Geometric variation of the frontal squama in the genus homo: Frontal bulging and the origin of modern human morphology

Abstract

The majority of studies of frontal bone morphology in paleoanthropology have analyzed the frontal squama and the browridge as a single unit, mixing information from different functional elements. Taking into account that the bulging of the frontal bone is often described as a species-specific trait of Homo sapiens, in this article we analyze variation in the midsagittal profile of the genus Homo, focusing on the frontal squama alone, using landmark-based superimpositions and principal components analysis. Our results demonstrate that anatomically modern humans are definitely separated from extinct human taxa on the basis of frontal bulging. However, there is minor overlap among these groups, indicating that it is necessary to exercise caution when using this trait alone to make taxonomic inferences on individual specimens. Early modern humans do not show differences with recent modern humans, and "transitional" individuals such as Jebel Irhoud 1, Maba, and Florisbad, show modern-like frontal squama morphology. The bulging of the frontal squama in modern humans may represent a structural consequence of more general cranial changes, or it could be a response to changes in the morphology of the underlying prefrontal brain elements. A subtle difference between Neandertals and the Afro-European Middle Pleistocene Homo sample is associated with flattening at bregma in the former group, a result that merits further investigation.

Neurocranial evolution in modern humans: the case of Jebel Irhoud 1

“…Its only neurocranial trait that departs from non-modern variation is the curvature of the frontal squama (Bruner et al, 2012). However, its degree of frontal bulging is only moderately higher than most extinct human taxa, and it could be within the normal range of the variability of the Middle Pleistocene species.…”

“…Its frontal morphology is generally comparable with the Middle Pleistocene variation, and a modern bulging of the frontal squama is likely to be partially related to the orientation of the bone as constrained by facial structures more than to the morphology of the bone per se. In fact, although modern humans are characterized by increased curvature of the frontal squama, this trait alone cannot provide a precise phylogenetic indication for individual specimens (Bruner et al, 2012).…”

Functional craniology and brain evolution: from paleontology to biomedicine

“…In terms of cranial profile, apart from a minor increase of the frontal curve (Bookstein et al, 1999; Bruner et al, 2013) the extent of globularity in Homo sapiens is due to bulging of the posterior vault surface, and is largely associated with a geometric dilation of the parietal bone (Bruner et al, 2004; Figure 4A ). Such globular shape of the neurocranium is a discrete feature of our species, and we have no evidence of any extinct taxon with a gradual or intermediate phenotype.…”

Shape analysis of spatial relationships between orbito‐ocular and endocranial structures in modern humans and fossil hominids

“…However, the frontal squama is more curved in modern humans when compared with non‐modern human species, which is likely a spatial consequence of reduction in the facial block, positioned below the braincase (Bruner et al. ). Also, a surface analysis of archaic and modern frontal lobes suggests that the degree of frontal bulging is proportional to the proximity between the frontal lobes and orbits, with extinct human species showing an intermediate morphotype between modern humans and living apes (Beaudet & Bruner, ).…”