Basicranial flexion, relative brain size, and facial kyphosis in nonhuman primates

Abstract

Numerous hypotheses explaining interspecific differences in the degree of basicranial flexion have been presented. Several authors have argued that an increase in relative brain size results in a spatial packing problem that is resolved by flexing the basicranium. Others attribute differences in the degree of basicranial flexion to different postural behaviors, suggesting that more orthograde animals require a ventrally flexed pre-sella basicranium in order to maintain the eyes in a correct forward-facing orientation. Less specific claims are made for a relationship between the degree of basicranial flexion and facial orientation. In order to evaluate these hypotheses, the degree of basicranial flexion (cranial base angle), palate orientation, and orbital axis orientation were measured from lateral radiographs of 68 primate species and combined with linear and volumetric measures as well as data on the size of the neocortex and telencephalon. Bivariate correlation and partial correlation analyses at several taxonomic levels revealed that, within haplorhines, the cranial base angle decreases with increasing neurocranial volume relative to basicranial length and is positively correlated with angles of facial kyphosis and orbital axis orientation. Strepsirhines show no significant correlations between the cranial base angle and any of the variables examined. It is argued that prior orbital approximation in the ancestral haplorhine integrated the medial orbital walls and pre-sella basicranium into a single structural network such that changes in the orientation of one necessarily affect the other. Gould's ("Ontogeny and Phylogeny." Cambridge: Belknap Press, 1977) hypothesis, that the highly flexed basicranium of Homo may be due to a combination of a large brain and a relatively short basicranium, is corroborated.

Nasal Septal and Premaxillary Developmental Integration: Implications for Facial Reduction in Homo

“…the chondrocranium plays a key integrative role in craniofacial development and evolution in genus Homo (Moss and Young, 1960;Enlow, 1990;Ross and Ravosa, 1993;Lieberman et al, 2000;McCarthy and Lieberman, 2001;Spoor, 2002, 2004;Bastir et al, , 2010Lieberman et al, 2008), the influence of the nasal septum, as one component of the chondrocranium, has not been as widely considered. There is, nevertheless, fossil evidence to suggest that an integrated nasal septal/premaxillary complex may account for variation in facial size between archaic and recent modern humans as Neandertal and recent human subadults exhibit taxonomic variation in the timing of premaxillary suture fusion.…”

“…A variety of ultimate causal mechanisms have been proposed to account for this evolutionary dynamic (see Lieberman, 2008); however, our ability to test hypotheses regarding morphological change in Homo relies on our understanding of the proximate causal mechanisms that underlie phenotypic variation in fossil forms. To this end, the development and integration of the chondrocranium has been a primary focus in detailing developmental (i.e., proximate) changes upon which evolutionary (i.e., ultimate) mechanisms operate (Moss and Young, 1960;Enlow, 1990;Ross and Ravosa, 1993;Lieberman et al, 2000;McCarthy and Lieberman, 2001;Spoor, 2002, 2004;Lieberman et al, 2008).…”

Brain expansion and comparative prenatal ontogeny of the non-hominoid primate cranial base

“…The hypothesis suggests that the apparently derived modern human basicranium resulted from an overall geometric rearrangement of the skull to house successive phylogenetic increases in brain size given the same, or a relatively shorter length of cranial base. This suggestion has been substantiated by numerous studies of cranial base flexion and relative brain size across extant adult primates, and consequently spatial-packing has now become an established concept (Ross and Ravosa, 1993;Ross and Henneberg, 1995;Spoor, 1997;Lieberman et al, 2000;McCarthy, 2001). Nevertheless, despite all these corroborating interspecific findings, we still know little about the structural interplay between the primate brain and skull during ontogeny.…”

“…The above idea derives from hypotheses used to explain evolutionary changes and interspecific differences in the primate cranial base, typically with regard to the varied mechanical demands imposed by differences of posture, mastication, vocalization and brain size (see extensive reviews in Ross and Ravosa, 1993;Ross and Henneberg, 1995;Spoor, 1997;Lieberman and McCarthy, 1999;Strait and Ross, 1999;Lieberman et al, 2000;McCarthy and Lieberman, 2001;Jeffery and Spoor, 2002). These works have given useful insights into the evolutionary mechanisms responsible for the emergence of cranial features unique to Homo sapiens.…”

“…The interspecific and ontogenetic evidence pertaining to each of these hypotheses and their predictions are presented here. More detailed reviews are given elsewhere (see Ross and Ravosa, 1993;Ross and Henneberg, 1995;Spoor, 1997;Lieberman et al, 2000;McCarthy, 2001).…”