A Geometric Morphometric Analysis of Hominin Lower Molars: Evolutionary Implications and Overview of Postcanine Dental Variation

Journal of Human Evolution 82 (2015) 34e50

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jhevol

A geometric morphometric analysis of hominin lower molars: Evolutionary implications and overview of postcanine dental variation

* Aida Gomez-Robles a, , Jose María Bermúdez de Castro b, María Martinon-Torres b, Leyre Prado-Simon c, Juan Luis Arsuaga d a Department of Anthropology, The George Washington University, 800 22nd St. NW, Washington, DC 20052, USA b Centro Nacional de Investigacion sobre la Evolucion Humana, Paseo de la Sierra de Atapuerca S/N, 09002 Burgos, Spain c Stomatology Department, Dentistry Faculty, University of Granada, Spain d Centro Mixto UCM-ISCIII de Evolucion y Comportamiento Humanos, Avd. Monforte de Lemos 5, Pabellon 14, 28029 Madrid, Spain article info abstract

Article history: Lower molars have been extensively studied in the context of hominin evolution using classic and Received 3 March 2014 geometric morphometric analyses, 2D and 3D approaches, evaluations of the external (outer enamel Accepted 17 February 2015 surface) and internal anatomy (dentine, pulp chamber, and radicular canals), and studies of the crown Available online 1 April 2015 and root variation. In this study, we present a 2D geometric morphometric analysis of the crown anatomy of lower first, second, and third molars of a broad sample of hominins, including Pliocene and Lower, Keywords: Middle, and Upper Pleistocene species coming from Africa, Asia, and Europe. We show that shape Atapuerca variability increases from first to second and third molars. While first molars tend to retain a relatively European Middle Pleistocene Generalized Procrustes analysis stable 5-cusped conformation throughout the hominin fossil record, second and third molars show Dental anthropology marked distal reductions in later Homo species. This trend to distal reduction is similar to that observed Homo antecesor in previous studies of premolars and upper second and third molars, and points to a correlated reduction Sima de los Huesos of distal areas across the whole postcanine dentition. Results on lower molar variation, as well as on Homo heidelbergensis other postcanine teeth, show certain trends in European Pleistocene populations from the Atapuerca Neanderthals sites. Middle Pleistocene hominins from Sima de los Huesos show Neanderthal affinities and strong dental reduction, especially in the most distal molars. The degree of dental reduction in this population is stronger than that observed in classic Neanderthals. Homo antecessor hominins from Gran Dolina-TD6 have primitive lower teeth that contrast with their more derived upper teeth. The evolutionary implications of these dental affinities are discussed in light of recent paleogenetic studies. © 2015 Elsevier Ltd. All rights reserved.

Introduction generally accepted utility of dental traits in the classification of specimens and species, as well as in describing possible evolu- The central role of dental anthropology in the broader frame- tionary scenarios (Suwa et al., 1994, 1996; Bailey, 2002a, 2004; work of paleoanthropology is demonstrated by the profusion of Bailey and Lynch, 2005; Guatelli-Steinberg and Irish, 2005; Kaifu articles aiming to address taxonomic and phylogenetic questions et al., 2005; Moggi-Cecchi et al., 2006; Martinon-Torres et al., via dental morphometry. Dental evolution is subject to several in- 2007a, b; Moggi-Cecchi and Boccone, 2007; Bailey et al., 2009; fluences, ranging from developmental constraints related to the Benazzi et al., 2011b; Gomez-Robles et al., 2013). Among studies serially homologous nature of the dentition to functional con- evaluating dental variation, analyses of lower molar morphology straints related to occlusion (Gomez-Robles and Polly, 2012). are especially common in the literature, and they range from classic Nevertheless, a significant phylogenetic signal remains in dental morphometric analyses (e.g., Wood and Abbott, 1983; Wood et al., morphology (Caumul and Polly, 2005), and this is the basis of the 1983; Bermúdez de Castro and Nicolas, 1995) to quantitative studies of form and shape variation (Benazzi et al., 2011a), sometimes based on 3D reconstructions (Skinner et al., 2008). More recently, some microCT-based studies have focused on the root morphology of mandibular molars (Kupczik and Hublin, 2010; * Corresponding author. E-mail address: [email protected] (A. Gomez-Robles). Emonet et al., 2012). http://dx.doi.org/10.1016/j.jhevol.2015.02.013 0047-2484/© 2015 Elsevier Ltd. All rights reserved. A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 35

