Geometric morphometric methods rely on the accurate identification and quantification of landmarks on biological specimens. As in any empirical analysis, the assessment of inter- and intra-observer error is desirable. A review of methods currently being employed to assess measurement error in geometric morphometrics was conducted and three general approaches to the problem were identified. One such approach employs Generalized Procrustes Analysis to superimpose repeatedly digitized landmark configurations, thereby establishing whether repeat measures fall within an acceptable range of variation. The potential problem of this error assessment method (the "Pinocchio effect") is demonstrated and its effect on error studies discussed. An alternative approach involves employing Euclidean distances between the configuration centroid and repeat measures of a landmark to assess the relative repeatability of individual landmarks. This method is also potentially problematic as the inherent geometric properties of the specimen can result in misleading estimates of measurement error. A third approach involved the repeated digitization of landmarks with the specimen held in a constant orientation to assess individual landmark precision. This latter approach is an ideal method for assessing individual landmark precision, but is restrictive in that it does not allow for the incorporation of instrumentally defined or Type III landmarks. Hence, a revised method for assessing landmark error is proposed and described with the aid of worked empirical examples.

Variation in the masticatory behavior of hunter-gatherer and agricultural populations is hypothesized to be one of the major forces affecting the form of the human mandible. However, this has yet to be analyzed at a global level. Here, the relationship between global mandibular shape variation and subsistence economy is tested, while controlling for the potentially confounding effects of shared population history, geography, and climate. The results demonstrate that the mandible, in contrast to the cranium, significantly reflects subsistence strategy rather than neutral genetic patterns, with hunter-gatherers having consistently longer and narrower mandibles than agriculturalists. These results support notions that a decrease in masticatory stress among agriculturalists causes the mandible to grow and develop differently. This developmental argument also explains why there is often a mismatch between the size of the lower face and the dentition, which, in turn, leads to increased prevalence of dental crowding and malocclusions in modern postindustrial populations. Therefore, these results have important implications for our understanding of human masticatory adaptation.O ne of the major differences categorizing human populations is variation in subsistence strategies and related paramasticatory behavior. A shift from a primarily hunting and gathering strategy to one based on extensive horticulture or animal husbandry is known to have occurred independently on several occasions in human prehistory, yielding a correlated shift in settlement pattern, demography, population expansion, and social reorganization (e.g., 1, 2). Given the wider cultural changes associated with increased food processing and, therefore, consumption of a more homogeneous and softer diet in agriculturalists, it has been hypothesized that the dietary changes associated with agriculture are likely to have had an important effect on the form of the cranium and mandible (e.g., 3-5). Although localized studies comparing hunter-gather and farming populations in Nubia (6), South America (7), the Ohio Valley (8), and the southern Levant (9) have found some support for an associated change between the masticatory apparatus and the initial transition to agriculture, it is currently unclear what effect agriculture has had on global patterns of human mandibular variation when compared against other wider microevolutionary factors, such as gene flow, migration, and natural selection. Hence, this study represents a global comparative analysis of the effects of subsistence strategy on modern human mandibular variation.In recent decades, it has become clear that the majority of modern human cranial shape variation is congruent with a null model of neutral evolution, with relatively few morphological regions being subject to diversifying selection (e.g., 10-19). However, there appear to be two major exceptions to this general pattern. Aspects of facial morphology, and particularly nasal morphology, are likely to have been subject to diversifying natural sele...

The extent to which past climate change has dictated the pattern and timing of the out-of-Africa expansion by anatomically modern humans is currently unclear [Stewart JR, Stringer CB (2012) Science 335:1317-1321. In particular, the incompleteness of the fossil record makes it difficult to quantify the effect of climate. Here, we take a different approach to this problem; rather than relying on the appearance of fossils or archaeological evidence to determine arrival times in different parts of the world, we use patterns of genetic variation in modern human populations to determine the plausibility of past demographic parameters. We develop a spatially explicit model of the expansion of anatomically modern humans and use climate reconstructions over the past 120 ky based on the Hadley Centre global climate model HadCM3 to quantify the possible effects of climate on human demography. The combinations of demographic parameters compatible with the current genetic makeup of worldwide populations indicate a clear effect of climate on past population densities. Our estimates of this effect, based on population genetics, capture the observed relationship between current climate and population density in modern huntergatherers worldwide, providing supporting evidence for the realism of our approach. Furthermore, although we did not use any archaeological and anthropological data to inform the model, the arrival times in different continents predicted by our model are also broadly consistent with the fossil and archaeological records. Our framework provides the most accurate spatiotemporal reconstruction of human demographic history available at present and will allow for a greater integration of genetic and archaeological evidence.human dispersals | colonization | population bottlenecks | net primary productivity | most recent common ancestor

