Quantitative Genetics, Pleiotropy, and Morphological Integration in the Dentition of Papio hamadryas

Sex-biased sampling may influence Homo naledi tooth size variation

A frequent source of debate in paleoanthropology concerns the taxonomic unity of fossil assemblages, with many hominin samples exhibiting elevated levels of variation that can be interpreted as indicating the presence of multiple species. By contrast, the large assemblage of hominin fossils from the Rising Star cave system, assigned to Homo naledi, exhibits a remarkably low degree of variation for most skeletal elements. Many factors can contribute to low sample variation, including genetic drift, strong natural selection, biased sex ratios, and sampling of closely related individuals. In this study, we tested for potential sex-biased sampling in the Rising Star dental sample. We compared coefficients of variation for the H. naledi teeth to those for eight extant hominoid samples. We used a resampling procedure that generated samples from the extant taxa that matched the sample size of the fossil sample for each possible Rising Star dental sex ratio. We found that variation at four H. naledi tooth positions-I2, M1, P4, M1-is so low that the possibility that one sex is represented by few or no individuals in the sample cannot be excluded. Additional evidence is needed to corroborate this inference, such as ancient DNA or enamel proteome data, and our study design does not address other potential factors that would account for low sample variation. Nevertheless, our results highlight the importance of considering the taphonomic history of a hominin assemblage and suggest that sex-biased sampling is a plausible explanation for the low level of phenotypic variation found in some aspects of the current H. naledi assemblage.

A tooth crown morphology framework for interpreting the diversity of primate dentitions

Variation in tooth crown morphology plays a crucial role in species diagnoses, phylogenetic inference, and the reconstruction of the evolutionary history of the primate clade. While a growing number of studies have identified developmental mechanisms linked to tooth size and cusp patterning in mammalian crown morphology, it is unclear (1) to what degree these are applicable across primates and (2) which additional developmental mechanisms should be recognized as playing important roles in odontogenesis. From detailed observations of lower molar enamel-dentine junction morphology from taxa representing the major primate clades, we outline multiple phylogenetic and developmental components responsible for crown patterning, and formulate a tooth crown morphology framework for the holistic interpretation of primate crown morphology. We suggest that adopting this framework is crucial for the characterization of tooth morphology in studies of dental development, discrete trait analysis, and systematics.

Assessment of the Relationship between the Total Occlusal Area of the Human Permanent Upper First and Second Molars and the Robusticity of the Facial Skeleton in Sex-Different Cranial Samples of Homo Sapiens: A Preliminary Study

The aim of this study was to establish whether there is a significant relationship between the total occlusal area (TOCA) of two types of permanent upper molars (first-M¹ and second-M²) and facial robusticity, as well as which of the examined facial regions indicate a relationship concerning the grade of their massiveness with the TOCA of analyzed molars in different sex adult Homo sapiens cranial samples. To obtain the values of the TOCA of the molars (n = 145), a morphometric method was performed based on the calibrated digital images of their occlusal surface using ImageJ software. The grades of the massiveness of six facial regions were assessed using qualitative scales of their expression, and an index of general facial robusticity was calculated. Two types of analyses were performed concerning standardized and non-standardized traits to the facial size, including Spearman's/or Pearson's correlations and partial rank correlations. The obtained results indicated the presence of a positive relationship between the relative TOCA of M²s and the relative general facial robusticity, as well as between the TOCA of both types of molars and the massiveness of trigone region of the facial skeleton in male crania. However, most of the obtained results were not consistent with the assumptions of the "localized masticatory stress hypothesis".

