What do brain endocasts tell us? A comparative analysis of the accuracy of sulcal identification by experts and perspectives in palaeoanthropology

Palaeoneurology is a complex field as the object of study, the brain, does not fossilize. Studies rely therefore on the (brain) endocranial cast (often named endocast), the only available and reliable proxy for brain shape, size and details of surface. However, researchers debate whether or not specific marks found on endocasts correspond reliably to particular sulci and/or gyri of the brain that were imprinted in the braincase. The aim of this study is to measure the accuracy of sulcal identification through an experiment that reproduces the conditions that palaeoneurologists face when working with hominin endocasts. We asked 14 experts to manually identify well-known foldings in a proxy endocast that was obtained from an MRI of an actual in vivo Homo sapiens head. We observe clear differences in the results when comparing the non-corrected labels (the original labels proposed by each expert) with the corrected labels. This result illustrates that trying to reconstruct a sulcus following the very general known shape/position in the literature or from a mean specimen may induce a bias when looking at an endocast and trying to follow the marks observed there. We also observe that the identification of sulci appears to be better in the lower part of the endocast compared to the upper part. The results concerning specific anatomical traits have implications for highly debated topics in palaeoanthropology. Endocranial description of fossil specimens should in the future consider the variation in position and shape of sulci in addition to using models of mean brain shape. Moreover, it is clear from this study that researchers can perceive sulcal imprints with reasonably high accuracy, but their correct identification and labelling remains a challenge, particularly when dealing with extinct species for which we lack direct knowledge of the brain

Frontal sinuses and human evolution

The frontal sinuses are cavities inside the frontal bone located at the junction between the face and the cranial vault and close to the brain. Despite a long history of study, understanding of their origin and variation through evolution is limited. This work compares most hominin species’ holotypes and other key individuals with extant hominids. It provides a unique and valuable perspective of the variation in sinuses position, shape, and dimensions based on a simple and reproducible methodology. We also observed a covariation between the size and shape of the sinuses and the underlying frontal lobes in hominin species from at least the appearance of Homo erectus. Our results additionally undermine hypotheses stating that hominin frontal sinuses were directly affected by biomechanical constraints resulting from either chewing or adaptation to climate. Last, we demonstrate their substantial potential for discussions of the evolutionary relationships between hominin species

