Anthropology Takes Control of Morphometrics

There has been a startling change over the last decade in the intellectual context of morphometrics. In the 1990’s, this field, which has not altered its focus upon the quantitative analysis of biomedical shape variation and shape change, was principally centered around concerns of medical image analysis; but now it is driven mainly by the demands of researchers in human variability, physical anthropology, primatology, and paleoanthropology instead. This essay celebrates that change and tries to account for it by reference to cognitive and intellectual aspects of the new home

Functional morphology and integration of corvid skulls – a 3D geometric morphometric approach

<p>Abstract</p> <p>Background</p> <p>Sympatric corvid species have evolved differences in nesting, habitat choice, diet and foraging. Differences in the frequency with which corvid species use their repertoire of feeding techniques is expected to covary with bill-shape and with the frontal binocular field. Species that frequently probe are expected to have a relatively longer bill and more sidewise oriented orbits in contrast to species that frequently peck. We tested this prediction by analyzing computed tomography scans of skulls of six corvid species by means of three-dimensional geometric morphometrics. We (1) explored patterns of major variation using principal component analysis, (2) compared within and between species relationships of size and shape and (3) quantitatively compared patterns of morphological integration between bill and cranium by means of partial least squares (singular warp) analysis.</p> <p>Results</p> <p>Major shape variation occurs at the bill, in the orientation of orbits, in the position of the foramen magnum and in the angle between bill and cranium. The first principal component correlated positively with centroid-size, but within-species allometric relationships differed markedly. Major covariation between the bill and cranium lies in the difference in orbit orientation relative to bill-length and in the angle between bill and cranium.</p> <p>Conclusion</p> <p>Corvid species show pronounced differences in skull shape, which covary with foraging mode. Increasing bill-length, bill-curvature and sidewise orientation of the eyes is associated with an increase in the observed frequency in probing (vice versa in pecking). Hence, the frequency of probing, bill-length, bill-curvature and sidewise orientation of the eyes is progressively increased from jackdaw, to Eurasian jay, to black-billed magpie, to hooded crow, to rook and to common raven (when feeding on carcasses is considered as probing). Our results on the morphological integration suggest that most of the covariation between bill and cranium is due to differences in the topography of the binocular fields and the projection of the bill-tip therein, indicating the importance of visual fields to the foraging ecology of corvids.</p

Virtual reconstruction of the Le Moustier 2 newborn skull.

Here we present a virtual skull reconstruction of the Le Moustier 2 neonate based on high-resolution computed tomographic data of the delicate original fragments. In the first step, the digital fragments were assembled based on anatomical criteria. If parts were missing on one side, or were better preserved on one side than the other, we used the software Avizo to reflect them across a midsagittal symmetry plane. Missing parts of the braincase were estimated based on complete reference crania using geometric morphometrics, so as to estimate endocranial volume (EV).When we compare our reconstruction of Le Moustier 2 to modern human neonates, we find that many morphological characteristics that separate Neandertal adults from modern human adults are already established at the time of birth. Neandertal features can already be detected in the shape of the orbit, the projection of the midface, the relative size and shape of the nose, the nasal bones, and the shape of the mandibular notch. The shape differences between Le Moustier 2 and modern human neonates in the cranial base are extremely subtle. Around the time of birth modern humans and Neandertals have very similar endocranial shapes and volumes. Our EV estimates for Le Moustier 2 range between 408–428 cc.Our reconstruction of Le Moustier 2 shows that most facial differences between modern humans and Neandertals develop prenatally as they are already established at the time of birth. Most shape differences in the braincase between modern humans and Neandertals, however, develop after birth. Our reconstruction of Le Moustier 2 therefore supports the notion that modern humans and Neandertals reach similar adult endocranial capacity through different postnatal ontogenetic pathways

Virtual reconstruction of the Le Moustier 2 newborn skull.