Given the relevance of dental morphology in understanding Nevertheless, a systematic evaluation of hominin lower molars in hominin evolution, we initiated in 2006 a systematic evaluation of the context of our 2D geometric morphometric series of analyses is postcanine dental variation in crown morphology (Martinon-Torres still lacking (Martinon-Torres et al., 2006; Gomez-Robles et al., et al., 2006). Our evaluations have included a comprehensive rep- 2007, 2008, 2011b). Therefore, the main objective of this study is resentation of the hominin fossil record, but they have paid special to complete the geometric morphometric analysis of the post- attention European Pleistocene groups. Our series of papers has canine dentition in order to investigate a more comprehensive provided evidence that several dental traits typify Middle and scenario of hominin dental evolution during the Plio-Pleistocene. Upper Pleistocene European groups. These features include a As in previous cases, our analysis is focused on the evaluation of change towards symmetry and distal reduction in upper premolars the European fossil record and the relationship of European fossils (Gomez-Robles et al., 2011b); a very characteristic skewed with modern humans, although Asian and African specimens are morphology in upper first molars that can be traced back to the late also included to place European variation into a broader context. As Lower Pleistocene (Gomez-Robles et al., 2007, 2011a; see also the final article in our series of papers, we discuss the implications Bailey, 2004); a strong reduction of distal cusps in upper second of dental variation across the whole poscanine dentition, with and, especially, third molars (Gomez-Robles et al., 2012); an special emphasis on the evolutionary relationships of the Lower acquisition of nearly symmetric shape and a strong reduction of the and Middle Pleistocene fossil samples from the Atapuerca sites of talonid in lower first premolars (Gomez-Robles et al., 2008); and a Gran Dolina-TD6 and Sima de los Huesos. general maintenance of an asymmetric conformation in lower second premolars (Martinon-Torres et al., 2006). Dental traits Material and methods related primarily to dental reduction are shared in general terms by Homo sapiens and Homo neanderthalensis (also referred to as Material modern humans and Neanderthals throughout the text, respec- fi tively), although the speci c features of these reductions differ in A sample of 132 lower first molars (M1), 126 lower second both species. These features are the ones that allow for the differ- molars (M2), and 108 lower third molars (M3) was analyzed entiation of these two species (and sometimes of their ancestral (Tables 1e3). The study of fossil teeth by means of 2D geometric populations) in spite of the general reduction that drives dental morphometrics based on image analysis ensured the availability of evolution in both cases. Dental morphology shows a wide range of variation within Table 1 List of lower first molars included in this study.a species, and its analysis across the closely related species that constitute the hominin clade does not allow for the identification of Species n Specimens unmistakably delineated species-specific morphologies. However, A. afarensis n ¼ 6 AL128-23, AL145-35, AL266-1, AL288, AL333- several trends can be identified and linked to particular population 1a, LH2 dynamics either via random factors such as genetic drift (Weaver A. africanus n ¼ 4 MLD2, STS24, STW498, Taung ¼ et al., 2007), or specific direct or indirect selective pressures P. robustus n 4 SK23, SKW5, SKX4446, TM1517 P. boisei n ¼ 2 KNM-ER3230, KNM-ER15930 (Grine, 1986; Bermúdez de Castro, 1989; Macho and Moggi-Cecchi, H. habilis s. l. n ¼ 6 KNM-ER1802, KNM-ER5431, OH7, OH13, OH16, 1992; Bermúdez de Castro and Nicolas, 1995; Bermúdez de Castro STW151b et al., 2003b; Irish and Guatelli-Steinberg, 2003; Lozano et al., H. ergaster n ¼ 3 KNM-ER820, OH22, KNM-WT15000 2008). Regardless of the origin of morphological diversification, it is Dmanisi n ¼ 2 D211, D2735 H. erectus n ¼ 11 Zhoukoudian: B3.9, 36, 137 evident in most studies of dental morphology that the ranges of Sangiran: S1b, S6, S7-20, S7-42, S7-43, S7-61, variation of different species overlap, although they can be distin- S7-62, S7-76 guished in their mean values/morphologies. This causes taxonomic H. antecessor n ¼ 3 Atapuerca-TD6: ATD6-5, ATD6-93, ATD6-96 classification based on dental morphology to be a probabilistic task European n ¼ 22 Arago: A13, A40, A89 that will give rise to the allocation of specimens to species with H. heidelbergensis Atapuerca-SH: AT-2 (Ind 2), AT-101 (Ind 3), AT- 1759 (Ind 6), AT-141 (Ind 10), AT-286 (Ind 11), certain probability (Bailey et al., 2009). AT-300 (Ind 12), AT-2276 (Ind 14), AT-2779 (Ind Our previous studies of upper and lower premolars, as well as 16), AT-829 (Ind 18), AT-576 (Ind 19), AT-4318 upper molars, have revealed not only particular morphological pat- (Ind 20), AT-607 (Ind 23), AT-1458 (Ind 24), AT- terns typical of European groups (Homo antecessor from Gran Dolina- 3934 (Ind 25), AT-561 (Ind 26), AT-792 (Ind 27), AT-950 (Ind 31) TD6, European Homo heidelbergensis,andH. neanderthalensis)but Montmaurin also a markedly derived character state of the Middle Pleistocene Pontnewydd 11 population from Atapuerca-Sima de los Huesos (SH) compared with H. neanderthalensis n ¼ 22 Arcy-sur-Cure (Grotte de l'Hyene): 3 other European Middle Pleistocene groups and classic Neanderthals Arcy-sur-Cure (Grotte du Renne): 35 (Gomez-Robles et al., 2012; Martinon-Torres et al., 2012, 2013). The Ehringsdorf Hortus: H1262 (Ind II), H988 (Ind V), H IV derived state of hominins from the SH collection is especially Krapina: B, C, D, E, G, J, 79 (Ind L), 84 (Ind N), 81 apparent in the most proximal premolars and the most distal molars. (Ind P), 80, 105 These morphological peculiarities are especially puzzling in light of Le Moustier 1 the recent DNA analysis of one SH hominin (Meyer et al., 2014), Malarnaud Petit-Puymoyen (two specimens) which has revealed a higher genetic mitochondrial affinity with Saint Cesaire Denisovans (Reich et al., 2010; Meyer et al., 2012)thanwithNean- Fossil H. sapiens n ¼ 10 Qafzeh: 4, H4 derthals. In terms of dental morphology, the scarce morphological Abri Pataud information on Denisovans shows that they are morphologically Isturitz: 106 different from both Neanderthals and modern humans, with metric Les Rois (three specimens including Les Rois A) Saint Germain-La Riviere: B3, B4, B5 and shape features similar to those observed in Homo habilis and Recent H. sapiens n ¼ 37 Heidenheim (AMNH): 15 individuals Homo erectus (Reich et al., 2010). La Torrecilla (LPA-UGR): 22 individuals We have previously evaluated lower second molar morphology a Ind: individual; AMNH: American Museum of Natural History (New York, USA); in the context of methodological approaches useful to deal with LPA-UGR: Laboratory of Physical Anthropology, University of Granada (Granada, evolutionary novelties and losses (Gomez-Robles et al., 2011c). Spain). 36 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 these relatively large samples. The analysis of crown morphology in Table 3 a these dental samples was based on occlusal photographs captured List of lower third molars included in this study. with a Nikon D80 or with a Nikon D1H digital camera fitted with an Species n Specimens

AF Micro Nikkor 105 mm, f/2.8D lens. The occlusal surface of molars A. afarensis n ¼ 7 AL188-1, AL266-1, AL288, AL333w-57, AL333-60, was placed as parallel to the lens as possible in order to minimize AL400-1a, LH4 the error due to the orientation of molars in 2D projections A. africanus n ¼ 8 MLD4, MLD18, MLD19, STS52, STW3, STW14, (Gharaibeh, 2005), which has been calculated to be approximately STW384, STW498 ¼ 3% of the distance between individual landmarks and the centroid P. robustus n 3 SK23, SKW5, TM1517 P. boisei n ¼ 4 KNM-ER729, KNM-ER3230, KNM-ER15930, Peninj: fi of the landmark con guration in lower premolars (Gomez-Robles 1 et al., 2008). This value is similar to the 1e4.5% error rate re- H. habilis s. l. n ¼ 3 OH4, OH13, OH16 ported by Bailey et al. (2004) in measuring cusp base areas in upper H. ergaster n ¼ 2 KNM-BK67, KNM-ER992 ¼ first molars, with comparable values in intra- and inter-observer Dmanisi n 1 D211 H. erectus n ¼ 5 Zhoukoudian: A2, G1.6, 52, 131 evaluations. Error rates in placing landmarks are lower in molars Sangiran: S1b than in premolars because their flatter occlusal surfaces make their H. antecessor n ¼ 2 Atapuerca-TD6: ATD6-96, ATD6-113 correct orientation less problematic (Gomez-Robles, 2010). Taxo- European n ¼ 23 Arago: A13, A89 nomic assignments followed the same guidelines laid out in pre- H. heidelbergensis Atapuerca-SH: AT-1 (Ind 1), AT-811 (Ind 4), AT-13 vious articles of this series and are shown in Tables 1e3. (Ind 7), AT-4147 (Ind 12), AT-222 (Ind 16), AT-2277 (Ind 18), AT-607 (Ind 23), AT-2385 (Ind 24), AT- 3943 (Ind 25), AT-30 (Ind 26), AT-143, AT-599, AT- Methods 942, AT-1468, AT-1473, AT-1945, AT-2273, AT- 2777, AT-3182 Montmaurin Dental morphology was studied by means of a set of landmarks Pontnewydd: 2, 21 located at cusp tips and groove intersections, and a set of equidis- H. neanderthalensis n ¼ 14 Arcy-sur-Cure (Grotte de l'Hyene): 5 tant sliding semilandmarks located at the molar periphery (Fig. 1). Breuil: 3 The employed landmarks were: 1) mesial or anterior fovea, or most Guattari: 2, 3 Hortus: H13 (Ind VI), H695 (IndV) mesial extreme of the central groove; 2) intersection between the Krapina: 106 (Ind E), H, 7 (Ind L), 9 central groove and the mesiobuccal groove; 3) intersection be- Petit-Puymoyen tween the central groove and the lingual groove; 4) intersection Saint Cesaire Shanidar: 2 Vindija: 206 ¼ Table 2 Fossil H. sapiens n 6 Qafzeh: H4 List of lower second molars included in this study.a Abri Pataud Isturitz: 107 Species n Specimens Les Rois (three specimens) Recent H. sapiens n ¼ 30 Heidenheim (AMNH): 10 individuals A. afarensis n ¼ 9 AL128-23, AL145-35, AL188-1, AL207-13, AL266-1, La Torrecilla (LPA-UGR): 14 individuals AL288, AL333-1b, AL333W-60, AL400-1a CENIEH: 6 individuals A. africanus n ¼ 4 MLD2, STS52, STW14, STW498 P. robustus n ¼ 4 SK23, SKW5, SKX4446, TM1517 a Ind: individual; AMNH: American Museum of Natural History (New York, USA); P. boisei n ¼ 3 L427-7, KNM-ER15930, KNM-ER3230 LPA-UGR: Laboratory of Physical Anthropology, University of Granada (Granada, H. habilis s. l. n ¼ 5 KNM-ER1802, KNM-ER5431, OH7, OH13, OH16 Spain); CENIEH: National Research Centre for Human Evolution (Burgos, Spain). H. ergaster n ¼ 3 KNM-ER992, OH22, KNM-WT15000 Dmanisi n ¼ 2 D211, D2735 H. erectus n ¼ 11 Zhoukoudian: A2, F1.5, G1.6, K1.96, 43, 44, 45 between the central groove and the distobuccal groove, also cor- Sangiran: S1b, S5, S7-64, S7-65 responding to the most anterior point of the fifth cusp or hypo- ¼ H. antecessor n 3 Atapuerca-TD6: ATD6-5, ATD6-96, ATD6-113 conulid; 5) tip of the mesiobuccal cusp or protoconid; 6) tip of the European n ¼ 24 Arago: A10, A13, A69, A89 H. heidelbergensis Atapuerca-SH: AT-1 (Ind 1), AT-3179 (Ind 2), AT- mesiolingual cusp or metaconid; 7) tip of the distobuccal cusp or 271 (Ind 3), AT-169 (Ind 10), AT-557 (Ind 11), AT- hypoconid; and 8) tip of the distolingual cusp or entoconid. 300 (Ind 12), AT-284 (Ind 14), AT-2193 (Ind 15), AT- The evaluation of first molars included the eight landmarks, 2763 (Ind 16), AT-941 (Ind 18), AT-946 (Ind 20), AT- whereas the evaluation of second molars did not include landmark 607 (Ind 23), AT-2396 (Ind 24), AT-3889 (Ind 25), 4 and the analysis of third molars excluded landmarks 1 and 4 due AT-1756 (Ind 26), AT-3176 (Ind 27), AT-792 (Ind 28) fi Montmaurin to the dif culty of identifying them in the most recent hominin Pontnewydd: 8, 15 species because of their strongly reduced morphology. The location H. neanderthalensis n ¼ 18 Arcy-sur-Cure (Grotte de l'Hyene): 4 of landmarks, especially those located on cusp tips, was assessed by Arcy-sur-Cure (Grotte du Renne): 5, 21 comparing photographs with the original molars or casts. Heavily Ehringsdorf Hortus: H IV, H930 (Ind V) worn molars were not included in our analyses, although a certain Krapina: 6 (Ind A), C, D, 10 (Ind E), G, J, 1 (Ind L), 86, degree of wear was accepted (up to wear stage 3e4; Molnar, 1971; 107 Supplementary Online Material [SOM] Fig. 1). Apart from these Le Moustier 1 landmarks, we digitized 39 sliding semilandmarks in counter- Petit-Puymoyen clockwise direction. The first semilandmark was located at the Saint Cesaire Fossil H. sapiens n ¼ 8 Qafzeh: H4, 11 point of the mesial margin directly opposite to the mesial fovea, or Abri Pataud at the point of the mesial margin intersected by the hypothetical Grimaldi prolongation of the central groove. Landmarks and semilandmarks Les Rois (three specimens) were digitized using TpsDig2 (Rohlf, 2005). The 39 semilandmarks Saint Germain-La Riviere: 3 Recent H. sapiens n ¼ 32 Heidenheim (AMNH): 13 individuals were slid in order to minimize the Procrustes distance between La Torrecilla (LPA-UGR): 19 individuals each individual and the target represented by the consensus morphology of the sample (Perez et al., 2006). We used this crite- a Ind: individual; AMNH: American Museum of Natural History (New York, USA); LPA-UGR: Laboratory of Physical Anthropology, University of Granada (Granada, rion to slide semilandmarks instead of other possible criteria (Gunz Spain). et al., 2005) because previous studies of hominin lower molars have A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 37

