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.

The endocast of the late Middle Paleolithic Manot 1 specimen, Western Galilee, Israel.

Studying endocasts has long allowed anthropologists to examine changes in the external topography and the overall size of the brain throughout the evolutionary history of hominins. The nearly complete calvaria of Manot 1 presents an opportunity to gain insights into the external brain morphology, vascular system, and dimensions of the brain of this late Middle Paleolithic hominin. Detailed size and shape analyses of the Manot 1 endocast indicate a modern Homo sapiens anatomy, despite the presence of some primitive features of the calvaria. Traits considered to be derived endocranial features for H. sapiens are present in Manot 1, including an elongated parietal sagittal chord with an elevated superior part of the hemisphere, a widened posterior part of the frontal lobes, a considerable development of the parietal reliefs such as the supramarginal lobules, and a slight posterior projection of the occipital lobes. These findings, together with data presented in previous studies, rule out the possibility of a direct Neanderthal ancestry for the Manot 1 hominin and instead confirm its affiliation with H. sapiens. The Manot 1 calvaria is more similar to that of later Upper Paleolithic H. sapiens than it is to the earlier Levantine populations of Skhul and Qafzeh. The late Middle Paleolithic date of Manot 1 provides an opportunity to analyze the recent developments in human cerebral morphology and organization.