Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China

Abstract

Objectives: The aim of this study is to explore the root and root canal morphology of Homo fossil occupying China during the Middle Pleistocene period. Human occupation and evolutionary dynamics in East Asia during the Middle Pleistocene period is one of the most intriguing issues in paleoanthropology, with the coexistence of multiple lineages and regional morphs suggesting a complex population interaction scenario. Although premolar root and canal morphology has certain phylogenetic, taxonomic, and functional implications, its morphological diversity, possible evolutionary trend and characteristics regarding Middle Pleistocene hominins inhabiting East Asia are still insufficiently understood; where these populations fits within the Homo lineage (with respect to root and pulp canal structure) needs to be explored.Materials and methods: Using microtomography, we directly observed and assessed the nonmetric variability of root and canal forms in maxillary and mandibular premolars of Chinese Middle Pleistocene Homo (N = 19), and compared our observed variations with Eurasian Early Pleistocene specimens from the Asia continent (N = 1) and Java (N = 2), as well as with Neanderthals (N = 28) and recent modern humans (N = 67).Results: A total number of nine types of root-canal forms were recorded. As a whole, the Chinese Middle Pleistocene record shows an evolutionary trend toward a modern human-like condition (a reduction of root/canal number and a simplification of root surface structure). We documented primitive signals like high percentage of Tomes' root in lower premolars. A considerable occurrence of incompletely separated root branches and bifid root and canal apices, representing evolutionary transformation from multi-root to single-root condition was also noticed. The results were compared with previous publications on Early and Middle Pleistocene Homo in East Africa, North Africa, and Eurasia.Conclusion: This work provides new original data, incorporates the latest human fossil discoveries and suggests that analyzing the variation of premolar root structural organization, notably integrating together root/canal form and number, could possibly contribute to taxonomic and phylogenetic assessments. The mid-Middle Pleistocene populations, or "classic" Homo erectus, in our study show closer affinity to Early and Middle Pleistocene hominins in Eurasia, than to East African early Homo, which supports the suggestion that at least some of the Early Pleistocene hominin groups in Eurasia contribute to the later population; on the other hand, it is still difficult to clearly trace the evolutionary fate of those late Middle Pleistocene populations (roughly assigned as archaic Homo sapiens through a craniodental perspective). More comparable materials from the Early to Middle Pleistocene period as well as precise chronological framework is needed to further explore the evolutionary trends of archaic hominins in the Asian continent before the arrival of modern humans.

Structural analysis of premolar roots in Middle Pleistocene hominins from China

“…Recent observations on the root-canal configuration (Pan and Zanolli, 2019) as well as the qualitative and quantitative comparisons of external root morphology presented here reveal a modern-like trend of root fusion and root number reduction for the East Asian mid-to late Middle Pleistocene Homo sp. specimens.…”

“…They differ from Chinese H. erectus in that the latter taxon generally shows stout premolar root(s) with thick cementum (Xing et al, 2018;Pan and Zanolli, 2019). Moreover, when multiple root branches exist, the upper premolar roots of East Asian H. erectus often spread in different directions (including H. erectus from Yiyuan site reported by Xing et al, 2016).…”

“…Information on the crown outer and inner morphology were available for some specimens in previous publications (see Table 1). Root and pulp canal form of these specimens was evaluated using crosssectional analyses (Pan and Zanolli, 2019).…”

“…sapiens" shows a modern-like morphology (premolars exhibit symmetrical occlusal contour, simplified occlusal conformations, lack of cingulum, and gracile roots), while some archaic features are only weakly expressed (Liu et al, 2013;Xing et al, 2019). East Asian mid-Middle Pleistocene H. erectus populations show closer affinity to other Early and Middle Pleistocene hominins in Eurasia than to East African early Homo (Pan and Zanolli, 2019). The results of this previous study supported the hypothesis that at least some of the Early Pleistocene hominin groups in Eurasia may have contributed to modern human populations (Kaifu et al, 2005;Martinón-Torres et al, 2007).…”

“…A recent non-metric investigation (Pan and Zanolli, 2019) using 2D virtual sections showed that among Chinese Middle Pleistocene human dental remains, the expression of root/canal form could be roughly aligned in a chronological sequence --the earliest specimens show an archaic morphology shared with H. erectus in Asia while the mid-to late Middle Pleistocene (296-194 Ka BP) group assigned to "archaic H.…”