An Analysis of Morphological Differences in the Femoral Diaphyseal Midshaft Between Fossil and Modern Humans

Abstract

The understanding of morphological variation between fossil and modern humans is critical to the discussion of evolutionary processes within the Homo lineage. Neanderthals are recognizably distinct in their morphology from Late Pleistocene Homo sapiens and recent modern humans, especially in the femoral midshaft cross-section. These lineages are often assumed to be independent when applying statistical methods to account for these morphological differences. The shared evolutionary history of fossil and modern humans, however, increases the likelihood that the distinction in observable traits cannot entirely be attributed to divergent selective pressures; morphological variation is obscured by phylogenetic signal and therefore violates assumptions of statistical independence. In order to understand the observed variation between these groups, phylogenetic signal must be taken into account.

To test for phylogenetic signal in the femoral cross-section in recent human evolution, geometric morphometric shape data was taken from fossil groups (Neanderthals and Late Pleistocene Homo sapiens) and compared to a global recent modern human sample. This shape data was isolated from other morphological constraints through Procrustes superimposition and mapped onto a phylogeny created from mitochondrial genomes from geographic and temporally comparable populations. The trends in femoral midshaft cross-sectional shape variation were examined through Principal Component Analyses and Canonical Variate Analyses and showed concentrated shape change in the region of the pilaster. A permutation test indicated that phylogenetic signal is present in the femoral midshaft shape. The presence of this signal between fossil and modern human groups stresses the importance of taking into account, and controlling for, shared evolutionary history in comparative analyses. In order to understand the nature of the phylogenetic signal present in the femoral midshaft, independent contrasts were calculated and a multivariate regression was performed to test for the impact of evolutionary allometry.

The results showed that allometric changes throughout modern and fossil human evolutionary history had an insignificant impact on changes in shape, explaining only 20% of shape variation. This, therefore, suggests that the observed shape changes at the femoral midshaft between these groups are not due to evolutionary allometry but can be attributed to other factors, such as behavioral, genetic, or environmental pressures.