Allometric change was evaluated both intraspecifically (static allometry) in H. heidelbergensis, H. neanderthalensis, and H. sapiens, and interspecifically (evolutionary allometry). For the latter, phylogenetic non-independence of data was accounted for by regressing independent contrasts of shape on independent contrasts of size (Klingenberg and Marugan-Lob on, 2013). Independent contrasts are calculated as weighted differences between pairs of sister species or nodes, and they were based on two different phylogenetic trees to explore the effect that the uncertainty in defining phylogenetic relationships has on these estimates. The first phylogenetic tree included Paranthropus as a monophyletic group and used a model of late divergence between Neanderthals and modern humans, whereas the second phylogenetic tree considered Paranthropus as paraphyletic and used a model of early divergence between Neanderthals and modern humans (SOM Fig. 2). Both trees also differed in the geometry of the relationships between H. ergaster, H. erectus, and the Dmanisi hominins (which were included as a separate species to increase the number of data points). Many other modifications of these phylogenetic trees are possible, but we restricted our analyses to these two because the aim of this study is not to do an exhaustive comparison of the effect of phylogenetic uncertainty in the estimation of allometric effects. Analyses were carried out with TpsRelw (Rohlf, 1998) and MorphoJ (Klingenberg, 2011). Figure 1. Landmarks and semilandmarks used in the lower first molar analysis. Landmark 1 has not been included in the lower second molar analysis. Landmarks 1 and 4 have not been included in the lower third molar analysis. B: buccal; L: lingual; Results M: mesial; D: distal. General morphological trends demonstrated a better performance of this approach in a similar M1 The first principal component (PC) corresponding to the context (Gomez-Robles et al., 2011c). morphological analysis of lower first molars reflects the reduction After performing a generalized Procrustes analysis (GPA) in or- of the fifth cusp, which is accompanied by a reduction of the der to remove all non-shape variation corresponding to position, mesiodistal diameter giving rise to a more squared morphology, as size, and orientation (Rohlf and Slice, 1990), several analyses were well as a change from the typical Dryopithecus pattern (also called carried out to assess the main morphological trends observed in the Y5 pattern) to a cruciform groove pattern (Fig. 2: top chart). The hominin clade. A first exploratory step was carried out using a second component shows mainly the distinction between relative warps analysis (Bookstein, 1991), which in this case is mesiodistally elongated molars (positive values) versus equivalent to a principal components analyses (PCA) of Procrustes- buccolingually broad molars (negative values). The main superimposed landmark coordinates. The PCAs were carried out on morphological change across the second PC affects the general complete samples and also on reduced samples including only proportion of the molars (rectangular shape versus approximately Homo species and grouping Dmanisi hominins, Homo ergaster, and squared shape in positive and negative values, respectively), but H. erectus under H. erectus s. l. The purpose of repeating analyses on this PC does not correspond with strong changes either in cusp reduced samples was to ascertain if important morphological fea- proportions or in the groove pattern. tures distinguishing Homo species might be masked by including The most notable observation in the distribution of individuals Australopithecus and Paranthropus species. A canonical variates in the PC space corresponding to PC1 and PC2 is the presence of two analysis (CVA), together with a comparison of species mean shapes outliers (two H. sapiens molars with only four cusps). This 4-cusp based on permutation tests, was employed in order to evaluate the conformation is part of the normal variability of H. sapiens, even performance of lower molar morphology in taxonomic classifica- though this pattern is a minority in the studied sample. Apart from tion and the general morphological distinctiveness of the three this, the PCA graph shows a complete overlapping of different most recent (and best represented) hominin species: species, including Australopithecus, Paranthropus, and Homo species H. heidelbergensis, H. neanderthalensis, and H. sapiens. The CVAs (Fig. 2: top chart). used the first ten principal components obtained in a PCA of these Lower first molars show a very homogeneous distribution of species (based in turn on GPAs including only these three species). variance such that no eigenvalue is clearly predominant versus the This number of principal components was chosen to ensure that other ones. Actually, PC1 and PC2 account only for 32.78% of total the number of variables was below the number of individuals per morphological variance. Therefore, PC3 and PC4 have also been group in all cases. Percentages of correct classification were cross- evaluated, and their combination does reveal certain interspecific validated by excluding each specimen from the analysis and later organization (Fig. 2: bottom chart). The most important feature of testing the group to which they are assigned. Technical details of the PCA graph showing PC3 against PC4 is the separation of these analyses are explained in previous articles in this series H. neanderthalensis and H. heidelbergensis from other hominins due (Gomez-Robles et al., 2007, 2008, 2011b, 2012). to their exclusive positive values (with only one exception corre- The relative contribution of allometric change to shape variation sponding to the Le Moustier 1 molar) on PC3. Positive scores on PC3 was evaluated by means of a multivariate regression of shape var- are associated with a general mesiodistally elongated shape, a well- iables on size. Size was measured as the logarithm of the centroid developed fifth cusp associated with a cruciform groove pattern, size, defined as the squared root of the sum of the squared distances and a mesial displacement of the anterior fovea caused by the between a set of landmarks and their centroid or gravity center. almost universal appearance in Neanderthal and pre-Neanderthal 38 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50

