Valeria Bernal

Differences between sliding semi-landmark methods in geometric morphometrics, with an application to human craniofacial and dental variation

Over the last decade, geometric morphometric methods have been applied increasingly to the study of human form. When too few landmarks are available, outlines can be digitized as series of discrete points. The individual points must be slid along a tangential direction so as to remove tangential variation, because contours should be homologous from subject to subject whereas their individual points need not. This variation can be removed by minimizing either bending energy (BE) or Procrustes distance (D) with respect to a mean reference form. Because these two criteria make different assumptions, it becomes necessary to study how these differences modify the results obtained. We performed bootstrapped‐based Goodalls F‐test, Footes measurement, principal component (PC) and discriminant function analyses on human molars and craniometric data to compare the results obtained by the two criteria. Results show that: (1) F‐scores and P‐values were similar for both criteria; (2) results of Footes measurement show that both criteria yield different estimates of within‐ and between‐sample variation; (3) there is low correlation between the first PC axes obtained by D and BE; (4) the percentage of correct classification is similar for BE and D, but the ordination of groups along discriminant scores differs between them. The differences between criteria can alter the results when morphological variation in the sample is small, as in the analysis of modern human populations.

Geometric morphometric approach to sex estimation of human pelvis

Sex estimation of skeletal remains is an important issue in both forensics and bioarchaeology. The chance of attaining a high level of accuracy regarding sex allocations is related to the skeletal component analyzed and the ability of the techniques employed to describe shape and size differences among the sexes. Current opinion regards the hip bone as the most reliable sex indicator because it is the most dimorphic bone, particularly in adult individuals. The aim of this study was therefore to analyze the greater sciatic notch and the ischiopubic complex morphology by employing geometric morphometric techniques, based on semilandmark and multivariate statistical methods, in order to develop a reliable and accurate technique for adult sex estimation. The sample analyzed consisted of 121 adult left hip bones randomly selected from the collection of documented skeletons housed at the Museu Antropologico de Coimbra. Morphometric analysis was based on coordinates of landmarks and semilandmarks of the ilium and ischiopubic regions that were digitized on 2D photographic images. Discriminant analysis with leave-one-out cross-validation and k-means clustering of shape and shape-size variables were used in order to classify individuals by sex. For the greater sciatic notch, average accuracy of 90.9% was achieved with both multivariate analyses based on shape variables. For the ischiopubic complex, the values obtained with shape variables were 93.4% and 90.1% for discriminant and k-means, respectively. Females were misclassified more frequently than males, especially for the ischiopubic complex. When multivariate statistical analyses were performed using shape-size variables, the percentages of correct classifications were lower than those obtained with shape variables. We conclude that the use of geometric morphometrics and multivariate statistics is a reliable method to quantify pelvic shape differences between the sexes and could be applied to discriminate between females and males.

The Uniqueness of the Human Anterior Dentition: A Geometric Morphometric Analysis

ABSTRACT: The analysis of bitemarks has a significant bearing on forensic odontology and has attracted an increasingly sophisticated array of techniques in its evaluation. Two postulates underlie all bitemark analyses: firstly, that the characteristics of the anterior teeth involved in the bite are unique, and secondly, that this uniqueness is accurately recorded in the material bitten. Here, we investigate the question of the uniqueness of the anterior dentition. To do this, we use geometric morphometric techniques based on landmark and semilandmark data. The incisor and canine occlusal surfaces of 50 randomly selected orthodontic casts of young individuals (17–20 years) of both sexes form the material for this study. We analyzed the sizes of these teeth by means of landmark and semilandmark analysis to calculate Procrustes distances between tooth outlines. In order to analyze shape variation among individuals, we carried out principal components analyses on the partial warp scores. These are derived from Partial Procrustes coordinates aligned by means of thin‐plate spline decomposition based on the bending energy matrix. The results indicate that there is no sexual dimorphism in the shape of the upper or lower arches. Plots of centroid size and first relative warps show less superposition among individuals than in shape analysis. This means that, when the size and shape are considered together, the difference between arches increases. Procrustes superimposition between the two individuals located most closely (0.0444) and the two most separated (0.1567) along the first axis of relative warp analyses show that individuals are not only differentiated by the relative position of their teeth but also by their arch shape. In conclusion, it appears that the incisal surfaces of the anterior dentition are in fact unique.

