Tooth crown tissue proportions and enamel thickness in Early Pleistocene Homo antecessor molars (Atapuerca, Spain)

Abstract

Tooth crown tissue proportions and enamel thickness distribution are considered reliable characters for inferring taxonomic identity, phylogenetic relationships, dietary and behavioural adaptations in fossil and extant hominids. While most Pleistocene hominins display variations from thick to hyper-thick enamel, Neanderthals exhibit relatively thinner. However, the chronological and geographical origin for the appearance of this typical Neanderthal condition is still unknown. The European late Early Pleistocene species Homo antecessor (Gran Dolina-TD6 site, Sierra de Atapuerca) represents an opportunity to investigate the appearance of the thin condition in the fossil record. In this study, we aim to test the hypothesis if H. antecessor molars approximates the Neanderthal condition for tissue proportions and enamel thickness. To do so, for the first time we characterised the molar inner structural organization in this Early Pleistocene hominin taxon (n = 17) and compared it to extinct and extant populations of the genus Homo from African, Asian and European origin (n = 355). The comparative sample includes 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 TD6 molars (tissue proportions, enamel thickness and distribution). TD6 permanent molars tend to exhibit on average thick absolute and relative enamel in 2D and 3D estimates, both in the complete crown and the lateral enamel. This condition is shared with the majority of extinct and extant hominin sample, except for Neanderthals and some isolated specimens. However, while the total crown percentage of dentine in TD6 globally resembles the low modern values, the lateral crown percentage of dentine tends to be much higher, closer to the Neanderthal signal. Similarly, the H. antecessor molar enamel distribution maps reveal a relative distribution pattern that is more similar to the Neanderthal condition (with the thickest enamel more spread at the periphery of the occlusal basin) rather than that of other fossil specimens and modern humans (with thicker cuspal enamel). Future studies on European Middle Pleistocene populations will provide more insights into the evolutionary trajectory of the typical Neanderthal dental structural organization.

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

“…On the contrary, studies on modern humans relate their relative thick enamel to a unique odontogenetic process and to the extreme dental reduction [29,[38][39][40]. Previous results on crown tissue proportions in H. antecessor (from the TD6 level of the Gran Dolina site, Atapuerca) and SH dentition [41,42] showed that TD6 molars shared with Neanderthals some histological aspects, such as the lateral enamel thickness and the enamel thickness distribution [41]. In addition, García-Campos and colleagues [42] described a Neanderthal-like, thin pattern of enamel thickness in TD6 and SH canines.…”

Endostructural assessment of a hominin maxillary molar (StW 669) from Milner Hall, Sterkfontein, South Africa

“…The thickest enamel in StW 669 is found along the oblique ridge connecting the metacone and the protocone, on the lingual aspect of the protocone and on the distal aspect of the hypocone. In these respects, the enamel distribution of StW 669 fits the fossil and extant human condition by sharing thick enamel on the lingual aspect of the crown 13,14,17 but differs from Australopithecus and Paranthropus that have thicker enamel at the cusp tips 9 .…”

“…1.26). 2 To discuss the taxonomic attribution of StW 669, qualitative and quantitative results derived from the endostructural three-dimensional (3D) analysis of StW 669 are compared to published data on tissue proportions, enamel distribution and enamel-dentine junction morphology of the permanent M 1 crowns of Australopithecus africanus 9,12 , Paranthropus robustus 9 , Homo erectus 13 , Homo antecessor 14 , Homo neanderthalensis 11,14,15 , European Middle Pleistocene Homo 16 and modern Homo sapiens 11,14,[17][18][19] . Unfortunately, to the best of our knowledge, there is no information available on the endostructure of the first molar of Homo naledi.…”

“…9 Three variables were measured in 3D and 2D 8,9,21 : volume (Ve, mm 3 ) and area (c, mm 2 ) of the enamel cap; volume (Vcdp, mm 3 ) and area (b, mm 2 ) of the coronal dentine that includes the coronal aspect of the pulp chamber; and surface (SEDJ, mm 2 ) and length (e, mm) of the enameldentine junction. We then calculated two indices of enamel thickness: 3D (3D AET, mm) and 2D (2D AET, mm) average enamel thickness, calculated Sources: Published data from Olejniczak et al 9 , Zanolli et al 11,13 , Martín-Francés et al 14 Ve, volume of the enamel cap; Vcdp, volume of the coronal dentine that includes the coronal aspect of the pulp chamber; SEDJ, surface of the enamel-dentine junction; 3D AET, 3D average enamel thickness; 3D RET, 3D relative enamel thickness Sources: Published data from Olejniczak et al 8,9 , Zanolli et al 13 , Martín-Francés et al 14 , Smith et al 16,19 C, area of the enamel; a, area of the coronal dentine that includes the coronal aspect of the pulp chamber; e, length of the enamel-dentine junction; 2D AET, 2D average enamel thickness; 2D RET, 2D relative enamel thickness as the ratio between Ve/c and SEDJ/e; 3D (3D RET) and 2D (2D RET) relative enamel thickness, obtained through the ratios 3D AET/(Vcdp) 1/3 *100 and 2D AET/b 1/2 *100 and allowing direct, scale-free comparisons. [8][9]21 Intraand inter-observer tests for measurement accuracy run by two observers in previous published studies using a similar analytical protocol revealed differences less than 5%.…”

Enamel thickness variation in the deciduous dentition of extant large‐bodied hominoids

“…The prevalence of teeth in the fossil record, coupled with the intimate connection between tooth crown anatomy and dietary proclivities, has resulted in enamel thickness featuring prominently in studies of hominoid evolution. However, this research has focused almost exclusively on the permanent dentition (e.g., for extant hominoids: Molnar & Gantt, 1977; Martin, 1985; Shellis et al, 1998; Gantt, 1986; Schwartz, 2000a; Kono, 2004; Smith, Olejniczak, Martin, & Reid, 2005; Smith, Olejniczak, Reid, Ferrell, & Hublin, 2006; Smith, Olejniczak, Reh, Reid, & Hublin, 2008; Smith, Kupczik, Machanda, Skinner, & Zermeno, 2012; Kono & Suwa, 2008; Olejniczak, Tafforeau, Feeney, & Martin, 2008; for fossil apes: Martin et al, 2003; Smith, Martin, & Leakey, 2003; Olejniczak, Smith, Wei, et al, 2008; Zanolli et al, 2015; for fossil hominins: Beynon & Wood, 1986; Grine & Martin, 1988; Conroy, 1991; Macho & Thackeray, 1992; Olejniczak, Smith, Skinner, et al, 2008; Olejniczak, Smith, Feeney, et al, 2008; Smith, Olejniczak, et al, 2012; Skinner et al, 2015; Pan et al, 2016; Martín‐Francés et al, 2018; Zanolli et al, 2018). Comparatively little research effort has been devoted to tissue proportions, including the amount of enamel, within deciduous teeth (Aiello, Montgomery, & Dean, 1991; Fornai et al, 2014; Gantt, Harris, Rafter, & Rahn, 2001; Grine, 2005; Mahoney, 2010, 2013).…”