Brain size and the human cranial base: A prenatal perspective

Abstract

Pivotally positioned as the interface between the neurocranium and the face, the cranial base has long been recognized as a key area to our understanding of the origins of modern human skull form. Compared with other primates, modern humans have more coronally orientated petrous bones and a higher degree of basicranial flexion, resulting in a deeper and wider posterior cranial fossa. It has been argued that this derived condition results from a phylogenetic increase in the size of the brain and its subcomponents (infra- and supratentorial volumes) relative to corresponding lengths of the cranial base (posterior and anterior, respectively). The purpose of this study was to test such evolutionary hypotheses in a prenatal ontogenetic context. We measured the degree of basicranial flexion, petrous reorientation, base lengths, and endocranial volumes from high-resolution magnetic resonance images (hrMRI) of 46 human fetuses ranging from 10-29 weeks of gestation. Bivariate comparisons with age revealed a number of temporal trends during the period investigated, most notable of which were coronal rotation of the petrous bones and basicranial retroflexion (flattening). Importantly, the results reveal significant increases of relative endocranial sizes across the sample, and the hypotheses therefore predict correlated variations of cranial base flexion and petrous orientation in accordance with these increases. Statistical analyses did not yield results as predicted by the hypotheses. Thus, the propositions that base flexion and petrous reorientation are due to increases of relative endocranial sizes were not corroborated by the findings of this study, at least for the period investigated.

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

“…Furthermore, Enlow and others have argued that increases in relative brain size also determine cranial base flexion during primate development (Enlow and Hunter, 1968;Enlow and McNamara, 1973;Enlow, 1976;Enlow, 1990). However, despite the intuitive appeal of Enlow's ontogenetic version of the hypothesis, a recent systematic and appropriately sampled study does not support the notion that spatial-packing influences the prenatal modern human cranial base (Jeffery and Spoor, 2002). Jeffery and Spoor showed that petrous orientation remains independent of significant increases in relative brain size from 12 to 29 weeks gestation.…”

“…Research shows that cranial base angulation and petrous orientation vary independently of increases in infratentorial volume relative to posterior cranial base length during the second and early third trimesters of human prenatal development (Jeffery and Spoor, 2002). The question is whether the same can be said about fetal nonhominoid primates.…”

“…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.…”

“…Thus, if cranial base flexion and petrous orientation are simply the result of spatialpacking, as suggested by the interspecific evidence, then intuitively the same principle should apply to primate ontogeny, during which the brain expands in a similar fashion to that seen across extant and extinct primate species. However, a recent ontogenetic investigation has shown that variation in the human fetal basicranium is independent of large increases in absolute and relative brain sizes (Jeffery, 1999;Jeffery and Spoor, 2002). The authors explained the inconsistency with the interspecific findings by proposing that as well as being multifactorial, the influences on the basicranium also vary in their efficacy over ontogenetic time.…”

“…This implies that while the range and number of factors influencing the basicranium can be the same in the fetus, the juvenile, and the adult, the proportion of influence exerted by each individual factor differs among these age groups. For instance, Jeffery and Spoor (2002) proposed that conserved patterning of morphology principally governs the embryonic basicranium, that upper airway enlargement dominates the fetal basicranium, and that brain growth is the greater influence on the perinatal and postnatal basicranium. However, before examining these propositions in detail it would clearly be of benefit to find out whether the propensity of the human fetal basicranium to vary independently of brain size is common to other primates or unique to humans.…”