Size does matter: metric variability of the upper second molars and differentiation of the Middle and Late Pleistocene Homo in Eurasia

Dental data are one of the most important sources of information on the evolution of ancient hominids. It preserves information about both the early stages of ontogenesis and the postnatal period of life. It is generally accepted that one of the main evolutionary trend within the genus Homo over the past two million years was reduction of teeth size. This trend substantially accelerated in modern Homo sapiens during the Upper Paleolithic and Neolithic. It is assumed that a number of factors could influence the teeth reduction process. However, no detailed understanding of its patterns, limits and chronological and territorial variability has been revealed so far. In this study, we analyze the variability of the upper second molar sizes in Neanderthals, Denisovans, Homo antecessor, Middle Pleistocene Homo from Sierra de Atapuerca (Gran Dolina), Homo erectus, Middle and Late Pleistocene Homo from China other than H. erectus, and compare them with the characteristics of the modern H. sapiens. The main goal of our analysis was to identify local chronological and geographical patterns in the evolution of the size of the second upper molars and assess their compliance with the general epochal trend. The results of the analysis showed that before the appearance of the Upper Paleolithic H. sapiens, two distinct local trends existed on the territory of Eurasia. The first trend, which can be detected in the European pre-sapiens humans, was a decrease in the molar size of the upper second molars. The second one, which got spread in the Middle Pleistocene in Asia, on the contrary, consisted of a sharp increase of the molar size, which formed the morphological specificity of the Denisovan lineage. No stable geographic differentiations of the size of the upper second molars are observed within H. sapiens sample.