| A quantitative assessment of the frontal bone morphology of Middle Pleistocene Homo was undertaken in order to address questions regarding the population structure and evolutionary history of these hominins. The results are interpreted within the framework of two broad evolutionary models that have been put forth to explain differences in trait distributions among populations of the Middle Pleistocene. The variation-based model explains morphological variability among regional groups as a product of population level processes such as isolation-by-distance and genetic drift, whereby geographically distinct groups differentiate morphologically but remain within one species. The other model interprets the pattern of morphological differences among Pleistocene Old World hominins as being indicative of separate evolutionary clades and therefore significant at the species level.;In this study, I analyzed 47 fossil and 234 modern human skulls. Using size-standardized measurements derived from Fourier analysis quantification of frontal bone outline tracings, I applied a partial-Mantel's distance matrix correlation test and calculated Moran's spatially autocorrelated I statistic to determine patterns of geographic differentiation in various features of the frontal bone in both fossil and modern populations. In addition, I used canonical variate analyses to determine which aspects of the frontal bone, if any, assisted in distinguishing among fossil groups.;The results of these analyses indicate that: (a) the various aspects of the frontal bone quantified here follow different patterns of geographic differentiation---namely, the sagittal aspects of the frontal bone follow isolation-by-distance, while the coronal aspects do not; (b) only the parasagittal features of the frontal bone demonstrate significant differences among taxonomic or temporal groups in discriminant function analyses, suggesting that they are more phylogenetically informative traits and should be given more weight---and also suggesting that it is necessary to be selective in the choice of frontal bone traits used to define and discriminate among fossil groups, and (c) the patterning of differentiation in these parasagittal aspects of frontal bone morphology of Middle Pleistocene Homo can be explained by a population structure characterized by isolation-by-distance. |