Dental size variation in admixed Latin Americans: Effects of age, sex and genomic ancestry

Dental size variation in modern humans has been assessed from regional to worldwide scales, especially under microevolutionary and forensic contexts. Despite this, populations of mixed continental ancestry such as contemporary Latin Americans remain unexplored. In the present study we investigated a large Latin American sample from Colombia (N = 804) and obtained buccolingual and mesiodistal diameters and three indices for maxillary and mandibular teeth (except third molars). We evaluated the correlation between 28 dental measurements (and three indices) with age, sex and genomic ancestry (estimated using genome-wide SNP data). In addition, we explored correlation patterns between dental measurements and the biological affinities, based on these measurements, between two Latin American samples (Colombians and Mexicans) and three putative parental populations: Central and South Native Americans, western Europeans and western Africans through PCA and DFA. Our results indicate that Latin Americans have high dental size diversity, overlapping the variation exhibited by the parental populations. Several dental dimensions and indices have significant correlations with sex and age. Western Europeans presented closer biological affinities with Colombians, and the European genomic ancestry exhibited the highest correlations with tooth size. Correlations between tooth measurements reveal distinct dental modules, as well as a higher integration of postcanine dentition. The effects on dental size of age, sex and genomic ancestry is of relevance for forensic, biohistorical and microevolutionary studies in Latin Americans.

A common mechanism drives the alignment between the micro- and macroevolution of primate molars

A central challenge for biology is to reveal how different levels of biological variation interact and shape diversity. However, recent experimental studies have indicated that prevailing models of evolution cannot readily explain the link between micro- and macroevolution at deep timescales. Here, we suggest that this paradox could be the result of a common mechanism driving a correlated pattern of evolution. We examine the proportionality between genetic variance and patterns of trait evolution in a system whose developmental processes are well understood to gain insight into how such alignment between morphological divergence and genetic variation might be maintained over macroevolutionary time. Primate molars present a model system by which to link developmental processes to evolutionary dynamics because of the biased pattern of variation that results from the developmental architecture regulating their formation. We consider how this biased variation is expressed at the population level, and how it manifests through evolution across primates. There is a strong correspondence between the macroevolutionary rates of primate molar divergence and their genetic variation. This suggests a model of evolution in which selection is closely aligned with the direction of genetic variance, phenotypic variance, and the underlying developmental architecture of anatomical traits.

Genetic influences on dentognathic morphology in the Jirel population of Nepal

Patterns of genetic variation and covariation impact the evolution of the craniofacial complex and contribute to clinically significant malocclusions in modern human populations. Previous quantitative genetic studies have estimated the heritabilities and genetic correlations of skeletal and dental traits in humans and nonhuman primates, but none have estimated these quantitative genetic parameters across the dentognathic complex. A large and powerful pedigree from the Jirel population of Nepal was leveraged to estimate heritabilities and genetic correlations in 62 maxillary and mandibular arch dimensions, incisor and canine lengths, and post-canine tooth crown areas (N ≥ 739). Quantitative genetic parameter estimation was performed using maximum likelihood-based variance decomposition. Residual heritability estimates were significant for all traits, ranging from 0.269 to 0.898. Genetic correlations were positive for all trait pairs. Principal components analyses of the phenotypic and genetic correlation matrices indicate an overall size effect across all measurements on the first principal component. Additional principal components demonstrate positive relationships between post-canine tooth crown areas and arch lengths and negative relationships between post-canine tooth crown areas and arch widths, and between arch lengths and arch widths. Based on these findings, morphological variation in the human dentognathic complex may be constrained by genetic relationships between dental dimensions and arch lengths, with weaker genetic correlations between these traits and arch widths allowing for variation in arch shape. The patterns identified are expected to have impacted the evolution of the dentognathic complex and its genetic architecture as well as the prevalence of dental crowding in modern human populations.

Genetic Correlation, Pleiotropy, and Molar Morphology in a Longitudinal Sample of Australian Twins and Families

This study aims to expand our understanding of the genetic architecture of crown morphology in the human diphyodont dentition. Here, we present bivariate genetic correlation estimates for deciduous and permanent molar traits and evaluate the patterns of pleiotropy within (e.g., m1–m2) and between (e.g., m2–M1) dentitions. Morphology was observed and scored from dental models representing participants of an Australian twin and family study (deciduous n = 290, permanent n = 339). Data collection followed Arizona State University Dental Anthropology System standards. Genetic correlation estimates were generated using maximum likelihood variance components analysis in SOLAR v.8.1.1. Approximately 23% of deciduous variance components models and 30% of permanent variance components models yielded significant genetic correlation estimates. By comparison, over half (56%) of deciduous–permanent homologues (e.g., m2 hypocone–M1 hypocone) were significantly genetically correlated. It is generally assumed that the deciduous and permanent molars represent members of a meristic molar field emerging from the primary dental lamina. However, stronger genetic integration among m2–M1/M2 homologues than among paired deciduous traits suggests the m2 represents the anterior-most member of a “true” molar field. The results indicate genetic factors act at distinct points throughout development to generate homologous molar form, starting with the m2, which is later replaced by a permanent premolariform crown.

