Faster growth corresponds with shallower linear hypoplastic defects in great ape canines

Linear enamel hypoplasia in Homo naledi reappraised in light of new Retzius periodicities

OBJECTIVES

Among low-latitude apes, developmental defects of enamel often recur twice yearly, linkable to environmental cycles. Surprisingly, teeth of Homo naledi from Rising Star in South Africa (241-335 kya), a higher latitude site with today a single rainy season, also exhibit bimodally distributed hypoplastic enamel defects, but with uncertain timing and etiology. Newly determined Retzius periodicities for enamel formation in this taxon enable a reconstruction of the temporal patterning of childhood stress.

METHODS

Using high resolution casts of 31 isolated anterior teeth from H. naledi, 82 enamel defects (linear enamel hypoplasia [LEH]) were identified. Seventeen teeth are assigned to three individuals. Perikymata in the occlusal wall of enamel furrows and between the onsets of successive LEH were visualized with scanning electron microscopy and counted. Defects were measured with an optical scanner. Conversion of perikymata counts to estimates of LEH duration and inter-LEH interval draws upon Retzius periodicities of 9 and 11 days.

RESULTS

Anterior teeth record more than a year of developmental distress, expressed as two asymmetric intervals centered on 4.5 and 7.5 months bounded by three LEH. Durations, also, show bimodal distributions, lasting 3 or 12 weeks. Short duration LEH are more severe than long duration. Relative incisor/canine rates of formation are indistinguishable from modern humans.

DISCUSSION

We invoke a disease and dearth model, with short episodes of distress reflecting onset of disease in young infants, lasting about 3 weeks, followed by a season of undernutrition, possibly intensified by secondary plant compounds, spanning about 12 weeks, inferably coincident with austral winter.

Differences in childhood stress between Neanderthals and early modern humans as reflected by dental enamel growth disruptions

Neanderthals' lives were historically portrayed as highly stressful, shaped by constant pressures to survive in harsh ecological conditions, thus potentially contributing to their extinction. Recent work has challenged this interpretation, leaving the issue of stress among Paleolithic populations highly contested and warranting in-depth examination. Here, we analyze the frequency of dental enamel hypoplasia, a growth disruption indicator of early life stress, in the largest sample of Neanderthal and Upper Paleolithic dentitions investigated to date for these features. To track potential species-specific patterns in the ontogenetic distribution of childhood stress, we present the first comprehensive Bayesian modelling of the likelihood of occurrence of individual and matched enamel growth disruptions throughout ontogeny. Our findings support similar overall stress levels in both groups but reveal species-specific patterns in its ontogenetic distribution. While Neanderthal children faced increasing likelihoods of growth disruptions starting with the weaning process and culminating in intensity post-weaning, growth disruptions in Upper Paleolithic children were found to be limited around the period of weaning and substantially dropping after its expected completion. These results might, at least in part, reflect differences in childcare or other behavioral strategies between the two taxa, including those that were advantageous for modern humans' long-term survival.

Meaningful measures of enamel hypoplasia: Prevalence and comparative intensity of developmental stress

OBJECTIVES

Developmental stress causing enamel thinning is an important topic in primate biology. Because taxa differ in growth rates and enamel thickness, the goal is to provide a new method allowing direct comparison of prevalence and salience of enamel defects among samples.

MATERIALS AND METHODS

Casts of ape teeth spanning the Late Pleistocene to Late Miocene from three site areas of increasing seasonality, equator (Sumatra) to 20° (Vietnam) and 25°N latitude (China), were examined for enamel defects among paleo-orangutans (n = 571, 222, respectively) and Lufengpithecus lufengensis (n = 198). Frequency of affected teeth and number of linear enamel hypoplasia were recorded. Defect dimensions were measured with a confocal microscope. Simple prevalence is compared to weighted prevalence (%), calculated by dividing "number of LEH from specific tooth groups" by "specific tooth sample size"; this quantity divided by "tooth-specific years of imbricational enamel formation." Defect dimensions are reduced to a dimensionless index termed "enamel deficit ratio" through dividing "daily enamel deficit" by "daily secretion rate."