Figure 2. Principal components analysis (relative warps analysis) of lower first molar morphology in the complete hominin sample. Top: PC1 versus PC2. Bottom: PC3 versus PC4. Orientation of TPS-grids is the same as in Figure 1. Relevant specimens discussed in the text are indicated. lower molars of a very well-defined mid-trigonid crest (Bailey, shows mainly negative values for PC1 and PC3 that are indicative of 2002a; Martinon-Torres, 2006; Bailey et al., 2011; Martinon- their general primitive morphology in lower first molars. Torres et al., 2012; Martínez de Pinillos et al., 2014). Austral- Homo antecessor shows morphologies close to the consensus of the opithecus, Paranthropus, and early Homo molars tend to plot in the sample. negative halves of the graphs for both PC1 and PC3, without a clear These patterns of variation are slightly different from the ones pattern of distribution along PC2 and PC4 (Fig. 2). Homo erectus also yielded by PCAs including only species of the genus Homo, a result A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 39 that is also driven by the exclusion from this reduced sample of the M2 The first PC corresponding to the shape analysis of lower H. sapiens outliers (Fig. 3). However, analyses of the complete and second molars reveals not only the reduction of the fifth cusp reduced samples are similar in not separating species in the com- observed in the most recent hominin species but also a general bination of the first and second PCs, but in the combination of PC3 reduction of the distal half of the molars associated with the and PC4. When excluding Australopithecus and Paranthropus spe- appearance of an X-pattern (Fig. 4: top chart). On the contrary, cies as well as the H. sapiens outliers, PC1 and PC2 are interchanged negative scores on PC1 correspond to mesiodistally long molars such that the reduction of the hypoconulid is recovered by PC2. with five well-developed cusps and the primitive Dryopithecus

Figure 3. Principal components analysis (relative warps analysis) of lower ﬁrst molar morphology in the reduced hominin sample (excluding Australopithecus and Paranthropus species, and H. sapiens outliers). Top: PC1 versus PC2. Bottom: PC3 versus PC4. Orientation of TPS-grids is the same as in Figure 1. 40 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 pattern. Negative scores on PC2 correspond to buccolingually humans and European groups (mainly SH hominins), whereas broad molars with peripherally located cusp apices and a earlier species occupy the whole morphospace along PC2. The three cruciform pattern, whereas positive scores on this PC correspond molars belonging to H. antecessor plot close to one another at the to more elongated molars along the mesioedistal axis with negative extreme of PC1, thus highlighting the well developed 5- more centrally located cusp apices and a mesially located cusped morphology typical in lower molars of the oldest Euro- anterior fovea. pean populations, similar to those observed in earlier species of the The ﬁrst PC shows a gradual separation of earlier molars genus Homo and even in some Australopithecus and Paranthropus belonging to Australopithecus, Paranthropus, and early Homo versus specimens. The three H. ergaster molars have intermediate scores recent molars belonging to later Homo species (Fig. 4: top chart). for PC2, but are distributed along the complete morphospace for The second PC shows primarily the separation between modern PC1, with the only exception of the region of the plot corresponding

Figure 4. Principal components analyses (relative warps analyses) of lower second molar morphology (top) and lower third molar morphology (bottom) in the complete hominin samples. Only the first two principal components are represented. Orientation of TPS-grids is the same as in Figure 1. Relevant specimens discussed in the text are indicated. A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 41 to strongly reduced molars. Both lower second molars from the Percentages of correct assignment are highest in the three cases Dmanisi site plot at the negative half of the graph for PC1, but they for H. sapiens and lowest for H. neanderthalensis, with intermediate differ along PC2. values for H. heidelbergensis. This reflects in part the morphological For lower second molars, morphological variations recovered by distinction of Neanderthal and SH molars due to the very anterior PC1 and PC2 in the analysis of the complete and reduced samples position of the mesial fovea, as well as the more peripheral position are very similar (Fig. 5: top). The distribution of the different spe- of cusp apices in H. sapiens (versus the more central position cies in the morphospace is also largely comparable in both analyses, observed in Neanderthals and, especially, in SH hominins). with European groups and H. sapiens separated along PC2, and Homo neanderthalensis and H. heidelbergensis molars are barely H. antecessor plotting at the negative extreme of PC1. differentiated in the position of the mesial fovea, but their main differences are the mesiodistally more elongated shape and the M Similar morphological patterns are observed in the analysis of 3 more central placement of cusp tips observed in H. heidelbergensis. lower third molars. The first PC shows the variation from unre- The observed relatively high percentages of correct classifica- duced molars with five well-defined cusps to strongly reduced tion are indicative of significant differences in mean shape between molars without a hypoconulid (c5) and with a small hypoconid the three species in the three molar positions. Procrustes distances and entoconid, which are sometimes obscured by the presence of between H. heidelbergensis and H. sapiens are greater than the accessory cusps (Fig. 4: bottom chart). The second PC also reflects distances between Neanderthals and H. heidelbergensis and be- the contrast between approximately squared molars with tween Neanderthals and modern humans for the three molars peripherally located cusp tips versus mesiodistally elongated (Table 5). Neanderthals and modern humans are expected to show molars with centrally compressed cusp apices. The exclusion in the greatest shape differences because the phylogenetic distance these molars of the landmark corresponding to the mesial between both species (the amount of time during which they have foveaddue to the difficulty in identify it in strongly reduced evolved independently) is longer that the phylogenetic distance molarsddoes not allow for an evaluation of its position. between either of them and H. heidelbergensis, regardless of the However, an a posteriori evaluation demonstrates that many phylogenetic position of the latter. Therefore, this result highlights molars plotting on the positive half of the graph for PC2 show a the morphological uniqueness of the H. heidelbergensis sample, mid-trigonid crest (and an associated mesial displacement of the mostly represented by Sima de los Huesos hominins, which is anterior fossa). especially evident in third molars. In the PCA graph corresponding to M3 morphology, the most negative scores correspond to P. boisei molars, which are generally Allometry the most expanded ones (both in size and in cusp number). The positive half of the graph for PC1, however, includes not only molars Allometric variation shows different patterns when evaluated at belonging to the most recent species but also some belonging to inter- and intraspecific levels (Table 6). Interspecifically, the three Australopithecus afarensis, Australopithecus africanus, H. habilis, H. molars show highly significant allometric variation when evaluated erectus, H. ergaster, and H. antecessor (Fig. 4: bottom chart). without considering phylogenetic relationships. These analyses Homo ergaster and Dmanisi molars plot close to the center of the show that allometric variation is stronger in second and third graph, whereas both H. antecessor third molars differ in their molars than in first molars. These results are more comparable to morphology: ATD6-113 has a well-defined 5-cusped conformation the ones presented in the other papers in this series (Gomez-Robles that makes it plot at the upper left quadrant (close to some et al., 2007, 2012), and they show that allometric variation is H. habilis molars). However, ATD6-96 has a strongly reduced generally smaller in lower molars than in upper molars. However, morphology that makes it plot at the lower right quadrant, in the upper and lower first molars show low allometric variation, with area mainly occupied by H. sapiens (although some Neanderthal, similar values in both cases. Phylogenetically corrected analyses Australopithecus, and H. erectus molars are also present in this area). yield different results when using different phylogenetic trees, this Removing Australopithecus and Paranthropus molars has a being the main limitation of this approach in the context of hom- noticeable effect in the PCA of the selected M sample (Fig. 5: 3 inin evolution. Lower first molars may or may not show significant bottom chart). Although PC1 still recovers distal reduction, the evolutionary allometry depending on the employed phylogenetic negative extreme values do not show the broadly expanded talonid tree, whereas second molars show significant allometry in both area typical of Paranthropus third molars (especially of P. boisei). The cases and third molars do not show significant allometry in either second PC, however, shows a very similar distinction between case (Table 6). It is important to note, however, that these results mesiodistally short or elongated molars, which is useful to distin- are susceptible to change if a different topology of evolutionary guish H. sapiens from H. neanderthalensis and H. heidelbergensis relationships across species (including different datings for species molars. and nodes) were proved to be more accurate. It has to be noted as well that these results are strongly influenced by evolutionary re- Discriminant ability of lower molar shape lationships among H. habilis, H. ergaster, Dmanisi hominins, and H. erectus, which are far from resolved (e.g., Lordkipanidze et al., The assignment tests based on canonical variates analyses of the 2013). three most recent groups (H. heidelbergensis, H. neanderthalensis, Other authors have addressed this problem by restricting allo- and H. sapiens) reveal that the lower first molar has the highest metric analyses to closely related species, i.e., Neanderthals and discriminant ability from the three lower molars. Discriminant modern humans (Bailey et al., 2014). Similarly, we also evaluated ability decreases towards the second and third molars, as intra- intraspecific allometric variation to avoid the potential confound- specific ranges of variation increase. The CVA graphs show how the ing effect of interspecific differences and phylogenetic uncertainty. ranges of distribution of the three species are generally well Intraspecific allometric change is significant or marginally signifi- defined in lower first molars, whereas they are blurred in second cant for the three molars in H. heidelbergensis, an effect that is and third molars (Fig. 6). As the range of variation of the three partially influenced by the inclusion in these samples of larger and species converge towards distal teeth, Neanderthals lose their morphologically less reduced molars from Arago, together with morphological identity and their percentage of correct assignment smaller and reduced molars from SH. Similarly, allometric effects drops to approximately 60% in second and third molars (Table 4). are significant for H. sapiens lower first molars, and they are also 42 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50

