Facial morphologies of Middle Pleistocene Europe: Morphological mosaicism and the evolution of Homo neanderthalensis

The phylogeny of the Middle Pleistocene hominins is a matter of intense scientific debate. Important phylogenetic and taxonomic uncertainties remain, not least due to conflicting results of phylogenetic analyses when methodologies or morphological focus differ. Geography has been proposed to play a key role in Middle Pleistocene hominin diversity, with a European group potentially ancestral to Neanderthals (Homo neanderthalensis) and an African group possibly ancestral to Homo sapiens, but the evidence is equivocal. In this study, we explore the connection between geography and facial morphology in Middle Pleistocene hominins with a particular emphasis on the potential Neanderthal affinities of the European group. Furthermore, to assess the impact of methodology on the results, we use a multimethod approach in which morphological affinities in both facial shape and discrete facial traits are assessed on a dataset consisting of 38 fossil and 20 recent hominin skulls divided into five groups (European and non-European Middle Pleistocene hominins, H. sapiens, H. neanderthalensis, and Homo erectus/Homo ergaster). Two main conclusions emerge from these analyses. First, methodological approach has a marked impact on the recorded pattern of morphological affinity, which may explain result discrepancies among previous studies. Second, this disparity may be caused by morphological mosaicism and polymorphism in the facial region of Middle Pleistocene hominins. The results provide some support for a closer connection between European Middle Pleistocene hominins and Neanderthals in terms of discrete facial traits, but not in overall facial shape, raising questions about the process of evolution of the Neanderthal facial phenotype. As a consequence of these results, we argue that greater attention needs to be paid to clarifying the broader evolutionary processes guiding hominin evolution during this period.