the degree of enamel thickness may reflect selective pressures related to the consumption of fallback foods (dietary items that may require complex processing and/or have low nutritional value) during times of preferred food scarcity. Others have speculated that enamel thickness reflects selection during mastication of foods with particular material properties (i.e., toughness and hardness). Orangutans prefer ripe fruit when available, but show interspecific and sex differences in the consumption of fallback foods (bark, leaves, and figs) and other preferred foods (certain seeds). Bornean orangutans (Pongo pygmaeus) have also been reported to masticate more mechanically demanding foods than Sumatran orangutans (Pongo abelii). To test these ecological models, we assessed two-dimensional enamel thickness in orangutan full dentitions using established histological and virtual quantification methods. No significant differences in average enamel thickness (AET) were found between species. We found significant differences in the components of enamel thickness indices between sexes, with males showing greater enamel-dentine junction lengths and dentine core areas, and thus relatively thinner enamel than females. Comparisons of individuals of known sex and species revealed a dentition-wide trend for Bornean females to show greater AET than Sumatran females. Differences between small samples of males were less evident. These data provide only limited support for ecological explanations of enamel thickness patterns within great ape genera. Future studies of dietary ecology and enamel thickness should consider sex differences more systematically.