Figure 5. Principal components analyses (relative warps analyses) of lower second molar morphology (top) and lower third molar morphology (bottom) in the reduced samples (excluding Australopithecus and Paranthropus). Only the first two principal components are represented. Orientation of TPS-grids is the same as in Figure 1. influenced by the inclusion in this sample of fossil and recent shortening of the mesiodistal diameter caused by the reduction of H. sapiens, the former being larger and less reduced. The most talonid cusps, especially of the hypoconulid. This reduction is important morphological change associated with size variation in associated in second and third molars with the frequent observa- the three molars at both inter- and intraspecific levels is a tion of þ and X groove patterns. A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 43

Discussion

Some particular morphological traits, such as certain additional cusps, crests, or foveas (Irish, 1998; Bailey, 2002b; Guatelli- Steinberg and Irish, 2005), and general size (Wood and Abbott, 1983; Suwa et al., 1994; Bermúdez de Castro and Nicolas, 1995) can help to distinguish lower first molars of different species. However, general morphology (measured as general proportions of molars, number of main cusps, relative area of these cusps, and organization of main grooves) shows relatively low variation throughout the hominin fossil record in these molars (Fig. 7). Several studies trying to evaluate morphological differences in lower molars of robust and gracile australopiths have observed certain trends, but not a clear separation of groups (Wood et al., 1983; Wood and Abbott, 1983; Suwa et al., 1994, 1996; Skinner et al., 2008). The similarity of lower first molars of Neanderthals and modern humans has been previously highlighted, with only some exceptions, the most important being the very frequently observed mid-trigonid crest in Neanderthals (Bailey, 2002a, b). This trait is also expressed with a very high frequency in the SH population (Martinon-Torres, 2006; Martinon-Torres et al., 2007b, 2012). Quantitative analyses of the cervical and crown outline of Nean- derthal and modern human first molars have also demonstrated that lower first molars yield a less clear separation of both groups as compared with upper first molars (Benazzi et al., 2011a). It has been suggested, however, that certain crown outline features can be useful to differentiate Neanderthal and modern human M1s once size information is regressed out, including a reduction of the buccodistal outline and a straighter lingual side in modern humans (Benazzi et al., 2011a). This relative stability in lower first molar morphology is also reflected in the homogeneous distribution of variance across principal components (Gomez-Robles and Polly, 2012). Actually, no separation of species or genera is observed in the combination of the two principal components that account for the highest percentage of variance. Different species overlap along PC1 and PC2, where individual distribution does not correspond to a taxonomic differentiation. The third principal component, however, does show interspecific separation. This component mainly reflects the separation of H. heidelbergensis and H. neanderthalensis molars due to the appearance of þ and X groove patterns. Non Y5-patterns appear to be more common in the SH population (44.4%; Martinon-Torres et al., 2012) than in Neanderthals (3% in Bailey, 2002b; and 17.9% in Martinon-Torres et al., 2012) or in modern humans (0e20% in Scott and Turner, 1997;0e12.5% in Bailey, 2002b; and 22.7e27.6% in Martinon-Torres et al., 2012). The second trait that causes a separation between Neanderthal and H. heidelbergensis molars and the other species is the relative displacement of the mesial fovea towards the mesial margin of the molars due to the frequent presence of the mid-trigonid crest (Bailey, 2002a, b; Martinon-Torres, 2006; Martinon-Torres et al., 2007b, 2012; Bailey et al., 2011; Martínez de Pinillos et al., 2014). This crest is observed not only in almost all the Neanderthals and H. heidelbergensis, but also in some molars from Sangiran (H. erectus), Dmanisi, and Gran Dolina-TD6 (H. antecessor; Martinon-Torres et al., 2007a). In some of these cases, however, the mid-trigonid crest is interrupted by the central groove, the course of which is attenuated at the point of intersection with the crest. Arago molars show the traits observed in SH molars to a lesser

Figure 6. Canonical variates analysis of lower ﬁrst, second, and third molars (represented from top to bottom). TPS-grids represent the deformation from the mean shape of the sample to the mean shape of H. heidelbergensis (bottom on each plot), of TPS-grids is the same as in Figure 1. Fossil and recent H. sapiens are represented with H. neanderthalensis (top on each plot), and H. sapiens (right on each plot). Orientation different symbols on the plot, but they have been grouped together in canonical variates analyses. 44 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50

Table 4 Percentage of correct classiﬁcations based on canonical variates analyses.

