Studying the evolution of brain form requires paleoneruologists to rely on casts from the cranial cavity from fossil species. Due to the lack of soft-tissue preservation in fossils, descriptions of macroanatomy and cytoarchitecture are taken from comparative non-human primates to serve as hypothetical models of early hominin brain form. Using extant non-human primates as models for fossil species ignores the separation of lineages, any specific adaptations and lineage-specific evolution since divergence. Furthermore, extant species risk being relegated as ‘living fossils’ with the issue worsened by the absence of identifiable fossils for either Pan or Gorilla. The untenable assumption is that extant chimpanzee anatomy should resembles the original form prior to the Pan–Homo split. Nonetheless, comparison among living hominoids is still mandatory to investigate the evolutionary radiation of this taxon.
Previous published descriptions of chimpanzee sulcal patterns occur in classic literature but were based on only a few post-mortem dissections. Recently, Falk and colleagues aimed to increase knowledge of chimpanzee sulcal variation by describing sulcal patterns present in in-vivo Magnetic Resonance Imaging (MRI) from eight chimpanzees. Results suggested that, contrary to previous opinion, two sulci do occur in both chimpanzees and humans. To elaborate, these two sulci are the middle-frontal sulcus located in the frontal lobe, and lunate sulcus located between the parietal and occipital lobes.
No quantitative analyses were conducted in this study, but Falk et al. (2018) provide detailed descriptions of the variation between individuals, highlighting why descriptions based on only one or two individuals cannot be used to reliably describe the brain anatomy of a species. The authors argue the presence of the middle-frontal sulcus and lunate sulcus in chimpanzees invalidates previous claims that these sulci represent derived states found only in the human lineage. Further quantitative analyses with much larger samples, including both extant and fossil species will aid in a better understanding of the brain anatomy of humans and other great ape species.
This month we have published a study featuring the cortical extension and anatomy of the precuneus, dealing with its metrics as well as with its sulcal pattern. The analysis was based on a MRI sample of 50 adult humans from both sexes. Our previous works concerned the variation, position, and surface, as well as the phylogenetic differences in the midsagittal plane. Instead, in this survey metrics was assessed on the coronal plane, “cutting” the precuneus in its anterior, middle and posterior sections, taking into account its curvature. The lateral (inner) extension of the precuneus was measured along the subparietal sulcus and its height was measured from the subparietal sulcus to the endocranial wall. Then a set of 10 two dimensional landmarks were digitized on the middle section along the outline of the parietal lobe, to analyze the correlation between outer brain profile and inner precuneus dimensions. We found that, on average, the precuneus extends 14 mm laterally and 36 mm vertically. It is wider on the anterior and middle sections, and usually larger on the right hemisphere, possibly due to the length of the fold (surface area) rather than to the thickness of the grey matter. The precuneus height influences the outer brain morphology (vertical stretching), but the subparietal size apparently has no influence on the external outline. Therefore, at least the former trait could be investigated in paleoneurology, by indirect inference on the inner dimensions as evaluated through their external effects. The lateral (parasagittal) surface of the upper parietal lobules seems to be unrelated to the size of the precuneus. Therefore, when a change of these areas is detected (like in Neandertals) the intraparietal sulcus is a better candidate as the cortical element involved in the morphological variations.
The sulcal pattern was analyzed on the average density projection of the 5 most sagittal stacks (5 mm of the cortex), a thickness which displays most of the sulcal features. Three characteristics were taken into consideration: the connections of the subparietal sulcus, the connections of other sulci in the precuneus, and the general sulcal scheme. Some of these features have been analyzed in other surveys, but the consideration of other folds beyond the subparietal one is specific of this study. Roughly half of the precuneus sulci that reach the external surface are not linked to the subparietal sulcus. Contrary to other studies which found higher frequencies of an H-like pattern of the subparietal sulcus, we found a larger proportion of an inverted-T pattern (subparietal sulcus connected with one precuneus sulcus). The differences between studies might be due to random effects of the samples, because of the relevant individual variability of these traits. The left hemisphere displays more sulci reaching the external surface while the right hemisphere displays deeper folding. The anterior area shows more sulcal complexity than the posterior one. There seems to be no relationship between the size of the subparietal sulcus and its folding pattern, and these characteristics might be hence influenced by genetics or folding biomechanics.