RESULTS

Weighted prevalence increases to the North, highlighting latitudinal similarities. In contrast, "enamel deficit ratio," designed to express comparative severity of developmental stress among samples, was least in the high latitude sample and differed little between paleo-orangutan samples.

DISCUSSION

The actual numbers generated are not as important as efficacy of the proposed methods for other taxa. Developmental stress appears least severe in the high latitude (Lufengpithecus) sample but affects a greater proportion, compared to paleo-orangutans. Regardless of findings, the proposed solutions to improve comparability of disparate samples, yield reasonable results.

Wild boar versus domestic pig-Deciphering of crown growth in porcine second molars

Based on the previously established periodicity of enamel growth marks, we reconstructed crown growth parameters of mandibular second molars from two wild boar and two domestic pigs of the Linderöd breed. Body weight gain and progression of dental development were markedly faster in the domestic pigs than the wild boar. While the final crown dimensions of the M₂ did not differ between domestic pigs and wild boar, mean crown formation time (CFT) of this tooth was considerably shorter in the domestic pigs (162 days) than in the wild boar (205 days). The difference in CFT was mainly attributable to a higher enamel extension rate (EER) in the domestic pig. Generally, EER was highest in the cuspalmost deciles of the length of the enamel-dentine-junction and markedly dropped in cervical direction, with lowest values occurring in the cervicalmost decile. In consequence, the cuspal half of the M₂ crown was formed about three times faster than the cervical half. In contrast to the EER, no marked difference in daily enamel secretion rate (DSR) was recorded between domestic pigs and wild boar. The duration of enamel matrix apposition as well as linear enamel thickness in corresponding crown portions was only slightly lower in the domestic pigs than the wild boar. Thus, the earlier completion of M₂ crown growth in the domestic pig was mainly achieved by a higher EER and not by an increased DSR. The more rapid recruitment of secretory ameloblasts in the course of molar crown formation of domestic pigs compared to wild boar is considered a side-effect of the selection for rapid body growth during pig domestication.

More severe stress markers in the teeth of flanged versus unflanged orangutans (Pongo spp.)

OBJECTIVES

We compared an early life stress indicator, linear enamel hypoplasia (LEH), in the canine teeth of two male orangutan (Pongo spp.) morphs. Flanged males have large bi-discoid cheek pads and a laryngeal throat pouch, and they exhibit either the same or higher levels of the stress hormone cortisol throughout development compared with unflanged males, which lack secondary sexual characteristics. Such "developmental arrest" is hypothesized to either reflect a response to experienced high stress (Hypothesis 1), or an adaptation to avoid elevated stress levels and/or having experienced lower stress levels (Hypothesis 2) during early life. As LEH defect depth has been shown to reflect the severity (i.e., intensity and/or duration) of early life stress events, we examined whether unflanged males have shallower LEH defects than flanged males.

MATERIALS AND METHODS

Flanging status was assessed by measuring the faces of preserved skins. Canine height (N = 37) was measured in the same individuals to assess commonality between morphs. LEH defect depths were analyzed using a standardized confocal profilometry method (N = 34).

RESULTS

Flanged males have significantly deeper LEH defects than unflanged adult males. Canine projected crown heights are similar across males regardless of morph.

CONCLUSIONS

Evidence from great apes shows that, when comparing canines with similar growth patterns, deeper defects reflect more severe stress events during development. Thus, our results suggest that "developmental arrest" of unflanged males is not a response to having experienced stress, but rather an adaptation to avoid the physiological impacts associated with chronic stress and/or experiencing lower stress levels.

Conventional microscopy makes perikymata count and spacing data feasible for large samples

OBJECTIVES

Current methods of quantifying defects of dental enamel (DDE) include either gross or low-level examination for linear enamel hypoplasia, histological analysis of striae of Retzius, or scanning electron microscopy (SEM) of a tooth or a tooth cast. Gross examination has been shown to miss many defects. Other methods can be destructive, require transporting samples, and are expensive. Here, we show that digital light microscopy (DLM) can be used for the analysis of DDE as indicated by widened perikymata spacing (WPS). This method takes advantage of high-power (100×) microscopy, but is non-destructive, can be implemented almost anywhere, and is inexpensive.