M1 M2 M3 Original Cross-validated Original Cross-validated Original Cross-validated

H. heidelbergensis 95.5% 81.8% 83.3% 75.0% 79.2% 70.8% H. neanderthalensis 86.4% 77.3% 83.3% 61.1% 71.4% 64.3% H. sapiens 93.6% 89.4% 95.1% 95.1% 86.1% 77.8% Total 92.3% 84.6% 89.2% 81.9% 81.1% 73.0%

Table 5 morphological distinction (5-cusped molars in early Homo versus Mean shape comparisons between H. heidelbergensis, H. neanderthalensis, and 4-cusped molars in later Homo) that may or may not be reﬂected in H. sapiens.a the morphospace depending on the set of landmarks employed H. heidelbergensis- H. heidelbergensis- H. neanderthalensis- (Gomez-Robles et al., 2011c). When landmarks demarcating the H. neanderthalensis H. sapiens H. sapiens ﬁfth cusp are not explicitly included, 5- versus 4-cusped molars

M1 PD ¼ 0.021 PD ¼ 0.029 PD ¼ 0.026 show a gradient across the morphospace, but not a clear disconti- P <0.001 P <0.001 P <0.001 nuity (Gomez-Robles et al., 2011c). Following Wood and colleagues ¼ ¼ ¼ M2 PD 0.022 PD 0.038 PD 0.034 (Wood et al., 1983; Wood and Abbott, 1983), lower molar shape in P ¼ 0.009 P <0.001 P <0.001 robust taxa is characterized by a combination of a relative reduction M3 PD ¼ 0.033 PD ¼ 0.047 PD ¼ 0.028 P ¼ 0.004 P <0.001 P ¼ 0.003 of the trigonid area and a relative enlargement of the talonid area.

a PD: Procrustes distance between the mean shape of listed species. P: p-value This trend is especially evident when comparing early hominin based on 10,000 random permutations (testing the null hypothesis of equal mean molars with late hominin molars. Later species show a very marked shapes in the compared species). reduction of the talonid cusps, which can contribute a signiﬁcant lower proportion to the total area of the molar than the trigonid cusps. degree, as demonstrated by their marginal location within the None of the Neanderthal second molars in our sample shows a range of variation of H. heidelbergensis (Fig. 2). 4-cusped conﬁguration, although two molars from SH and one from Pontnewydd have only four cusps (compared with 72.5% of

M2 H. sapiens). While this may be the result of a sampling bias, it un- derscores the strong trend to reduction of SH dentitions, which The results of the classic works by Wood and colleagues reveal seems to be even stronger that that observed in Neanderthals that shape and size variation of lower second molars corresponding (Bermúdez de Castro and Nicolas, 1995; Gomez-Robles et al., 2012). to Plio-Pleistocene African species show a continuous distribution, Extremely reduced lower second molars with only four cusps and the extremes of which correspond to P. boisei, and H. habilis and non-Y patterns have also been described in H. erectus specimens H. ergaster (Wood et al., 1983; Wood and Abbott, 1983). This from Sangiran dated to the Lower Pleistocene or to the early Middle gradient is also observed in our results, although this variation is Pleistocene (Zanolli, 2013). Three second molars from Arago are masked due to the inclusion of Asian and European later groups. located far from most SH molars in the PCA plot, and they delim- Actually, the inclusion of later Homo species reveals a clear itate the area of distribution of H. heidelbergensis in the

Table 6 Inter- and intraspeciﬁc allometric analysis.a

Interspeciﬁc without phylogeny Independent contrasts (tree 1) Independent contrasts (tree 2) H. heidelbergensis H. neanderthalensis H. sapiens

M1 4.28% 13.26% 18.37% 7.79% 6.54% 7.30% P <0.001 P ¼ 0.153 P ¼ 0.024 P ¼ 0.076 P ¼ 0.153 P ¼ 0.001

M2 7.08% 21.30% 29.76% 10.63% 6.17% 3.28% P <0.001 P ¼ 0.001 P ¼ 0.008 P ¼ 0.014 P ¼ 0.390 P ¼ 0.194

M3 9.74% 7.97% 10.31% 9.10% 13.07% 2.79% P <0.001 P ¼ 0.433 P ¼ 0.267 P ¼ 0.024 P ¼ 0.105 P ¼ 0.455

a Percentage of shape variance explained by allometric effects. P: p-value based on 10,000 random permutations.

Figure 7. Morphological diversity of lower first molars. H. habilis (A: STW 151), H. heidelbergensis (B: AT-3934), H. neanderthalensis (C: Petit Puymoyen), and H. sapiens (D: Saint Germain-La Riviere B3). Orientation of molars is the same as in Figure 1. Scale bars are 5 mm. A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 45 morphospace (Fig. 4: top chart). As in most mandibular teeth, identified. However, the taxonomic utility of these differences is H. antecessor has a primitive conformation in lower second molars, limited. One of the most evident observations related to the lower characterized by the large size of the distal half and by the gener- third molar analysis is the strong structural reduction of the SH alized presence of additional cusps (Bermúdez de Castro et al., hominins. The Procrustes distance between the mean shape of 1999b, 2008; Fig. 8). Middle Pleistocene European molars (H. heidelbergensis) and Both molars from Dmanisi show very different morphologies. H. sapiens is considerably higher than the distance observed be- D211 has a general rounded shape and a Y5 pattern. The hypo- tween Neanderthals and modern humans. Lower third molars from conulid (c5) begins at the central fossa, before the point of inter- Arago plot at a marginal area of the distribution range of section between the central groove and the lingual groove, a H. heidelbergensis, and they have been described as intermediate in morphology that is not observed in australopiths. This groove morphology between Lower Pleistocene European groups and the configuration is also typical of some H. erectus and it is similar to SH molars (Bermúdez de Castro et al., 2003a). In spite of the that observed in some H. habilis, such as OH16. The lower second primitive morphologies observed in the lower dentition of molar of D2735, however, has a rectangular and mesiodistally H. antecessor (Bermúdez de Castro et al., 1999b; Gomez-Robles elongated shape. The evaluation of mesiodistal and buccolingual et al., 2008; Gomez-Robles, 2010), the hemi-mandible ATD6-96 diameters (Martinon-Torres et al., 2008; Macaluso, 2010) reveals (Carbonell et al., 2005) shows one of the most reduced third molars that these different shapes are the result of the shorter mesiodistal observed in the complete sample (including recent H. sapiens). This and larger buccolingual diameters of D211. Although not molar has a nearly circular shape, and cusps are reduced to small completely clear due to the presence of additional cusps that enamel ridges (Fig. 9B). This molar is located within the range of obscure the identification of the main ones, D2735 seems to have a distribution of H. sapiens in the PCA plot, close to other earlier cruciform groove pattern (Martinon-Torres et al., 2008). The exis- molars that also show a certain degree of reduction (Fig. 4: bottom tence of secondary cusps (c6 and c7) that distort the location of the chart). The other third molar belonging to H. antecessor, which is main ones, then altering the groove pattern, is also observed in located in the mandibular fragment ATD6-113 (Bermúdez de Castro Australopithecus, Paranthropus, and H. habilis (Wood and Abbott, et al., 2008), has a much more primitive aspect with five well- 1983; Wood et al., 1983; Tobias, 1991). The derived aspect of defined cusps arranged in a Y-pattern, in spite of the complex OH22, which plots at the area of the morphospace only occupied by array of secondary grooves (Fig. 9A). The third molar of the Plio- recent species, is also apparent in Wood et al. (1983). In their Pleistocene mandible D211 is strongly reduced in size, but it still principal components analyses (based either on relative cusp areas preserves the five main cusps plus two accessory cusps (c6 and c7; or on the morphology of the groove pattern), OH22 appears as an Margvelashvili, 2006; Martinon-Torres et al., 2008), thus demon- outlier and plots far from other African molars, including H. ergaster strating that size reduction in this case is not associated with KNM-ER992. reduction in cusp number. As for australopiths and early Homo,it has been shown elsewhere that there is considerable overlap in

M3 their ranges of variation and that complex morphologies with accessory cusps are observed in these groups (Wood et al., 1983). Lower third molars show a different pattern of reduction than that observed in upper third molars (Gomez-Robles et al., 2012). Serial variation and EvoDevo Reduction in upper molars is very consistent in H. heidelbergensis, H. neanderthalensis, and H. sapiens: when only one cusp is reduced First molars are less variable than second and, especially, third or lost, this is usually the hypocone; if a second cusp is reduced, that molars. The morphological stability of first molars can be the result reduction affects the metacone; mesial cusps (protocone and par- of a canalization process reducing morphological variation or of acone) are more stable (Kondo and Yamada, 2003; Takahashi et al., stabilizing selection operating in first molars due to their central 2007). Cusp reduction in lower third molars is less consistent and developmental and functional role (Gomez-Robles and Polly, 2012). shows a weaker relationship with size reduction than that observed The strongest morphological change observed in lower molars is in upper third molars (Williams and Corruccini, 2007; Gomez- the reduction of distal cusps, which have been suggested to Robles et al., 2012). Actually, H. sapiens lower third molars do not constitute a developmental module partially independent of that show significant intraspecific allometric variation, whereas in up- formed by mesial cusps of the three molars (Hlusko et al., 2004). per third molars both the Neanderthal and modern human lineages Distal reduction in third molars is first observed in early repre- show significant and similar intraspecific allometric trajectories sentatives of the genus Homo, although, in these groups, strongly (Gomez-Robles et al., 2012). reduced molars appear only occasionally. The proportion of In spite of this unordered pattern of cusp reduction and the high reduced versus non-reduced molars seems to be reversed in recent intraspecific variation, significant differences in mean shapes of Homo species, with a majority of individuals losing one or more H. heidelbergensis, H. neanderthalensis, and H. sapiens have been cusps, although a certain proportion retain the primitive 5-cusped