Recent applications of population genetic models to human craniodental traits have revealed a strong neutral component to patterns of global variation. However, little work has been undertaken to determine whether neutral processes might also be influencing the postcranium, perhaps due to substantial evidence for selection and plastic environmental responses in these regions. Recent work has provided evidence for neutral effects in the pelvis, but has been limited in regard to shape data (small numbers of linear measurements) and restricted only to males. Here, we use geometric morphometric methods to examine population variation in the human os coxae (pelvic bone) in both males and females. Neutrality is examined via apportionment of variance patterns and fit to an Out-of-Africa serial founder effect model, which is known to structure neutral genetic patterns. Moreover, we compare males and females directly, and the true versus false pelvis, in order to examine potential obstetrical effects. Our results indicate evidence for substantial neutral population history effects on pelvic shape variation. They also reveal evidence for the effect of obstetrical constraints, but these affect males and females to equivalent extents. Our results do not deny an important role for selection in regard to specific aspects of human pelvic variation, especially in terms of features associated with body size and proportions. However, our analyses demonstrate that at a global level, the shape of the os coxae reveals substantial evidence for neutral variation. Our analyses thus indicate that population variation in the human pelvis might be used to address important questions concerning population history, just as the human cranium has done.

Recent studies have demonstrated that the shape of the human temporal bone is particularly strongly correlated with neutral genetic expectation, when compared against other cranial regions, such as the vault, face, and basicranium. In turn, this has led to suggestions that the temporal bone is particularly reliable in analyses of primate phylogeny and human population history. While several reasons have been suggested to explain the temporal bone's strong fit with neutral expectation, the temporal bone has never systematically been compared against other individual cranial bones defined using the same biological criteria. Therefore, it is currently unknown whether the shapes of all cranial bones possess reliable information regarding neutral genetic evolution, or whether the temporal bone is unique in this respect. This study tests the hypothesis that the human temporal bone is more congruent with neutral expectation than six other individual cranial bones by correlating population affinity matrices generated using neutral genetic and 3D craniometric data. The results demonstrate that while the temporal bone shows the absolute strongest correlation with neutral genetic data compared with all other bones, it is not statistically differentiated from the sphenoid, frontal, and parietal bones in this regard. Potential reasons for the temporal bone's consistently strong fit with neutral expectation, such as its overall anatomical complexity and/or its contribution to the architecture of the basicranium, are examined. The results suggest that future phylogenetic and taxonomic studies would benefit from considering the shape of the entire cranium minus those regions that deviate most from neutrality.

Recent studies comparing craniometric and neutral genetic affinity matrices have concluded that, on average, human cranial variation fits a model of neutral expectation. While human craniometric and genetic data fit a model of isolation by geographic distance, it is not yet clear whether this is due to geographically mediated gene flow or human dispersal events. Recently, human genetic data have been shown to fit an iterative founder effect model of dispersal with an African origin, in line with the out-of-Africa replacement model for modern human origins, and Manica et al. (Nature 448 (2007) 346-349) have demonstrated that human craniometric data also fit this model. However, in contrast with the neutral model of cranial evolution suggested by previous studies, Manica et al. (2007) made the a priori assumption that cranial form has been subject to climatically driven natural selection and therefore correct for climate prior to conducting their analyses. Here we employ a modified theoretical and methodological approach to test whether human cranial variability fits the iterative founder effect model. In contrast with Manica et al. (2007) we employ size-adjusted craniometric variables, since climatic factors such as temperature have been shown to correlate with aspects of cranial size. Despite these differences, we obtain similar results to those of Manica et al. (2007), with up to 26% of global within-population craniometric variation being explained by geographic distance from sub-Saharan Africa. Comparative analyses using non-African origins do not yield significant results. The implications of these results are discussed in the light of the modern human origins debate.

No abstract

No abstract