Conserved and Taxon-Specific Patterns of Phenotypic Modularity in the Mammalian Dentition

Previous genotype:phenotype mapping of the mouse and primate dentition revealed the presence of pre- and post-canine modules in mice and anthropoid primates, as well as molar and premolar submodules in anthropoid primates. We estimated phenotypic correlation matrices for species that sample broadly across Mammalia to test the hypothesis that these modules exist across a broader range of taxa and thereby represent a conserved mammalian trait. We calculated phenotypic correlation matrices from linear dental measurements of 419 individual specimens representing 5 species from 4 mammalian orders: Artiodactyla (Odocoileus hemionus), Carnivora (Canis latrans and Ursus americanus), Didelphimorphia (Didelphis virginiana), and Primates (Colobus guereza). Our results based on hierarchical clustering indicate a generally higher correlation within incisors and among post-canine teeth. However, the post-canine phenotypic correlation matrices do not consistently exhibit the premolar and molar submodularity observed in anthropoid primates. Additionally, we find evidence of sex differences in the Odocoileus phenotypic correlation matrices: Males of this species exhibit overall higher inter-trait correlations compared to females. Our overall findings support the interpretation that incisors and post-canine dentition represent different phenotypic modules, and that this architecture may be a conserved trait for mammals.

Relative tooth size, Bayesian inference, and Homo naledi

OBJECTIVES

Size-corrected tooth crown measurements were used to estimate phenetic affinities among Homo naledi (~335-236 ka) and 11 other Plio-Pleistocene and recent species. To assess further their efficacy, and identify dental evolutionary trends, the data were then quantitatively coded for phylogenetic analyses. Results from both methods contribute additional characterization of H. naledi relative to other hominins.

MATERIALS AND METHODS

After division by their geometric mean, scaled mesiodistal and buccolingual dimensions were used in tooth size apportionment analysis to compare H. naledi with Australopithecus africanus, A. afarensis, Paranthropus robustus, P. boisei, H. habilis, H. ergaster, H. erectus, H. heidelbergensis, H. neanderthalensis, H. sapiens, and Pan troglodytes. These data produce equivalently scaled samples unaffected by interspecific size differences. The data were then gap-weighted for Bayesian inference.

RESULTS

Congruence in interspecific relationships is evident between methods, and with many inferred from earlier systematic studies. However, the present results place H. naledi as a sister taxon to H. habilis, based on a symplesiomorphic pattern of relative tooth size. In the preferred Bayesian phylogram, H. naledi is nested within a clade comprising all Homo species, but it shares some characteristics with australopiths and, particularly, early Homo.

DISCUSSION

Phylogenetic analyses of relative tooth size yield information about evolutionary dental trends not previously reported in H. naledi and the other hominins. Moreover, with an appropriate model these data recovered plausible evolutionary relationships. Together, the findings support recent study suggesting H. naledi originated long before the geological date of the Dinaledi Chamber, from which the specimens under study were recovered.

The genetic architecture of anterior tooth morphology in a longitudinal sample of Australian twins and families

OBJECTIVE

This study presents a quantitative genetic analysis of human anterior dental morphology in a longitudinal sample of known genealogy. The primary aim of this work is to generate a suite of genetic correlations within and between deciduous and permanent characters to access patterns of integration across the diphyodont dental complex.

DESIGN

Data were recorded from casted tooth crowns representing participants of a long-term Australian twin and family study (deciduous n = 290, permanent n = 339). Morphological trait expression was observed and scored following Arizona State University Dental Anthropology System standards. Bivariate genetic correlations were estimated using maximum likelihood variance decomposition models in SOLAR v.8.1.1.