Figure 8. Morphological diversity of lower second molars. H. antecessor (A: ATD6-113), H. heidelbergensis (B: AT-1), H. neanderthalensis (C: Arcy-sur-Cure: Grotte de l'Hyene 4), and recent H. sapiens (D: LT102). Orientation of molars is the same as in Figure 1. Scale bars are 5 mm. 46 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50

Figure 9. Morphological diversity of lower third molars. H. antecessor (A: ATD6-113; B: ATD6-96), H. heidelbergensis (C: AT-2385), and recent H. sapiens (D: CENIEH 32). Orientation of molars is the same as in Figure 1. Scale bars are 5 mm. conformation. This reflects an increasing trend towards reduction Sima de los Huesos fossils Although some teeth from SH have that was initiated in the Lower Pleistocene. This trend is especially transitional morphologies between Lower Pleistocene populations evident in the later developing cusps (distal), thus pointing to a and Neanderthals (i.e., first molars and second premolars), some direct link between developmental dynamics and evolutionary dental classes in the SH hominins show more derived morphol- change (Jernvall, 2000). ogies than in Neanderthals. This is especially evident in distal molar Although with species-specific particularities, both Neanderthal morphologies, where the proportion of individuals showing and modern human lineages show a strong trend towards distal reduction of one or more cusps is not only higher than that found in reduction in their premolars and distal molars (Gomez-Robles and Neanderthals but is also higher than the frequencies of reduction Polly, 2012). One possible explanation for this is inheritance from found in many modern human populations (Scott and Turner, 1997; an also distally reduced common ancestor (Gomez-Robles et al., Bailey, 2002b; Martinon-Torres, 2006; Martinon-Torres et al., 2012). 2013). It can also be hypothesized, however, that distal reduction Moreover, Bermúdez de Castro and Nicolas (1995, 1996) have noted has evolved in parallel in both lineages. If this were the case, either the extreme reduction in size of the postcanine dentition in the SH shared adaptive pressures or developmental constraints would be hominins, a feature that is likely related to a reduction in the causing the same morphological variation in both lineages. It has morphological complexity of these teeth (Williams and been suggested that restrictions related to space in the jaw and/or Corruccini, 2007). Apart from these high frequencies of reduction developmental timing can be involved in the structural reduction of of the distal molars, the SH hominins seem to show more derived the distal cusps of later developing teeth (premolars and distal morphologies in upper and lower first premolars (Gomez-Robles molars; Bermúdez de Castro, 1989; Gomez-Robles et al., 2012; et al., 2008, 2011b). This observation highlights again the greater Gomez-Robles and Polly, 2012). Space restrictions could play a variability corresponding to the elements most distant from the role in reduction in modern humans, but it is unlikely to be oper- first molars (Gomez-Robles and Polly, 2012) ating in the reduction observed in the Neanderthal lineage. Time The derived morphology of the SH hominins has made some restrictions can be the result of a postdisplacement event causing a authors (Endicott et al., 2010; Stringer, 2012) question the delay in the onset of dental development and giving rise to a revised chronology of ca. 600 kyr (thousands of years ago) paedomorphic molar that does not completely develop its potential (Bischoff et al., 2007), and favor a more conservative chronology (Bermúdez de Castro, 1989; Jernvall, 2000). This heterochronic of ca. 350 kyr (Bischoff et al., 2003). Recent new data have phenomenon could be related to the appearance of a modern life yielded a minimum age estimate for the SH hominins of ca. 430 history pattern (Bermúdez de Castro, 1989; Gomez-Robles et al., ka (Arnold et al., 2014; Arsuaga et al., 2014). This discrepancy in 2012), a trait that is thought to have appeared approximately SH chronology, however, does not shed light on the strong dental 1Ma(Bermúdez de Castro et al., 1999a, 2010). If a modern pattern reduction of the SH hominins in comparison with Neanderthals. of dental development already existed in the late Lower Pleistocene If a progressive and gradual change towards more derived mor- and it is related to dental structural reduction, there would be a phologies (from a presumed less reduced ancestral condition) is common ancestry to this reduction, even if it has further developed expected along the Neanderthal lineage, it is difficult to explain following slightly different trajectories in the phylogenetic that the morphology of the SH teeth is even more derived than branches leading to Neanderthals and modern humans. The that of Neanderthals in aspects related to dental reduction. This morphological trend observed in our series of papers (general distal holds even in light of the recently published higher DNA affinity reduction in both lineages, but with fine-scale particularities of the SH hominins with Denisovans than with Neanderthals differentiating both groups) is fully consistent with this hypothesis. (Meyer et al., 2014). First, previous DNA analyses have demonstrated that affinities based on mitochondrial DNA do not Overview of postcanine dental variation necessarily match affinities observed through the high-coverage analysis of the nuclear genome. Mitochondrial DNA analysis first As this is the last article in our series of papers analysing variation revealed Denisovans to be the sister group of the in postcanine teeth, the final part of this discussion is devoted to H. neanderthalensis-H. sapiens clade (Reich et al., 2010), whereas summarizing the most relevant phylogenetic implications of these nuclear DNA has shown Denisovans to be the sister group of morphological trends across the whole postcanine dentition. The Neanderthals but not of modern humans (Meyer et al., 2012). most important interpretations of these morphological patterns in Second, even if future DNA analyses of the SH hominins the context of the interaction between evolution, development, and demonstrated that they are indeed closer to Denisovans than to function have been discussed elsewhere (Gomez-Robles and Polly, Neanderthals, this would still imply that the SH hominins have a 2012). Here we focus on the most critical phylogenetic inferences, closer phylogenetic relationship with Neanderthals than with some of which have been outlined in the context of ancestral state modern humans since, as mentioned above, Denisovans and reconstruction (Gomez-Robles et al., 2013), paying special attention Neanderthals form a clade to the exclusion of modern humans to the European Pleistocene populations coming from Sima de los (Meyer et al., 2012). In terms of dental morphology, Denisovan Huesos and Gran Dolina-TD6 sites. upper third molar size and shape are not only quite different A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 47 from those observed in Neanderthals and modern humans (Reich de Castro et al., 1999b). This can be the result of the stronger et al., 2010) but also very different from those of the Sima de los integration and more static pattern of evolution undergone by Huesos hominins (Personal observation). the mandibular dentition versus the weaker integration of the The derived condition of the SH hominins can be observed not maxillary dentition, which is associated with a less static only in general conformations but also in discrete dental traits evolutionary pattern (Gomez-Robles and Polly, 2012). In addi- (Martinon-Torres et al., 2012). A possible explanation for this is that tion, although it has been claimed that the mandibles from Gran phylogenetic signal (Klingenberg and Gidaszewski, 2010) is relaxed Dolina-TD6 lack all the apomorphies characteristic of Neander- in the evolution of the most distal and mesial elements of the ar- thals (Rosas and Bermúdez de Castro, 1999; Carbonell et al., cade, and/or that dental morphology of the SH hominins shows 2005; Bermúdez de Castro et al., 2008), the ATD6-96 mandible extreme morphologies within the probably higher range of varia- (the only H. antecessor mandible that preserves the complete tion of Middle Pleistocene groups, perhaps associated with other ramus) displays a well-developed medial pterygoid tubercle on biological particularities of this population. However, it is impor- theinnersurfaceoftheramus(Carbonell et al., 2005). This trait tant to note that many other skeletal features in the SH hominins do has been proposed to be an apomorphy of the Neanderthal not show such derived character states as dental morphology lineage (Rak et al., 1994; Weaver, 2009), although a recent study (Arsuaga et al., 2014). For example, cranial (including temporal) and has shown that it can be observed at low frequencies in taxa facial morphology (Arsuaga et al., 1997; Martínez and Arsuaga et al., other than Neanderthals (Bermúdez de Castro et al., 2015). 1997), as well as some postcranial features (Carretero et al., 1997; Additionally, humeral morphology in H. antecessor has recently Gomez-Olivencia et al., 2007; Lorenzo, 2007; Gomez-Olivencia, been demonstrated to show a Neanderthal derived morphology 2009), show intermediate or primitive conditions with respect to (Bermúdez de Castro et al., 2012). later Neanderthals. This mosaic pattern of evolution would corre- Part of this apparent disagreement can be explained by the fact spond to a scenario where Neanderthal dental features would have that some characters supporting the existence of a H. antecessor-H. arisen early on in Neanderthal evolution together with other traits neanderthalensis-H. sapiens clade are based on highly fragmentary related to masticatory functions (Arsuaga et al., 2014), giving rise to evidence (e.g., morphology of the temporal squama and cranial a period during which dental morphology changed only slightly or capacity). Moreover, the high cranial capacity of H. antecessor has even faded in its Neanderthal apomorphies. Not surprisingly, been supported for some time by the alleged attribution of the incipient Neanderthal dental features seem to be one of the first Ceprano calvaria (with an adult cranial capacity of approximately acquisitions of Lower Pleistocene European populations (see 1185 cc) to H. antecessor (Manzi et al., 2001). This evidence, below). The clear derived state of the SH dentitions, as well as the however, was weakened by more recent dating of the Ceprano incipiently derived morphology of Gran Dolina-TD6 hominins to- fossil to ca. 0.45 Ma (millions of years ago) (Muttoni et al., 2009). wards Neanderthal conformations (Gomez-Robles et al., 2013), Regarding midfacial topology, classic concerns have focused on the reveals that dental traits can be leading the Neanderthalization juvenile status of the ATD6-69 maxilla (e.g., Rightmire, 2001). All process in populations that remain undifferentiated (TD6) or not in all, the meaning of facial morphology of H. antecessor remains fully differentiated (SH) in other traits. This model is plausible if controversial. While some studies point to clear similarities with considering the correlation of dental evolution in different dental H. sapiens (Lacruz et al., 2013), others suggest that those similar- classes, which would be evolving as a strongly integrated unit ities are part of a generalized pattern of facial architecture shared (Gomez-Robles and Polly, 2012). with Middle Pleistocene populations (Vialet, 2005, 2009; Freidline Gran Dolina-TD6 hominins Gran Dolina-TD6 fossils were proposed et al., 2013), or they question the phylogenetic utility of the canine to belong to the last common ancestral species of Neanderthals and fossa, claiming that the depressed infraorbital morphology is an modern humans (H. antecessor) according to certain allometric secondary result of small size (Maddux and Franciscus, synapomorphic traits shared by the three species (Bermúdez de 2009). Castro et al., 1997): a high convex anterior border of the temporal These observations emphasize that, while it is possible to match squama, a gracile mandibular corpus without alveolar individual derived characters from the Gran Dolina-TD6 hominins prominence at the M1 level, a cranial capacity above 1000 cc, a with later fossil species, it is not easy to know which of these modern midfacial topography, and a nearly vertical and anteriorly characters have the most phylogenetic valence. It is important to positioned incisive canal (Bermúdez de Castro et al., 2004). stress that genetic coalescence, population separation, and Further detailed monographic studies of cranial and postcranial phenotypic differentiation of two taxa represent chronologically remains of H. antecessor either supported the position of distinct events (Hublin, 2009). Different characters may coalesce in H. antecessor as the last common ancestor of H. neanderthalensis the ancestral species in different ways, giving rise to different and H. sapiens (Arsuaga et al., 1999; Rosas and Bermúdez de scenarios depending on the studied trait. Some characters (those Castro, 1999) or were unable to rule it out (Carretero et al., 1999; with a strong phylogenetic signal) will mirror the topology of the Lorenzo et al., 1999). Later studies showed similar conclusions real phylogeny, whereas others will follow atypical coalescence (Carbonell et al., 2005; Bermúdez de Castro et al., 2008; García- patterns. If some traits diversify before separation of the lineages, Gonzalez et al., 2009; Gomez-Olivencia et al., 2010), although the ancestral species will show phenotypic polymorphisms, leading some questions have been raised about the evolutionary to the identification of apomorphic traits of one of the lineages in continuity or discontinuity of Gran Dolina and Sima de los the incipiently diversifying populations of the ancestral species. Huesos populations (Bermúdez de Castro et al., 2003a). This scenario would be facilitated by geographic fragmentation of Nevertheless, the study of dental morphology has revealed the ancestral species in different (and perhaps isolated) areas the existence of certain similarities between H. antecessor and (Dennell et al., 2011). H. neanderthalensis (and European H. heidelbergensis), but not Focusing on dental morphology, the suite of traits that has between H. antecessor and H. sapiens (Martinon-Torres, 2006, been the subject of detailed analysis in this series of papers, the 2006; Martinon-Torres et al., 2007a, 2007b; Gomez-Robles phylogenetic position of H. antecessor can be interpreted in et al., 2007, 2011a, b; Gomez-Robles, 2010). These studies show different ways. First, it can be proposed that H. antecessor re- a higher resemblance in upper dentition morphology between mains are located at the very beginning of the Neanderthal H. antecessor and Neanderthals, although the morphology of lineage. However, this hypothesis is in deep disagreement with lower teeth tends to keep a rather primitive pattern (Bermúdez divergence times suggested by the majority of molecular studies 48 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50