MATERIALS AND METHODS

As proof of concept, we created photomontages of labial surfaces of five human canines from images made using DLM and SEM. We counted and measured the widths of all visible perikymata for each imaging modality and fit measurements to a negative curve representing the expected values for each tooth. We calculated residuals for each measurement. WPS were defined when R² was in the 90th percentile, and were considered matched in DLM and SEM images when observed within the same decile of the tooth surface.

RESULTS

There were more WPS detected in the images derived from DLM than from SEM. Overall, the data derived from the two imaging modalities provided similar information about the frequency and timing of stress during dental development.

CONCLUSIONS

The method described here allows for DDE data acquisition as WPS from large samples, making feasible population-level studies that reflect sophisticated understanding of dental development.

3D enamel profilometry reveals faster growth but similar stress severity in Neanderthal versus Homo sapiens teeth

Early life stress disrupts growth and creates horizontal grooves on the tooth surface in humans and other mammals, yet there is no consensus for their quantitative analysis. Linear defects are considered to be nonspecific stress indicators, but evidence suggests that intermittent, severe stressors create deeper defects than chronic, low-level stressors. However, species-specific growth patterns also influence defect morphology, with faster-growing teeth having shallower defects at the population level. Here we describe a method to measure the depth of linear enamel defects and normal growth increments (i.e., perikymata) from high-resolution 3D topographies using confocal profilometry and apply it to a diverse sample of Homo neanderthalensis and H. sapiens anterior teeth. Debate surrounds whether Neanderthals exhibited modern human-like growth patterns in their teeth and other systems, with some researchers suggesting that they experienced more severe childhood stress. Our results suggest that Neanderthals have shallower features than H. sapiens from the Upper Paleolithic, Neolithic, and medieval eras, mirroring the faster growth rates in Neanderthal anterior teeth. However, when defect depth is scaled by perikymata depth to assess their severity, Neolithic humans have less severe defects, while Neanderthals and the other H. sapiens groups show evidence of more severe early life growth disruptions.

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.

Insights on patterns of developmental disturbances from the analysis of linear enamel hypoplasia in a Neolithic sample from Liguria (northwestern Italy)

OBJECTIVE

To assess developmental disturbances through the analysis of linear enamel hypoplasia (LEH) frequency and to infer environmental stress and life history within Neolithic communities from Liguria (Italy).

MATERIALS

43 unworn/minimally worn permanent anterior teeth of 13 individuals recovered from nearby caves and dated to c. 4800-4400 cal. BCE.

METHODS

LEH defects were identified with high-resolution macrophotos of dental replicas, age at LEH was calculated via perikymata counts. LEH defects matched between two or more teeth were considered as systemic disturbances. LEH frequency by age classes was analyzed via GLZ and Friedman ANOVA.

RESULTS

Number of matched defects per individual range between 2-12. The mean LEH per individual was highest in the 2.5-2.99 age category, with a significant increase relative to earlier growth stages, followed by a decline.

CONCLUSION

LEH may reflect life-history in the local ecology of Neolithic Liguria, where several individuals with osteoarticular tuberculosis have been recorded. Disease burden may have triggered developmental disturbances around the time of weaning. Age at first defect was negatively correlated with age at death and positively with the total number of defects, suggesting that early stress may have affected survivorship.

SIGNIFICANCE

The study contributes to the reconstruction of ecological pressures among Neolithic people of Liguria, and informs on environmental challenges during the Neolithic adaptive expansion.

LIMITATIONS

The visual examination of macrophotos is prone to observer error; mid-crown tends to display more visible LEH due to tooth architecture.

SUGGESTIONS FOR FURTHER RESEARCH

Apply different quantitative methods to examine severity and duration of disturbances.