Kamath, Mulky Shruti; (2021) Virtual Analysis of the Lower Third Premolar Crown Morphology of the Atapuerca Hominins: Taxonomic and Phylogenetic Inferences for the Early and Middle Pleistocene in Europe. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Atapuerca sites continue to figure prominently in the understanding of European hominin evolution. The dental studies of these Early Pleistocene (Homo antecessor) and Middle Pleistocene [Sima de los Huesos (SH)] hominins have been particularly valuable in providing phylogenetic contexts for the last common ancestor (LCA) of modern humans and Neanderthals and the investigation of Neanderthal roots in the Middle Pleistocene. Advances in micro-computed tomography (μCT) have offered further opportunities to examine the outer and inner structural morphology of these specimens. However, the taxonomically diagnostic lower premolars have yet to be fully explored by these means. This research project considers the lower third premolar (P3) crown morphology of the Atapuerca hominins through the analysis of the enamel dentine junction (EDJ) discrete traits, and the dental tissue proportions, to understand their taxonomic and phylogenetic position. The EDJ discrete traits of the H. antecessor P3s are morphologically distinct from those of the SH hominins and the Neanderthals. H. antecessor preserves primitive features whereas SH hominins show closer affinities with Neanderthals, ratifying their phylogenetic link. The study of the dental tissue proportions show that the thin enamel condition, which was classically considered a distinctive trait for the Neanderthal clade, was already present in H. antecessor. In contrast, the dental tissue proportions of the SH hominins are more similar to that of modern humans, despite being phylogenetically closer to the Neanderthals. We propose several factors that may contribute to the relatively thick enamel condition of the SH hominins, such as the relative dentine reduction due to small postcanine dentition in the SH population, and a general derived condition of the SH P3s compared to the LCA. Our study also corroborates previous findings in showing the presence of sexual dimorphism in dental tissue proportions in a modern human population. However, after accounting for allometric scaling, this dimorphism is significantly reduced. Accordingly, this study suggests that what is perceived as sexual dimorphism may be the result of allometric scaling. Furthermore, when comparing the modern human population to the Middle Pleistocene (MP) hominins and Late Pleistocene (LP) groups, significant differences between populations are noted even after accounting for non-isometric scaling. Several factors such as small sample sizes of comparative MP and LP populations, or differences in sex composition of these comparative populations, may contribute to such results. 3 Our study is compatible with H. antecessor being closely related to the LCA as it shows a general primitive conformation in the EDJ and a Neanderthal-like tissue distribution that links the Early Pleistocene populations from Europe to the Neanderthal clade. Regarding theMiddlePleistocene,theP3 morphologyofseveralkeyspecimensfromtheEurasianMP, including Mauer, show morphological similarities to the SH hominins and Neanderthals. As such, and based on the P3 morphology alone, H. heidelbergensis as a taxon may not be considered distinctive. However, we acknowledge the high cranial and mandibular variability in the Middle Pleistocene record, so the validity of this taxon remains a matter of deliberation.

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