RESULTS

Genetic correlation estimates indicate high levels of integration between antimeres but low to moderate levels among traits within a tooth row. Only 9% of deciduous model comparisons were significant, while pleiotropy was indicated for one third of permanent trait pairs. Canine characters stood out as strongly integrated, especially in the deciduous dentition. For homologous characters across dentitions (e.g., deciduous i¹ shoveling and permanent I¹ shoveling), ∼70% of model comparisons yielded significant genetic correlations.

CONCLUSIONS

Patterns of genetic correlation suggest a morphological canine module that spans the primary and secondary dentition. Results also point to the existence of a genetic mechanism conserving morphology across the diphyodont dental complex, such that paired deciduous and permanent traits are more strongly integrated than characters within individual tooth rows/teeth.

Morphological integration of the canine region within the hominine alveolar arch

The early hominin record is characterized by numerous shifts in dental proportions (e.g., canine reduction and megadontia) linked to changes in diet and social behavior. Recent studies suggest that hominins exhibit a reduction in the magnitude of covariation between the anterior and posterior dental components compared with other extant great apes. They point toward, but do not directly test, the relative independence of canine morphology within the hominin alveolar arch. This study focuses specifically on the how the canine region covaries with other regions of the dental arch because the canine region has drastically reduced in size and changed in shape across human evolution. We examine extant primate species most commonly used as a comparative framework for fossil hominin morphology: Gorilla gorilla (n = 27), Pan troglodytes (n = 27), and Homo sapiens (n = 30). We used geometric morphometric methods to test for size and shape covariation between the canine region with other dental regions. We also examined the influence of sexual dimorphism and allometry on intraspecific and interspecific patterns of covariation. The analysis of size and shape covariation between the mandibular canine and other individual tooth regions elucidated complex, species-specific, and sex-specific morphological relationships in the mandibular alveolar arch. There was little evidence to support different patterns of morphological integration between humans on the one hand and nonhuman apes on the other. Canine region morphology was relatively independent from other dental regions across species based on shape and did not significantly covary more with either the incisor or postcanine region in any species. The size correlations between the canine and other dental regions were moderate to high. The species-specific results of this study question the ability to make a priori assumptions about morphological integration in the extant hominin mandibular alveolar arch and its application to the fossil record.

Genetic correlations in the rhesus macaque dentition

Quantitative genetic analyses can indicate how complex traits respond to natural selection by demonstrating the genetic relationships between features that constrain their evolution. Genetic correlations between dental measurements have been estimated previously in baboons, humans, and tamarins and indicate variable patterns of modularity by tooth type across these taxa. Here, heritabilities of, and genetic correlations between, linear dental measurements were estimated from the Cayo Santiago rhesus macaques (Macaca mulatta). Relationships between the genetic correlation matrix and matrices designed to test hypotheses of modularity by tooth type, region, function, and development were assessed using a random skewers approach. Dental measurements were found to be moderately to highly heritable, with 24 of 28 heritability estimates differing significantly (p < 0.05) from zero. Almost all genetic correlations between dental dimensions were positive. The genetic correlation matrix was most similar to a regionally modular matrix, with distinct anterior and postcanine tooth modules. This pattern is consistent with previous quantitative genetic analyses of baboons and previous phenotypic analyses of cercopithecoid primates. The existence of a genetic module for the canines and honing premolar was not supported. Ongoing selection pressures, rather than strong genetic constraints, are likely necessary to preserve functional relationships between the canines and honing premolar based on these findings. The genetic correlation matrix of the Cayo Santiago rhesus macaques mirrors patterns of phenotypic correlations observed for cercopithecoid primates broadly and demonstrates that genetic contributions to these patterns may be fairly stable over the course of cercopithecoid evolution. The quantitative genetic study of additional taxa will be necessary to determine whether the regional modularity of baboons and macaques, or the integrated pattern of humans and tamarins, is shared more broadly across primates.

