Long anterior mandibular tooth roots in Neanderthals are not the result of their large jaws.

Tooth root length has been shown to taxonomically distinguish Neanderthals from modern humans. However, this may result from differences in jaw size between both taxa, although most previous studies have revealed a very low or non-existent correlation between tooth size and jaw size in recent modern humans. We therefore investigated, within a broader taxonomical frame, to what extent measurements on the anterior tooth roots and the symphyseal region covary. Our samples comprise permanent mandibular incisors and canines from Mauer, Neanderthals, and extant and fossil modern humans sensu lato. Using micro-computed tomography, we took linear and cross-sectional surface area measurements of the roots and the symphyseal region and calculated the root volume. We also measured 3D landmarks to quantify the overall size of the mandible using centroid size. Furthermore, we analyzed the relationship between root size and symphyseal shape, based on Procrustes shape variables of semi-landmarks along the symphyseal outline. Our results show that Neanderthals have significantly larger anterior tooth roots than recent modern humans in terms of root length, mid-sagittal surface area and volume, even after correction for mandibular size. In contrast, symphyseal height and width do not differ significantly between both taxa, whereas, without scaling, the mid-sagittal symphyseal surface area and the centroid size of the mandible do differ. Importantly, no significant correlation was found between any of the root and symphyseal measurements after correction for overall mandibular size. The shape analyses revealed that Neanderthals have a vertical symphyseal profile with an evenly-thick symphysis, whereas recent modern humans display an unevenly-thick symphysis, comprising a pronounced incurvatio mandibularis and a bony chin. These results suggest a negative evolutionary allometry for the recent modern human anterior root size. Therefore, root length and other root dimensions can be considered taxonomically relevant for distinguishing Neanderthals from modern humans.