The cranial functional axis

Pereira Pedro and Bruner J Anat 2021

The head is a complex biomechanical system, formed by a delicate network of soft and hard tissues. Furthermore, its functional and structural relationships extend to the whole body, influencing crucial adaptations ranging from breathing to locomotion. Modern humans are characterized by large parietal bones, at least in part associated with the enlargement of the parietal cortex. Such parietal expansion tilts the orientation of the brain that, in turn, is expected to influence the orientation of the head. In a new geometric morphometric analysis, we investigate the relationships between frontal bone, parietal bone, orbits and cranial base, in a sample of adult modern humans, to test whether and how the size of the parietal region is correlated with the morphological features of the orbital block. Results suggest that the dimension of the parietal bone exerts an influence on the orientation and rotation of the orbital region, but not on its form. In this case, we should hence consider the conceptual and evolutionary differences between a strict morphological integration (a reciprocal influence on shape) and a topological integration (a reciprocal influence on position). This survey supports further the hypothesis that an expansion of the parietal region in modern humans could have involved or required a rearrangement of the head functional axis, namely a rotation of the facial block and changes in the face vs braincase spatial organization. More info in The Skull Box.

Eyes and tools

Eye and tools (Bruner 2021)

This week, we present a new research line for the lab, we have been developing in the last couple of years: visual attention and cognitive archaeology. After our previous surveys on haptics and Lower Paleolithic stone tools, we have now begun to investigate how early human technology influences our visual exploration patterns, by using eye-tracking to analyze the schemes of saccades and fixations according to the dwell time and fixation counts. We analyzed the pattern of visual exploration in choppers and handaxes, through vision-only or associated with hand manipulation. The two tool types trigger different visual exploration patterns. In general, the central region of the tool is the more explored one, followed by the tool tip. The knapped region triggers more visual attention than the raw cortex. We found no differences in the visual exploration patterns when the tool is handled, or between males and females. Beyond the central issue of visual attention and cognition, this research line has two main wider targets. The first one is to promote experimental and quantitative approaches in cognitive archaeology, a field that is still largely based on theoretical perspectives but rarely brought into labs and stats. Quantification is mandatory to evaluate theories and hypotheses according to falsification criteria and experimental procedures.  The second aim deals with theories on extended cognition, trying to understand if, how much, and in what way tools are integrative parts of our cognitive system. If humans have evolved a specialized prosthetic capacity, cues of its visuospatial and body integration functions should be still detectable in the changes associated with the paleontological and archaeological record.

Arothron

Arothron (Profico et al 2021)

Arothron is a new R package for geometric morphometrics and virtual anthropology. It is “an R software suite meant to process and analyze digital models of skeletal elements [that] includes tools to digitally extract virtual cavities such as cranial endocasts, to statistically align disarticulated or broken bony elements, and to visualize local variations between surface meshes and landmark configurations”. Among the examples provided in this presentation paper, there is a morphological comparison between the right hemisphere in Neanderthals and modern humans.

Craniosynostotic vessels

Premature fusion of the sutures deforms cranial morphology and disrupt skull organization, with mild to severe effects. The fine morphogenetic balance between hard and soft tissues is altered, generating spatial, structural, and functional conflicts during growth and development. The vascular system is also influenced, suffering consequences associated with oxygenation, thermal regulation, or endocranial pressure. After our extensive survey on the presence and expression of craniovascular traits in normal adult humans, now we have published a study dealing with the same features in craniosynostotic skulls. We considered those craniosynostoses that deform the skulls into a rounder (brachycephalic) and longer (dolichocephalic) shape, and check for differences in the prevalence of endocranial vascular traces commonly investigated in bioarchaeology and paleoneurology. Craniosynostotic skulls display a larger vascular diversity, when compared with normal subjects. The posterior middle meningeal artery is more developed in the dolichocephalic phenotype, probably as a passive consequence of spatial redistribution. The most interesting differences probably deal with the drainage system. The craniosynostotic skulls present a frequent occipito-marginal venous drainage, and larger or more frequent emissary canals. These features are likely to be functional responses to special physiological conditions associated with endocranial pressure and blood flow constraints. Because of their involvement in many structural and functional aspects, these features and their variations have direct implications in human evolution, population biology, and medicine.

Laszlo Zaborszky

It’s with sadness and gratitude that we announce Professor Laszlo Zaborszky has stepped down from his role as Editor-in-Chief of Brain Structure and Function as of 30th June 2021, after over fourteen years at the helm of the journal. As many of you will know, Laszlo co-founded the journal back in 2007 along with the late Professor Karl Zilles, when it relaunched from the previous title of Anatomy & Embryology. Between the two of them, and in collaboration with the journal’s original Springer publishing editor Andrea Pillman and wider Editorial Board, they managed to successfully develop the journal and grow it not just in submissions, but in Impact Factor and reputation throughout the neuroscience community …

[Keep on reading this article in Brain Structure and Function]