A genotype:phenotype approach to testing taxonomic hypotheses in hominids

Paleontology has long relied on assumptions about the genetic and developmental influences on skeletal variation. The last few decades of developmental genetics have elucidated the genetic pathways involved in making teeth and patterning the dentition. Quantitative genetic analyses have refined this genotype:phenotype map even more, especially for primates. We now have the ability to define dental traits with a fair degree of fidelity to the underlying genetic architecture; for example, the molar module component (MMC) and the premolar-molar module (PMM) that have been defined through quantitative genetic analyses. We leverage an extensive dataset of extant and extinct hominoid dental variation to explore how these two genetically patterned phenotypes have evolved through time. We assess MMC and PMM to test the hypothesis that these two traits reveal a more biologically informed taxonomy at the genus and species levels than do more traditional measurements. Our results indicate that MMC values for hominids fall into two categories and that Homo is derived compared with earlier taxa. We find a more variable, species-level pattern for PMM. These results, in combination with previous research, demonstrate that MMC reflects the phenotypic output of a more evolutionarily stable, or phylogenetically congruent, genetic mechanism, and PMM is a reflection of a more evolutionarily labile mechanism. These results suggest that the human lineage since the split with chimpanzees may not represent as much genus-level variation as has been inferred from traits whose etiologies are not understood.

Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions

The dentition is an extremely important organ in mammals with variation in timing and sequence of eruption, crown morphology, and tooth size enabling a range of behavioral, dietary, and functional adaptations across the class. Within this suite of variable mammalian dental phenotypes, relative sizes of teeth reflect variation in the underlying genetic and developmental mechanisms. Two ratios of postcanine tooth lengths capture the relative size of premolars to molars (premolar-molar module, PMM), and among the three molars (molar module component, MMC), and are known to be heritable, independent of body size, and to vary significantly across primates. Here, we explore how these dental traits vary across mammals more broadly, focusing on terrestrial taxa in the clade of Boreoeutheria (Euarchontoglires and Laurasiatheria). We measured the postcanine teeth of N = 1,523 boreoeutherian mammals spanning six orders, 14 families, 36 genera, and 49 species to test hypotheses about associations between dental proportions and phylogenetic relatedness, diet, and life history in mammals. Boreoeutherian postcanine dental proportions sampled in this study carry conserved phylogenetic signal and are not associated with variation in diet. The incorporation of paleontological data provides further evidence that dental proportions may be slower to change than is dietary specialization. These results have implications for our understanding of dental variation and dietary adaptation in mammals.

Unpacking the heritability of body mass index and other ratios

OBJECTIVES

Ratios of weight to height, especially body mass index (BMI = kg/m² ), are often used in epidemiological and genetic studies of health, but the limitations of quantitative genetic analysis of ratios are not widely known. The heritability of these ratios can be closely approximated from a bivariate quantitative genetic model of weight and height which clarifies how BMI heritabilities change.

METHODS

I explored this bivariate approximation and alternative measures through simulated datasets fit with linear mixed models. Simulated data were based on published heritabilities and other statistics for BMI and related anthropometric dimensions from four human samples.

RESULTS

Inspection of the bivariate approximation and analysis of simulated data show the heritability of weight/height crucially depends on the phenotypic (r_P ) and genetic correlations (r_A ) between weight and height. Changes in these correlations can have dramatic effects on the heritability of BMI. For example, when r_P  ≪ r_A heritability of BMI is reduced to 35-50% of its value when the correlations are equal.

DISCUSSION

Increasing adiposity likely decreases the phenotypic correlations more than the genetic correlation resulting in reduced heritability of the ratio. This contrasts with the commonly reported stability or increase of BMI heritability and implies it may result from increased genetic variance in weight in obesogenic environments. The bivariate model offers other advantages over ratios, including estimating the conditional genetic variance or heritability of weight that is unassociated with height, which may prove useful in quantitative and molecular genetic studies.