Morphological co-variation of the cranium and endocast in the genus Homo

Il existe chez les espèces du genre Homo une diversité morphologique crânienne et cérébrale importante, et les interactions de ces deux éléments sont complexes. De manière générale, au cours de l’évolution de ce taxon, le neurocrâne prend une importance croissante par rapport au bloc facial en raison d’une expansion cérébrale marquée. Cependant, les modalités de cette expansion sont multiples, et elle se met en place chez les différentes espèces via des modifications morphologiques qui leur sont propres. Mise à part l’augmentation du volume cérébral, l’endocrâne témoigne de réorganisations neuroanatomiques. Ces différents facteurs - volume et organisation – ainsi que les contraintes morpho-fonctionnelles diverses exercées sur la face externe du crâne, sont susceptibles de résulter en une variété de relations morphologiques et spatiales entre le neurocrâne et l’endocrâne. Il est donc pertinent de documenter ces relations afin de pouvoir par la suite mieux appréhender la variabilité et les mécanismes évolutifs à l’oeuvre chez les différents taxons du genre Homo. Nous explorons dans ce travail de thèse les variations jointes du neurocrâne et de l’endocrâne dans le genre Homo et chez Homo sapiens. Cette contribution est basée sur l’analyse de modèles virtuels de crânes et d’endocrânes à l’aide de méthodes géométriques et d’une méthode innovative de déformations de surfaces. Nous avons étudié des données morphologiques issues de populations actuelles afin d’éclaircir la nature des relations entre le neurocrâne et l’endocrâne chez Homo sapiens. Pour cela, nous avons comparé les asymétries des hémisphères de l’endocrâne – qui reflètent celles des hémisphères cérébraux – aux asymétries de la voûte crânienne. Les schémas d’asymétrie bilatérale relevés sont identiques sur le crâne et sur l’endocrâne. Cela s’explique par une morphologie de la voûte du crâne calquée sur celle de l’endocrâne, malgré un effet « tampon » de l’os qui n’enregistre pas sur sa face externe toutes les asymétries cérébrales. Les possibles corrélations entre le degré d’asymétrie et des facteurs tels que la conformation générale du crâne, la robustesse des superstructures osseuses, le sexe et le volume endocrânien ont également été explorées. Nous avons ensuite analysé les schémas de co-variation entre neurocrâne et endocrâne au sein du genre Homo. Nous avons ainsi pu mettre en évidence des éléments de co-variation qui concernent l’ensemble du genre Homo, et d’autres qui sont spécifiques à certains taxons, notamment aux Néandertaliens ou à Homo sapiens. Ainsi, si la conformation de la voûte crânienne est très semblable à la morphologie endocrânienne, les interactions crâne-endocrâne dans la zone occipitale et cérébelleuse apparaissent plus variables, et semblent inféodées au degré de globularisation de l’ensemble du cerveau et du neurocrâne. Ces résultats mettent en évidence certaines interactions entre réorganisations cérébrales et morphologie crânienne chez les différentes espèces du genre Homo, et soulignent le caractère crucial du croisement des données et des méthodes pour l’interprétation du registre fossile.Species of the genus Homo display cranial and endocranial morphological variations, with complex interactions between these two elements. Generally speaking, throughout the evolution of this taxon the neurocranium becomes increasingly important by comparison with the facial skeleton, due to a marked cerebral expansion. The modalities of this expansion differ accross species and occur at least partly through species-specific morphological processes. Apart from the increase in cerebral volume, the endocast bears the traces of neuroanatomical reorganisations. These two factors – volume and organisation – as well as various morpho-functional constraints on the external face of the cranium, may result in a variety of morphological and spacial relationships between the neurocranium and the endocranium. It is therefore important to document these relations in order to better apprehend the variability and the evolutionary mechanisms behind the morphologies of the different Homo species.This doctoral thesis explores the joint morphological variations of the neurocranium and endocast in the genus Homo and within Homo sapiens through multiple approaches. We offer a contribution to this topic based on shape analyses of virtual crania and endocasts, using geometric morphometrics and an innovative technique of surface deformations. We analysed morphological data from extant populations in order to clarify the nature of the relationship between the neurocranium and the endocast in Homo sapiens. One of the lines of evidence investigated is the correspondence between neurocranial and endocranial (and therefore cerebral) gross asymmetries. Our results show that the patterns of bilateral asymmetries of the neurocranium are identical to those of the endocranium. There is a close correspondance between the morphologies of the endocranial and cranial vaults, despite the neurocranium not displaying the full extent of cerebral asymmetries on its external vault. Correlations between asymmetry and factors including sex, endocranial volume and importance of the bony superstructures were also tested. Co-variation patterns between neurocranial and endocranial morphologies in the genus Homo were analysed. Our results highlight elements of co-variation between the neuro and endocranium which are shared accross the genus Homo, and others which are species-specific. While the cranial vault closely follows endocranial morphology, interaction patterns between the endo and neurocranium in the occipito-cerebellar area appear more variable and linked to the overall degree of globularisation of the brain and neurocranium. These results highlight some of the interactions between cerebral reorganisations and cranial morphology in Homo species, and underline the importance of crossing data and methods in order to interpret the fossil record

Of Tongues and Men: A Review of Morphological Evidence for the Evolution of Language

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Trends in cerebral organisation in Upper Palaeolithic and recent humans.

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The role of allometry and posture in the evolution of the hominin subaxial cervical spine

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The development of bite force resistance in modern humans and Neanderthals

