Dental variability of the Pleistocene hominins from Sierra de Atapuerca. Expression of trigonid crest patterns in Homo antecessor and evolutionary inferences.

Teeth are considered a valuable source of morphological characters with taxonomic and phylogenetic utility. Here we will focus on the dentition of the human fossils recovered from the Gran Dolina-TD6 site, assigned to Homo antecessor species and dated to MIS 21 or MIS 25. To date, Gran Dolina-TD6 stratum has provided more than 150 human remains characterized by a unique combination of plesiomorphic and apomorphic traits. This unique combination, as well as new hominin fossil findings in Africa and Eurasia, has helped us to refine the phylogenetic position of H. antecessor regarding the Early and Middle Pleistocene hominins from those continents. The main purpose of our study is to contribute to a better understanding of the human evolution in Europe during the Early and Middle Pleistocene transition with the help of the valuable Atapuerca fossil samples. Thus, using micro-computed tomography (microCT) and knowing that the trigonid crest pattern expression bears a significant taxonomic and phylogenetic value, we have analysed the enamel and dentine surfaces of different hominins. Our sample includes H. antecessor, Sima de los Huesos (SH), H. neanderthalensis and H. sapiens specimens. Regarding the trigonid crest pattern, our results confirm that SH and Neanderthal samples share the highest frequencies of the typical continuous middle trigonid crest (MdTC) at both the enamel and the enamel-dentine junction, a feature that has been considered as a ”Neanderthal feature”. The identification of this feature in the H. antecessor dental samples means that a continuous MdTC cannot be considered as a Neanderthal apomorphy. However, the lower frequencies of expression suggest that H. antecessor is phenetically closer to H. sapiens who would have preserved a primitive pattern. Recent studies suggest that H. antecessor is at (or close to) the node of divergence of H. sapiens and H. neanderthalensis, and this would explain that TD6 hominins share features with both the modern humans and the Neanderthal lineages.