Jeffery, Nathan; (1999) Fetal development and evolution of the human cranial base. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

One of the principal morphological features that distinguishes Homo sapiens from other primates is the major reorganisation of its cranial base. Pivotally positioned as the interface between the neurocranium and the face, the cranial base has long been recognised as a key area to our understanding of the origins of modern human skull form. A number of authors have hypothesised that the unique architectural features of the modern human cranial base result from changes in the size of the brain. However, the ontogenetic mechanisms underlying this relationship have yet to be investigated. This study assesses a number of evolutionary hypotheses regarding the relationship between brain and cranial base morphology in a sample of human fetuses. Forty-two formalin preserved human fetuses, 53-205mm crown-rump length (11-23 weeks gestation +/- 1 week), were imaged using high resolution magnetic resonance imaging. This approach was used in this study in order to non-invasively image soft-tissues as well as base morphology. Transverse T2 (TE = 40msecs; TR = 9secs) weighted images were acquired contiguously through the head, interpolated and then resampled. Cranial base angle, angle of palate orientation, orientation of the petrous temporal bones, orientation of the orbits, sagittal orientation of the tentorium cerebelli, and infra and supratentorial brain volumes were measured using the images produced. Bivariate analysis of the data indicates that whilst many parts of the fetal cranial base are indeed interrelated with brain development some are not. In particular the study shows that the substantial 47-fold increase of brain size during the period studied does not correlate with an increase in base flexion. Thus, these results suggest that there are indeed ontogenetic relationships between brain development and cranial base morphology, but rather than being a simple continuous process these interactions likely involve different parts of the cranial base at different times.

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