New techniques for old bones: Morphometric and diffeomorphometric analysis of the bony labyrinth of the Reilingen and Ehringsdorf Neandertals

Abstract

Neandertals are known to possess very distinctive traits in their bony labyrinth morphology, such as an inferiorly positioned posterior canal and a very low number of turns in the cochlea. Hence, the inner ear has been often used to assess the Neandertal status of fragmentary fossils. However, the evolutionary processes leading to this "classic Neandertal" morphology are still debated. Middle Pleistocene earliest Neandertal specimens such as Reilingen and Ehringsdorf are key to resolving this issue; however, their phylogenetic affinities remain contentious. Here, we analyze the bony labyrinth of these two individuals to provide new insights into the evolution of the Neandertal clade. We employed diffeomorphic deformation analysis to study the semicircular canal and vestibule shape, and morphometric variables to analyze the cochlea, comparing them to a sample including Sima de los Huesos humans, Krapina Neandertals, and late Neandertals. Our results reveal that both Reilingen and Ehringsdorf possess almost a fully derived, late Neandertal-like inner ear morphology. This finding challenges a simple anagenetic model of trait accumulation and allows refining the mosaic pattern of inner ear evolution. The advanced morphology of these early specimens, coupled with the unique mosaic of features in the Krapina sample, suggests the existence of a highly variable Middle Pleistocene metapopulation, with the classic Neandertal labyrinthine morphology that emerged early in their evolution, but was fixed only post-Krapina, possibly after a bottleneck. This study underscores the complex, mosaic nature of Neandertal evolution, highlighting a disconnect between the evolutionary trajectories of the external cranium and the inner ear.

Keywords: cochlea; diffeomorphism; human evolution; inner ear; semicircular canals.