Morphometric analysis of molars in a Middle Pleistocene population shows a mosaic of 'modern' and Neanderthal features

Abstract

Previous studies of upper first molar (M1) crown shape have shown significant differences between Homo sapiens and Homo neanderthalensis that were already present in the European Middle Pleistocene populations, including the large dental sample from Atapuerca-Sima de los Huesos (SH). Analysis of other M1 features such as the total crown base area, cusp proportions, cusp angles and occlusal polygon have confirmed the differences between both lineages, becoming a useful tool for the taxonomic assignment of isolated teeth from Late Pleistocene sites. However, until now the pattern of expression of these variables has not been known for the SH sample. This fossil sample, the largest collection from the European Middle Pleistocene, is generally interpreted as being from the direct ancestors of Neanderthals, and thus is a reference sample for assessing the origin of the Neanderthal morphologies. Surprisingly, our study reveals that SH M(1) s present a unique mosaic of H. neanderthalensis and H. sapiens features. Regarding the cusp angles and the relative occlusal polygon area, SH matches the H. neanderthalensis pattern. However, regarding the total crown base area and relative cusps size, SH M(1) s are similar to H. sapiens, with a small crown area, a strong hypocone reduction and a protocone enlargement, although the protocone expansion in SH is significantly larger than in any other group studied. The SH dental sample calls into question the uniqueness of some so-called modern traits. Our study also sounds a note of caution on the use of M(1) occlusal morphology for the alpha taxonomy of isolated M(1) s.

Keywords: Atapuerca; Sima de los Huesos; cusp angle; cusp area; dental reduction; metrics; occusal polygon area; teeth.

Fig. 1

Upper first molar from Sima de los Huesos site (left) showing the separation of the different cusp areas (middle), the occlusal polygon and the cusp angles (right) at the protocone (angle A), paracone (angle B), metacone (angle C) and hypocone (angle D). Mesial: top of the figure; distal: bottom; buccal: right; lingual: left. White bar: 5 mm.

Fig. 2

Principal component analysis of the individual absolute cusp areas, showing the first two principal components, the variance explained by each, and the vectors of the variable loadings. The first component (PC1) is associated with increase of all the variables (general size). The second component is mostly associated with the inverse proportions of the hypocone and the protocone.

Fig. 3

Schematic representation of evolution of the cusp angles. Arrows represent the main cusp shift in each of the represented groups. ERE, Homo erectus; SAP, Homo sapiens; ANT, Homo antecessor; SH, Sima de los Huesos; NEA, Homo neanderthalensis.