Modern human and Neanderthal faces present clear morphological differences at all ontogenetic stages. Their post-natal ontogenetic allometric trajectories diverge [1] and in both species, as the various components of the mid-face develop and grow, bone facial remodelling is thought to play a key role in adapting them to their final adult form. In modern humans, maxillary growth is characterised by bone resorption on the anterior surface, whereas in Neanderthals extensive bone deposition is the common finding [2]. This morphogenetic difference is present by approximately 5 years of age [2]. During development, crania are loaded by forces applied to the masticatory system in feeding and manipulation. These change over time, as diet (e.g. weaning) and paramasticatory behaviour change. Differences are known to exist in adults between Homo sapiens and Neanderthals, with Homo sapiens relatively more efficient at generating bite forces but less able to support the strains and deformations produced by such forces [3,4]. Differences in mid-facial form between these species might be expected to be influenced and to influence the distribution and magnitude of strains experienced during masticatory system loading. Since bone adapts to loads, such differences might underlie and contribute to the distinctive distributions of facial remodelling fields in both species and so, to differences in craniofacial growth. The present study explores the ontogeny of modern human and Neanderthal biting resistance as a preliminary to assessing potential associations between the distributions of facial strains arising from biting and facial remodelling among hominins. Our aim is to see if any differences in strains exist during post-natal ontogeny. We test the null hypothesis that modes and magnitudes of cranial deformation do not differ between modern humans and Neanderthals at each age stage when exposed to similar constraints. We used ontogenetic series of Neanderthals and modern humans ranging from newborn to adult. Using 44 landmarks and 201 sliding semilandmarks multivariate regressions of cranial shape on size were used to create three surfaces representing the mean infant, juvenile, and adult stages. These surfaces were converted into finite element models and constrained and loaded in a standardised way to simulate right first incisor and P4/dm2 biting. Applied forces and material properties were identical among models to control all variables except craniofacial form. We compared the resulting deformations, maps of von Mises strains and tensile and compressive strains in the maxilla. The resulting deformations differ in both mode and magnitude between modern humans and Neanderthals. In both incisor and P4/dm2 biting simulations, modelled strains decrease between infants and adults, as is to be expected given differences in size. The infant modern human presents higher strains than the infant Neanderthal over the anterior and inferior maxilla in both biting simulations. This is reversed in the juvenile models and the strains are more similar in adults. Finally, for both biting simulations, modern humans and Neanderthals deform differently, reflecting the differences in developed strains at each age stage. These findings reflect differences in the dynamics of facial growth between modern humans and Neanderthals. Moreover, the differences in strains in the infant, juvenile maxillae in modern humans and Neanderthals model may to some extent underlie and explain the differences in maxillary surface remodelling in these two species. Further work on a wider range of models and loading scenarios is needed to explore this issue further. Acknowledgments: We would like to thanks D. Shapiro, Joan T. Richtsmeier, G. Holoborow, S. Black and L. Scheuer for the information and access to Bosma and Dundee-Scheuer human skeletal collections. For the access and permission to their fossil materials, we would like to thanks the different institutes and their collaborators: Musée national d’Histoire naturelle, Musée de l’Homme (Paris, France), Musée national de la Préhistoire and his director J.J. Cleyet Merle, Museum of Natural History (London, UK), Patrick Semal and the Institut Royal des Sciences naturelles de Belgique (Bruxelle, Belgique), Jean Jacques Hublin and Philipp Gunz from Max Planck Institute for Evolutionary Anthropology (Leizpig, Germany), Luca Bondioli and the Pigorini Museum (Università di Padova, Italy).We would also like to thank the Dan David Center of Human Evolution and Biohistory Research, Shmunis family anthropological institute, Sackler Faculty of Medicine, Tel Aviv University (Tel Aviv, Israel) for granting access to Amud 1. Finally, we thank the NESPOS platform for access to modern and fossil material. References: [1] Krovitz, G. E., 2003. Shape and growth differences between Neandertals and modern humans: grounds for a species-level distinction?. Cambridge Studies in Biological and Evolutionary Anthropology, 320-342. [2] Lacruz, R.S., Bromage, T.G., O’Higgins, P., Arsuaga, J.L., Stringer, C., Godinho, R.M., Warshaw, J., Martínez, I., Gracia-Tellez, A., De Castro, J.M.B. and Carbonell, E., 2015. Ontogeny of the maxilla in Neanderthals and their ancestors. Nature communications, 6(1), 1-6. [3] Godinho, R. M.,Fitton, L. C., Toro-Ibacache, V., Stringer, C. B., Lacruz, R. S., Bromage, T. G., O'Higgins, P., 2018. The biting performance of Homo sapiens and Homo heidelbergensis. Journal of Human Evolution, 118, 56-71. [4] O'Connor, C. F., Franciscus, R. G., Holton, N. E., 2005. Bite force production capability and efficiency in Neandertals and modern humans. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists, 127(2), 129-151

The brain of René Descartes (1650): A neuro-anatomical analysis

International audienceThe skull of René Descartes is held in the National Museum of Natural History since the 19th c. Up to date, only anthropological examinations were carried out, focusing on the cranial capacity and phrenological interpretation of the skull morphology. Using CT-scan based 3D technology, a reconstruction of the endocast was performed, allowing for its first complete description and inter-disciplinary analysis: assessment of metrical and non-metrical features, retrospective diagnosis of anatomical anomalies, and confrontation with neuro-psychological abilities of this well-identified individual

Did Cro-Magnon 1 have neurofibromatosis type 1?

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The brain of René Descartes (1650): A neuro-anatomical analysis

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