Here we present a virtual skull reconstruction of the Le Moustier 2 neonate based on high-resolution computed tomographic data of the delicate original fragments. In the first step, the digital fragments were assembled based on anatomical criteria. If parts were missing on one side, or were better preserved on one side than the other, we used the software Avizo to reflect them across a midsagittal symmetry plane. Missing parts of the braincase were estimated based on complete reference crania using geometric morphometrics, so as to estimate endocranial volume (EV).When we compare our reconstruction of Le Moustier 2 to modern human neonates, we find that many morphological characteristics that separate Neandertal adults from modern human adults are already established at the time of birth. Neandertal features can already be detected in the shape of the orbit, the projection of the midface, the relative size and shape of the nose, the nasal bones, and the shape of the mandibular notch. The shape differences between Le Moustier 2 and modern human neonates in the cranial base are extremely subtle. Around the time of birth modern humans and Neandertals have very similar endocranial shapes and volumes. Our EV estimates for Le Moustier 2 range between 408–428 cc.Our reconstruction of Le Moustier 2 shows that most facial differences between modern humans and Neandertals develop prenatally as they are already established at the time of birth. Most shape differences in the braincase between modern humans and Neandertals, however, develop after birth. Our reconstruction of Le Moustier 2 therefore supports the notion that modern humans and Neandertals reach similar adult endocranial capacity through different postnatal ontogenetic pathways

Evolution of the base of the brain in highly encephalized human species

The increase of brain size relative to body size-encephalization-is intimately linked with human evolution. However, two genetically different evolutionary lineages, Neanderthals and modern humans, have produced similarly large-brained human species. Thus, understanding human brain evolution should include research into specific cerebral reorganization, possibly reflected by brain shape changes. Here we exploit developmental integration between the brain and its underlying skeletal base to test hypotheses about brain evolution in Homo. Three-dimensional geometric morphometric analyses of endobasicranial shape reveal previously undocumented details of evolutionary changes in Homo sapiens. Larger olfactory bulbs, relatively wider orbitofrontal cortex, relatively increased and forward projecting temporal lobe poles appear unique to modern humans. Such brain reorganization, beside physical consequences for overall skull shape, might have contributed to the evolution of H. sapiens' learning and social capacities, in which higher olfactory functions and its cognitive, neurological behavioral implications could have been hitherto underestimated factors. © 2011 Macmillan Publishers Limited. All rights reserved.Peer Reviewe

Structural organization of the cranial vault in Neanderthals and present-day humans: Can endocranial shape characteristics be explained by vault thickness?

In studies of hominin brain evolution the shape of the interior braincase (or endocranium) is often used to infer cortical organization. However, the braincase is not only shaped by brain, but also by the interplay of evolutionary and developmental changes of facial size and shape, the development of soft tissues, and neurocranial bone thickness. It therefore remains unclear to what extent the well-documented endocranial shape differences between the more globular present-day humans and the ..

Covariation of endocranial shape and cranial vault thickness in present-day humans and Neandertals

A better understanding of the patterns of brain growth and development in extinct but also in extant great apes informs discussions about the evolution of cognitive abilities and behaviors in the human lineage. In fossils, brain shape and its cortical organization can usually only be inferred from the shape of the endocranial cavity and from the brain imprints in the cranial bone. However, the shape of the braincase results from different mechanisms: the patterns of brain growth and development [1], the evolutionary and developmental changes affecting facial size and shape [2], the development of soft tissues [3] and the pattern of growth and development of the entire neurocranium [4]. It has been shown that present-day humans and Neandertals achieved similar endocranial capacities via different developmental pathways, suggesting underlying differences in the tempo and mode of brain growth and development [5].Here, we assess the influence of bone thickness on endocranial shape. Specifically, we examined to what extent differences in bone thickness of the cranial vault can explain the endocranial shape differences between present-day humans and Neanderthals. Our sample comprises 75 computed tomographic scans of adult present-day humans and 6 Neandertals. Endocranial shape was measured using 935 landmarks and semilandmarks and analyzed after a Procrustes registration. Cranial vault thickness (CVT) was computed from 472 landmarks and semilandmarks as the distance between the endocranial and the ectocranial surfaces. We first quantified CVT standardized for the size. Second, we explored the covariation between endocranial shape and the cranial vaultthickness using a two-blocks partial least-squares analysis (PLS). Last, we established a predictive regression model of endocranial shape using cranial thickness as an input variable and endocranial shape as an output. We built this model from the present-day human sample only, and measured the fitness of the model in explaining the endocranial shape that characterizes the Neandertal individuals.Our results demonstrate that even though Neandertals tend to have a thicker cranial vault, these values are still comprised within the range of variation of present-day humans. The first dimension of covariation in the PLS analysis was driven by variation within present-day humans. Individuals displaying elongated shapes showed an overall thinner CVT, while those with rounded vaults had a thicker cranial vault. Scores along the second axis of covariation displayed a shift between present-day humans and Neandertals. Along this axis, present-day humans were characterized by bulged, vertically stretched and thin parietal bones, while Neandertals displayed vertically shorter, wider and thicker parietal bones. Finally, our regression model failed to predict the Neandertal endocranial shape from their CVT values. Altogether, our results suggest that endocranial shape differences between present-day humans and Neandertals are not likely to be explained by their CVT, and strengthen the hypothesis of different brain shapes between these two human groups