Nesher Ramla

A new fossil from Israel: Nesher Ramla. There is a right parietal bone and fragments of the left one (NR-1), and a mandible (NR-2). They likely belong to the same individual. The bones are dated to 126 ka. These fossils display features that are typical of Middle Pleistocene archaic humans (i.e., Homo heidelbergensis and Homo erectus), but with additional traits that can be frequently found in Neanderthals. The parietal bone is large, but with an archaic morphology, if we consider its general form, the bone thickness, the anatomy of the vascular traces, or the gross proportions. A cluster analysis of its principal anatomical features suggests a morphological similarity with Arago. Taking into account its age and the admixture of archaic and derived features, the main hypothesis concerns the possibility that this fossil could represent a remnant population ancestral to Neanderthals. Mosaic evolution is not only an anatomical or taxonomic concept, but has also a geographic aspect, suggesting differential rates and times of change on a spatial (and rather local) scale. Diversity is a multidimensional issue, and any phylogenetic unit has its own evolutionary fingerprint. Here a post by Ann Gibbons, and a commentary by Marta Mirazón Lahr.

Paranthropus

A recent shape analysis on the endocasts of OH5 and SK1585 (two members of the genus Paranthropus) suggests that their overall morphology is perfectly within the range of variation of extant chimps. In contrast, STS5 (a specimen that is representative of the genus Australopithecus) displays an endocranial shape which is intermediate (exactly halfway!) between chimps and modern humans, and out of the range of both genera. Therefore, this similarity between Paranthropus and Pan may suggest a lack of derived endocranial traits in the robust australopiths, at least in relationship to the human lineage. Of course, paranthrops had a very specialized (and odd!) skull, so we must consider that their important bony superstructures may have introduced very peculiar and autoapomorphic constraints in the morphogenetic relationships between face and braincase, influencing the brain gross morphology in some specific regions (like for example, at the frontal lobes).

Galvanic shapes

Silva Gago et al_AJHB2021

Our first research article on electrodermal activity concerned arousal/attention differences when handling different Paleolithic tools. The second was on the influence of hand size and sex. Now, the American Journal of Human Biology has published the last one of the trilogy, on the influence of tool shape. The main geometric features in Lower Paleolithic tools are i) elongation and ii) the position of the maximum thickness. Interestingly, none of these two factors seem to influence the level or arousal or attention when grasping the tools during haptic exploration. The features that trigger an electrodermal reaction are the general tool size (a spatial issue), the tool weight (a gravitational issue), and the morphology of the tool base (a grasping issue). Such electrophysiological responses are supposed to be associated with cognitive brain-body feedback, and possibly with those sensing capacities that support a good prosthetic ability. These studies are part of a wider frame investigating visuospatial integration in cognitive archaeology.

The great mediator

Corpus callosum

Dmanisi

Marcia Ponce de León and colleagues have now published a very comprehensive study of the endocasts from Dmanisi. The article includes a part on sulcal patterns and brain-skull relationships, and a part on endocranial shape variation. The endocast shape analysis is very extensive, on apes and humans, a valuable study in paleoneurology, indeed. It gives an inclusive view of the endocranial variability in hominoids, at both intra-and interspecific level. In this shape analysis, the Dmanisi endocasts show a morphology definitely within the early human range (Homo ergaster/erectus). The sulcal analysis shows, instead, that the Dmanisi precentral sulcus crosses the coronal suture, as in apes, while in humans it is positioned behind this cranial reference. They conclude that the Dmanisi people (and hence early Homo) had not evolved a large frontal cortex (possibly, a lack of expansion of the Broca’s region). So, they suggest that an expansion of the frontal lobe happened after the origin of our genus. Such expansion, associated with later hominids after 1.5 My, would have involved the frontal and the parietal lobes at once.

The spatial relationship between the brain sulci and the skull bones is a very interesting topic, because of its relevance in functional craniology, and because it is something that can be directly investigated in fossils. However, it must be taken into account that the spatial relationships between skull and brain are not only due to brain anatomy, but also to multiple factors (facial development, cranial base flexion and so on) that generate a large and complex set of spatial constraints. In fact, the same team published two years ago a study suggesting caution when considering together the cranial and cerebral landmarks to make paleoneurological inferences. The frontal lobe morphology is precisely a good case-study, because it is influenced by the spatial relationships with the orbits and upper face. Wider frontal lobes in modern humans and Neandertals could be, for example, a structural consequence of having the frontal cortex right above the orbital roof, and not a sign of real neurobiological changes. It is not trivial, in this sense, that the endocasts from Dmanisi, although having such a plesiomorph pattern between sulci and suture, then display an overall “human-like” brain geometry. This relationship between the precentral sulcus and the coronal suture is really interesting, but maybe more caution should be put when giving for granted that it is due to brain cortical evolution alone. Needless to say that, furthermore, the taxonomy of Dmanisi is still uncertain. There are still doubts on whether they are all from the same species, or even whether they are really part of the human genus. Here a commentary by Amélie Beaudet, a latest news article by Michael Price, and a ScienceNews post.


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