Discrepancy between cranial and DNA data of early Americans: implications for American peopling.

Currently, one of the major debates about the American peopling focuses on the number of populations that originated the biological diversity found in the continent during the Holocene. The studies of craniometric variation in American human remains dating from that period have shown morphological differences between the earliest settlers of the continent and some of the later Amerindian populations. This led some investigators to suggest that these groups—known as Paleomericans and Amerindians respectively—may have arisen from two biologically different populations. On the other hand, most DNA studies performed over extant and ancient populations suggest a single migration of a population from Northeast Asia. Comparing craniometric and mtDNA data of diachronic samples from East Central Argentina dated from 8,000 to 400 years BP, we show here that even when the oldest individuals display traits attributable to Paleoamerican crania, they present the same mtDNA haplogroups as later populations with Amerindian morphology. A possible explanation for these results could be that the craniofacial differentiation was a local phenomenon resulting from random (i.e. genetic drift) and non-random factors (e.g. selection and plasticity). Local processes of morphological differentiation in America are a probable scenario if we take into consideration the rapid peopling and the great ecological diversity of this continent; nevertheless we will discuss alternative explanations as well.

Ontogeny of Robusticity of Craniofacial Traits in Modern Humans: A Study of South American Populations

To date, differences in craniofacial robusticity among modern and fossil humans have been primarily addressed by analyzing adult individuals; thus, the developmental basis of such differentiation remains poorly understood. This article aims to analyze the ontogenetic development of craniofacial robusticity in human populations from South America. Geometric morphometric methods were used to describe cranial traits in lateral view by using landmarks and semilandmarks. We compare the patterns of variation among populations obtained with subadults and adults to determine whether population-specific differences are evident at early postnatal ontogeny, compare ontogenetic allometric trajectories to ascertain whether changes in the ontogeny of shape contribute to the differentiation of adult morphologies, and estimate the amount of size change that occurs during growth along each population-specific trajectory. The results obtained indicate that the pattern of interpopulation variation in shape and size is already established at the age of 5 years, meaning that processes acting early during ontogeny contribute to the adult variation. The ontogenetic allometric trajectories are not parallel among all samples, suggesting the divergence in the size-related shape changes. Finally, the extension of ontogenetic trajectories also seems to contribute to shape variation observed among adults.

Spatial regression techniques for inter-population data: studying the relationships between morphological and environmental variation.

Understanding the importance of environmental dimensions behind the morphological variation among populations has long been a central goal of evolutionary biology. The main objective of this study was to review the spatial regression techniques employed to test the association between morphological and environmental variables. In addition, we show empirically how spatial regression techniques can be used to test the association of cranial form variation among worldwide human populations with a set of ecological variables, taking into account the spatial autocorrelation in data. We suggest that spatial autocorrelation must be studied to explore the spatial structure underlying morphological variation and incorporated in regression models to provide more accurate statistical estimates of the relationships between morphological and ecological variables. Finally, we discuss the statistical properties of these techniques and the underlying reasons for using the spatial approach in population studies.

ENTIERROS HUMANOS DEL NORESTE DE PATAGONIA: NUEVOS FECHADOS RADIOCARBÓNICOS

* ABSTRACT In this paper we present a set of radiocarbon determinations obtained for individuals belonging to archaeological sites from northeast Patagonia. These sites are located near Laguna del Juncal, in the lower valley of Negro river, and San Blas peninsula. The 14C results suggest that the sites containing human burials cover, in this area, a temporal span ranging from 3.000 to 500 years BP. It has also been observed that chronological differences between burials are associated with different kinds of artifi cial cranial deformation.

