Revealing the evolutionary processes which resulted in the derived morphologies that characterize the Neanderthal clade has been an important task for paleoanthropologists. One critical method to quantify evolutionary changes in the morphology of hominin populations is through evaluating morphological phenotypic diversity (i.e., disparity) in phylogenetically informative bones as a close proxy to neutral evolutionary processes. The goal of this study is to quantify the degree of disparity in the Neanderthal clade. We hypothesize that a reduction in bony labyrinth disparity is indicative of the underlying genetic variation resulting from bottleneck events. We apply a deformation-based geometric morphometric approach to investigate semicircular canal and vestibule shape of a chronologically broad sample of individuals belonging to the Neanderthal lineage. Our results identify a significant reduction in disparity after the start of Marine Isotope Stage 5 supporting our hypothesis of a late bottleneck, possibly leading to the derived morphology of Late Pleistocene Neanderthals. In some structures, morphological phenotypic disparity can reflect underlying genetic variation. Here, the authors apply deformation-based geometric morphometrics to the Neanderthal clade bony labyrinth to identify a disparity reduction around MIS5, possibly pointing to a population bottleneck.

Hence, the evidence provided by semicircular canal shape variation suggests the need to reconsider the scenario for the origin and evolution of the Neanderthal clade and highlights the role played by population structure. It also suggests that paleodemes might have started to locally evolve and diverge more frequently than previously understood, posing the possibility that some of them (i.e., Krapina individuals) might not have a direct phyletic relationship with later Neanderthal populations.