The Neanderthal problem: Three-dimensional geometric morphometric models of cranial shape variation within and among species (Homo sapiens)

The taxonomic position of Neanderthals as a separate species or subspecies of H. sapiens is a matter of wide disagreement and has implications for modern human origins. Both morphological and genetic evidence suggest a wide separation between Neanderthals and modern humans. However, the interpretation of the genetic data has been questioned, while several authors see evidence for continuity or interbreeding between Neanderthals and Upper Paleolithic Europeans. The objectives of this study include: (a) the quantitative evaluation of Neanderthal features which are usually described qualitatively, achieved with the use of geometric morphometrics; and (b) the development of models of both intra- and inter-specific variation, to be applied to a comparison between Neanderthals and modern humans, in order to clarify the Neanderthal taxonomic position. Two such models were developed, based on modern human populations and on chimpanzee species and subspecies.; Geometric morphometrics enables the quantification of features that are difficult to measure with traditional caliper measurements. Landmarks were digitized on the temporal and occipital bone, and ridge curves along the posterior cranial profile. The data were processed using Generalized Procrustes Analysis, and the fitted coordinates were analyzed using multivariate statistical methods, including Principal Components, Canonical Variates and Discriminant Function analyses.; The temporal bone landmarks separate Neanderthals from modern humans most strongly. Some overlap exists in the posterior cranial profile, and extensive overlap is observed in the occipital bone landmarks. Neanderthals are consistently more distant from modern humans than the two chimpanzee species or subspecies, or any two recent human populations, are from each other. No morphological affinities were detected between Neanderthals and Upper Paleolithic Europeans. The latter are often quite distant from recent populations, probably due to high levels of cranial robusticity, but also very distant from Neanderthals. Limited similarities were found between Neanderthals and individual Central European Upper Paleolithic specimens. The Qafzeh/Skhul specimens are very distant from both Neanderthals and modern humans, probably due to retentions of primitive characters.; These findings support the hypothesis that Neanderthals represent a separate species from modern humans, although not unequivocally. Further investigation of the position of the Upper Paleolithic and Skhul/Qafzeh specimens is warranted.