Brain Size and the Human Cranial Base: A Prenatal Perspective Nathan Jeffery* and Fred Spoor Evolutionary Anatomy Unit, Department of Anatomy and Developmental Biology, University College London, London WC1E 6JJ, UK KEY WORDS fetal ontogeny; basicranium; relative encephalization; MRI; human evolution ABSTRACT Pivotally positioned as the interface be- tween 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 ori- entated 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 re- sults from a phylogenetic increase in the size of the brain and its subcomponents (infra- and supratentorial vol- umes) 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 pre- natal 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 in- creases of relative endocranial sizes across the sample, and the hypotheses therefore predict correlated variations of cranial base flexion and petrous orientation in accor- dance 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. Am J Phys Anthropol 118: 324 –340, 2002. © 2002 Wiley-Liss, Inc. The cranial base of modern humans differs mark- edly from that of other primates (Fig. 1). The petrous pyramids are more coronally oriented, the foramen magnum faces more inferiorly, and it shows a higher degree of midline basicranial flexion, i.e., the basi- occipital has a more vertically inclined orientation relative to the anterior cranial base (e.g., Keith, 1910; Duckworth, 1915; Zuckerman, 1955; Cam- eron, 1930; Ashton, 1957). Consequently, modern human crania have a deeper and wider posterior cranial fossa, than other primates, including the extant African apes (Dean, 1988; Ross and Ravosa, 1993; Spoor, 1997). Analysis of the evolutionary fac- tors underlying this uniquely derived morphology is complex, owing to the cranial base’s central position in the head as the interface between the face, the neurocranium, and the neck. Based on specific struc- tural demands that appear to follow from these re- lations a multitude of hypotheses have been pro- posed, which phylogenetically link the modern human morphology with, among others, brain ex- pansion (i.e., phylogenetic encephalisation), obliga- tory bipedalism, and facial orthognathism (e.g., Biegert, 1963; Du Brul, 1977; Dean, 1988; Ross and Ravosa, 1993). Most recently, some of these hypo- thetical associations have been assessed by analyz- ing interspecific trends among extant and fossil pri- mates (e.g., Ross and Ravosa, 1993; Ross and Henneberg, 1995; Spoor, 1997; Strait and Ross, 1999; Lieberman et al., 2000). Phylogeny can be seen as the product of successive ontogenies, as the accumulation of developmental change (Garstang, 1922). Indeed, a number of stud- ies advanced possible ontogenetic mechanisms un- derlying the phylogenetic origin of the modern hu- man cranial base (e.g., Ford, 1956; Moss, 1958; Scott, 1958; Knowles, 1963; Enlow and Hunter, 1968; Enlow, 1976). Such hypotheses generally take a mechanistic approach, in which the cranial base responds during ontogeny to the differential growth and development of closely related structures, such as the brain, or aspects of the face. These mechanis- Grant sponsor: Medical Research Council; Grant sponsor: Wellcome Trust; Grant sponsor: University of London Intercollegiate NMR Re- search Service Scheme, Queen Mary and Westfied College. *Correspondence to: Nathan Jeffery, Evolutionary Anatomy Unit, Department of Anatomy and Developmental Biology, University Col- lege London, Rockefeller Building, University St., London WC1E 6JJ, UK. E-mail ucgansj@ucl.ac.uk Received 30 April 2001; accepted 11 October 2001. DOI 10.1002/ajpa.10040 Published online in Wiley InterScience (www.interscience.wiley. com). AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 118:324 –340 (2002) © 2002 WILEY-LISS, INC.