Endostructural Morphology in Hominoid Mandibular Third Premolars: Geometric Morphometric Analysis of Dentine Crown Shape

In apes, the mandibular third premolar (P3) is adapted for a role in honing the large upper canine. The role of honing was lost early in hominin evolution, releasing the tooth from this functional constraint and allowing it to respond to subsequent changes in masticatory demands. This led to substantial morphological changes, and as such the P3 has featured prominently in systematic analyses of the hominin clade. The application of microtomography has also demonstrated that examination of the enamel-dentine junction (EDJ) increases the taxonomic value of variations in crown morphology. Here we use geometric morphometric techniques to analyze the shape of the P3 EDJ in a broad sample of fossil hominins, modern humans, and extant apes (n = 111). We test the utility of P3 EDJ shape for distinguishing among hominoids, address the affinities of a number of hominin specimens of uncertain taxonomic attribution, and characterize the changes in P3 EDJ morphology across our sample, with particular reference to features relating to canine honing and premolar ‘molarization’. We find that the morphology of the P3 EDJ is useful in taxonomic identification of individual specimens, with a classification accuracy of up to 88%. The P3 EDJ of canine-honing apes displays a tall protoconid, little metaconid development, and an asymmetrical crown shape. Plio-Pleistocene hominin taxa display derived masticatory adaptations at the EDJ, such as the molarized premolars of Australopithecus africanus and Paranthropus, which have well-developed marginal ridges, an enlarged talonid, and a large metaconid. Modern humans and Neanderthals display a tall dentine body and reduced metaconid development, a morphology shared with premolars from Mauer and the Cave of Hearths. Homo naledi displays a P3 EDJ morphology that is unique among our sample; it is quite unlike Middle Pleistocene and recent Homo samples and most closely resembles Australopithecus, Paranthropus and early Homo specimens

From fossils to mind

Fossil endocasts record features of brains from the past: size, shape, vasculature, and gyrification. These data, alongside experimental and comparative evidence, are needed to resolve questions about brain energetics, cognitive specializations, and developmental plasticity. Through the application of interdisciplinary techniques to the fossil record, paleoneurology has been leading major innovations. Neuroimaging is shedding light on fossil brain organization and behaviors. Inferences about the development and physiology of the brains of extinct species can be experimentally investigated through brain organoids and transgenic models based on ancient DNA. Phylogenetic comparative methods integrate data across species and associate genotypes to phenotypes, and brains to behaviors. Meanwhile, fossil and archeological discoveries continuously contribute new knowledge. Through cooperation, the scientific community can accelerate knowledge acquisition. Sharing digitized museum collections improves the availability of rare fossils and artifacts. Comparative neuroanatomical data are available through online databases, along with tools for their measurement and analysis. In the context of these advances, the paleoneurological record provides ample opportunity for future research. Biomedical and ecological sciences can benefit from paleoneurology's approach to understanding the mind as well as its novel research pipelines that establish connections between neuroanatomy, genes and behavior