Ontogenetic Allometry and Cranial Shape Diversification Among Human Populations From South America

Modifications of ontogenetic allometries play an important role in patterning the shape differentiation among populations. This study evaluates the influence of size variation on craniofacial shape disparity among human populations from South America and assesses whether the morphological disparity observed at the interpopulation level resulted from a variable extension of the same ontogenetic allometry, or whether it arose as a result of divergences in the pattern of size‐related shape changes. The size and shape of 282 adult and subadult crania were described by geometric morphometric‐based techniques. Multivariate regressions were used to evaluate the influence of size on shape differentiation between and within populations, and phylogenetic comparative methods were used to take into account the shared evolutionary history among populations. The phylogenetic generalized least‐squares models showed that size accounts for a significant amount of shape variation among populations for the vault and face but not for the base, suggesting that the three modules did not exhibit a uniform response to changes in overall growth. The common slope test indicated that patterns of evolutionary and ontogenetic allometry for the vault and face were similar and characterized by a heightening of the face and a lengthening of the vault with increasing size. The conservation of the same pattern of shape changes with size suggests that differences in the extent of growth contributed to the interpopulation cranial shape variation and that certain directions of morphological change were favored by the trait covariation along ontogeny. Anat Rec, 2011.

Spatial patterns and evolutionary processes in southern South America: A study of dental morphometric variation

The purpose of this article is to examine the patterns of evolutionary relationships between human populations from the later Late Holocene (1,500-100 years BP) of southern South America on the basis of dental morphometric data. We tested the hypotheses that the variation observed in this region would be explained by the existence of populations with different phylogenetic origin or differential action of gene flow and genetic drift. In this study, we analyzed permanent teeth from 17 samples of male and female adult individuals from throughout southern South America. We measured mesiodistal and buccolingual diameters at the base of the crown, along the cement-enamel junction. The results of multiple regression analysis and a mantel correlogram indicate the existence of spatial structure in dental shape variation, as the D(2) Mahalanobis distance between samples increases with increasing geographical distance between them. In addition, the correlation test results show a trend toward reduction of the internal variation of samples with increasing latitude. The detected pattern of dental variation agrees with the one expected as an outcome of founder serial effects related to an expansion of range during the initial occupation of southern South America.

Evolutionary Relationships Among Prehistoric Human Populations: An Evaluation of Relatedness Patterns Based on Facial Morphometric Data Using Molecular Data

ABSTRACT The study of evolutionary relationships among human populations is fundamental to inferring processes that determine their structure and history. Among the different data types used to infer such relationships, molecular data, particularly nuclear and mitochondrial DNA, are preferred because of their high heritability and the low probability of changes during development. However, although the reliability of relatedness patterns based on other traits is discussed, except in unusual circumstances most prehistoric populations remain within the domain of morphological study. Therefore the primary goal of this study is to test the reliability of relatedness patterns constructed on the basis of craniometric data on a regional scale. In particular, we analyze samples from populations belonging to the Chaco, Pampa, and Patagonia regions of South America for which craniometric and molecular data are available. We compare a strongly supported relatedness pattern based on molecular data with the results obtained through landmark-based and semilandmark-based facial data. The matrices based on Euclidean distance for morphometric data and DA distances for molecular data were used to perform principal coordinates analyses and to obtain reticulograms. Finally, a principal components analysis of all individuals was performed with morphometric data. The results indicate that ordination analyses yield slightly different results depending on the morphometric data used. However, the reticulograms obtained with both landmark-based and semilandmark-based data allow the separation of the Chubut samples from the Chaco samples, with the Pampa sample in between the others; this pattern is congruent with molecular-based analyses. As a consequence, our results indicate that facial morphometric data allow the inference of the structure and history of the prehistoric populations for the studied region.