Dental integration and modularity in pinnipeds

Morphological integration and modularity are important for understanding phenotypic evolution because they constrain variation subjected to selection and enable independent evolution of functional and developmental units. We report dental integration and modularity in representative otariid (Eumetopias jubatus, Callorhinus ursinus) and phocid (Phoca largha, Histriophoca fasciata) species of Pinnipedia. This is the first study of integration and modularity in a secondarily simplified dentition with simple occlusion. Integration was stronger in both otariid species than in either phocid species and related positively to dental occlusion and negatively to both modularity and tooth-size variability across all the species. The canines and third upper incisor were most strongly integrated, comprising a module that likely serves as occlusal guides for the postcanines. There was no or weak modularity among tooth classes. The reported integration is stronger than or similar to that in mammals with complex dentition and refined occlusion. We hypothesise that this strong integration is driven by dental occlusion, and that it is enabled by reduction of modularity that constrains overall integration in complex dentitions. We propose that modularity was reduced in pinnipeds during the transition to aquatic life in association with the origin of pierce-feeding and loss of mastication caused by underwater feeding.

Genetic contributions to dental dimensions in brown-mantled tamarins (Saguinus fuscicollis) and rhesus macaques (Macaca mulatta)

OBJECTIVES

The use of dental metrics in phylogenetic reconstructions of fossil primates assumes variation in tooth size is highly heritable. Quantitative genetic studies in humans and baboons have estimated high heritabilities for dental traits, providing a preliminary view of the variability of dental trait heritability in nonhuman primate species. To expand upon this view, the heritabilities and evolvabilities of linear dental dimensions are estimated in brown-mantled tamarins (Saguinus fuscicollis) and rhesus macaques (Macaca mulatta).

MATERIALS AND METHODS

Quantitative genetic analyses were performed on linear dental dimensions collected from 302 brown-mantled tamarins and 364 rhesus macaques. Heritabilities were estimated in SOLAR using pedigrees from each population, and evolvabilities were calculated manually.

RESULTS

Tamarin heritability estimates range from 0.19 to 0.99, and 25 of 26 tamarin estimates are significantly different from zero. Macaque heritability estimates range from 0.08 to 1.00, and 25 out of 28 estimates are significantly different from zero.

DISCUSSION

Dental dimensions are highly heritable in captive brown-mantled tamarins and free-ranging rhesus macaques. The range of heritability estimates in these populations is broadly similar to those of baboons and humans. Evolvability tends to increase with heritability, although evolvability is high relative to heritability in some dimensions. Estimating evolvability helps to contextualize differences in heritability, and the observed relationship between evolvability and heritability in dental dimensions requires further investigation.

Morphological variation and covariation in mandibular molars of platyrrhine primates

Molars are highly integrated biological structures that have been used for inferring evolutionary relationships among taxa. However, parallel and convergent morphological traits can be affected by developmental and functional constraints. Here, we analyze molar shapes of platyrrhines in order to explore if platyrrhine molar diversity reflects homogeneous patterns of molar variation and covariation. We digitized 30 landmarks on mandibular first and second molars of 418 extant and 11 fossil platyrrhine specimens to determine the degree of integration of both molars when treated as a single module. We combined morphological and phylogenetic data to investigate the phylogenetic signal and to visualize the history of molar shape changes. All platyrrhine taxa show a common shape pattern suggesting that a relatively low degree of phenotypic variation is caused by convergent evolution, although molar shape carries significant phylogenetic signal. Atelidae and Pitheciidae show high levels of integration with low variation between the two molars, whereas the Cebinae/Saimiriinae, and especially Callitrichinae, show greater variation between molars and trend toward a modular organization. We hypothesize that biomechanical constraints of the masticatory apparatus, and the dietary profile of each taxon are the main factors that determine high covariation in molars. In contrast, low molar shape covariation may result from the fact that each molar exhibits a distinct ecological signal, as molars can be exposed to distinct occlusal loadings during food processing, suggesting that different selective pressures on molars can reduce overall molar integration.

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.

