Crown tissue proportions and enamel thickness distribution in the Middle Pleistocene hominin molars from Sima de los Huesos (SH) population (Atapuerca, Spain).

Laura Martín-Francés,Cecilia García-Campos,Clément Zanolli,Juan Luis Arsuaga,Mario Modesto-Mata,Marina Martínez De Pinillos,José María Bermúdez De Castro,María Martinón-Torres,Priscilla Bayle,Cyril Charles

doi:10.1371/journal.pone.0233281

Abstract

Dental enamel thickness, topography, growth and development vary among hominins. In Homo, the thickness of dental enamel in most Pleistocene hominins display variations from thick to hyper-thick, while Neanderthals exhibit proportionally thinner enamel. The origin of the thin trait remains unclear. In this context, the Middle Pleistocene human dental assemblage from Atapuerca-Sima de los Huesos (SH) provides a unique opportunity to trace the evolution of enamel thickness in European hominins. In this study, we aim to test the hypothesis if the SH molar sample approximates the Neanderthal condition for enamel thickness and/or distribution. This study includes 626 molars, both original and comparative data. We analysed the molar inner structural organization of the original collections (n = 124), belonging to SH(n = 72) and modern humans from Spanish origin (n = 52). We compared the SH estimates to those of extinct and extant populations of the genus Homo from African, Asian and European origin (estimates extracted from literature n = 502). The comparative sample included maxillary and mandibular molars belonging to H. erectus, East and North African Homo, European Middle Pleistocene Homo, Neanderthals, and fossil and extant H. sapiens. We used high-resolution images to investigate the endostructural configuration of SH molars (tissue proportions, enamel thickness and distribution). The SH molars exhibit on average thick absolute and relative enamel in 2D and 3D estimates, both in the complete crown and the lateral enamel. This primitive condition is shared with the majority of extinct and extant hominin sample, except for Neanderthals and some isolated specimens. On the contrary, the SH molar enamel distribution maps reveal a distribution pattern similar to the Neanderthal signal (with thicker enamel on the lingual cusps and more peripherally distributed), compared to H. antecessor and modern humans. Due to the phylogenetic position of the SH population, the thick condition in molars could represent the persistence of the plesiomorphic condition in this group. Still, more data is needed on other Early and Middle Pleistocene populations to fully understand the evolutionary meaning of this trait.

Highlights

The European Middle Pleistocene record is central to understand the timing and pattern of the emergence of the Neanderthals [1,2,3]
More data is needed on other Early and Middle Pleistocene populations to fully understand the evolutionary meaning of this trait
Tissue proportions assessed for the Sima de los Huesos (SH) maxillary and mandibular molars and the comparative samples are shown in Tables 2–4

Summary

Introduction

The European Middle Pleistocene record is central to understand the timing and pattern of the emergence of the Neanderthals [1,2,3]. The Atapuerca-Sima de los Huesos (SH) population provides a unique opportunity to trace the Neanderthal signature in the Middle Pleistocene. The human fossil assemblage comprises more than 6500 remains from at least 28 individuals recovered from a single stratigraphic level, LU6, [1]. The possible exclusion of SH population is due to the lack of shared cranial features with other European Middle Pleistocene fossils, such as Ceprano (see below) and Arago [1], and the noticeable differences between the Mauer and the SH mandibular samples [12]. While the mitochondrial DNA revealed that SH is the closest group related to the eastern Siberian-Denisovans [23], the nuclear DNA [18] confirmed the long maintained assumption that SH belonged to the Neanderthal clade [1, 6, 7, 17]

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Conclusion