The evolution of anthropoid molar proportions

BACKGROUND

Developmental processes that underpin morphological variation have become a focus of interest when attempting to interpret macroevolutionary patterns. Recently, the Dental Inhibitory Cascade (DIC) model has been suggested to explain much of the variation in mammalian molar size proportions. We tested the macroevolutionary implications of this model using anthropoid primate species (n=100), focusing on overall morphological patterns, as well as predictions made about molar size variability, direct developmental control, and diet.

RESULTS

Of the species sampled, 56 % had centroids that fell within regions of molar proportion morphospace consistent with the DIC model. We also found that the third molar had greater variation in size than either the first or second molars, as expected by the model. Some DIC model predictions were not supported, however, such as the expected proportion of M 2/M 1 when the third molar is absent. Furthermore, we found that some variability in third molar size could not be explained by the influence of the inhibitory cascade. Overall, we found considerable clade-specific differences in relative molar sizes among anthropoid primates, with hominoids and cercopithecins strongly divergent from DIC model predictions, and platyrrhines, colobines, and papionins more consistent with the inhibitory cascade. Finally, we investigated reasons why some clades deviated from DIC model expectations. Adaptations for frugivory (e.g., bunodont cusp relief) appeared to be one driver of relatively larger second molars and have evolved independently in multiple lineages of anthropoids.

CONCLUSIONS

The DIC model explains some of the variation in anthropoid primate molar proportions. However, there are interesting deviations away from this broad mammalian pattern, particularly in hominoids and cercopithecins, which suggest the model is only one of multiple mechanisms determining morphological variability in mammalian teeth.

Modeling the dental development of fossil hominins through the inhibitory cascade

The inhibitory cascade is a mathematical model for interpreting the relative size of the occlusal surfaces of mammalian molars in terms of developmental mechanisms. The cascade is derived from experimental studies of mouse molars developed in culture, and has been tested and applied to the dentitions of rodents, ungulates, carnivores, and platyrrhines. Results from such applications have provided new information regarding the origins of plesiomorphic traits in mammalian clade and how derived morphologies may arise. In this study we apply the inhibitory cascade model to the postcanine dentition of a sample of Old World primates that includes fossil hominins. The results of this study suggest that the inhibitory cascade (i.e. M1 < M2 < M3 ) describes the relative sizes of the molar occlusal areas of Old World primates and is likely the plesiomorphic condition for this clade. Within that clade, whereas most Old World monkeys have a M1 < M2 < M3 pattern, most apes have a M1 < M2 ≈ M3 pattern. This modified cascade suggests that greater levels of inhibition (or less activation) are acting on the posterior molars of apes, thus facilitating the reduction of M3 s within the apes. With the exception of the baboon genus Papio, extant congeners typically share the same molar inhibitory cascade. The differences in the relative size relationships observed in the molar and premolar-molar cascades of the species included in the fossil hominin genus Paranthropus suggest that although large postcanine teeth are a shared derived trait within this genus, the developmental basis for postcanine megadontia may not be the same in these two Paranthropus taxa. Our results show that phenotypic characters such as postcanine megadontia may not reflect common development.

Modularity in the mammalian dentition: mice and monkeys share a common dental genetic architecture

The concept of modularity provides a useful tool for exploring the relationship between genotype and phenotype. Here, we use quantitative genetics to identify modularity within the mammalian dentition, connecting the genetics of organogenesis to the genetics of population-level variation for a phenotype well represented in the fossil record. We estimated the correlations between dental traits owing to the shared additive effects of genes (pleiotropy) and compared the pleiotropic relationships among homologous traits in two evolutionary distant taxa-mice and baboons. We find that in both mice and baboons, who shared a common ancestor >65 Ma, incisor size variation is genetically independent of molar size variation. Furthermore, baboon premolars show independent genetic variation from incisors, suggesting that a modular genetic architecture separates incisors from these posterior teeth as well. Such genetic independence between modules provides an explanation for the extensive diversity of incisor size variation seen throughout mammalian evolution-variation uncorrelated with equivalent levels of postcanine tooth size variation. The modularity identified here is supported by the odontogenic homeobox code proposed for the patterning of the rodent dentition. The baboon postcanine pattern of incomplete pleiotropy is also consistent with predictions from the morphogenetic field model.