(e.g., Krings et al., 1997, 1999; Ovchinnikov et al., 2000; Ho et al., Appendix A. Supplementary data 2005; Green et al., 2010). The second possibility is that H. antecessor is a dead evolutionary lineage (replaced or geneti- Supplementary data related to this article can be found at http:// cally absorbed during the Middle Pleistocene by other pop- dx.doi.org/10.1016/j.jhevol.2015.02.013. ulations, Schwartz and Tattersall, 2005) sharing some phenetic similarities with Neanderthals. An intermediate explanation References would set H. antecessor as part of the first populations where incipient Neanderthal characters would be arising, although Arnold, L.J., Demuro, M., Pares, J.M., Arsuaga, J.L., Aranburu, A., Bermúdez de reproductive isolation from other penecontemporary pop- Castro, J.M., Carbonell, E., 2014. Luminescence dating and palaeomagnetic age constraint on hominins from Sima de los Huesos, Atapuerca. Spain. J. Hum. Evol. ulations (caused by geographic isolation in Western European 67, 85e107. regions) would not be complete (Gomez-Robles et al., 2013). Arsuaga, J.L., Martínez, I., Gracia, A., Lorenzo, C., 1997. The Sima de los Huesos crania (Sierra de Atapuerca, Spain). A comparative study. J. Hum. Evol. 33, 219e281. Arsuaga, J.L., Martínez, I., Lorenzo, C., Gracia, A., Munoz,~ A., Alonso, O., Gallego, J., Conclusions 1999. The human cranial remains from Gran Dolina Lower Pleistocene site (Sierra de Atapuerca, Spain). J. Hum. Evol. 37, 431e457. Arsuaga, J.L., Martínez, I., Arnold, L.J., Aranburu, A., Gracia-Tellez, A., Sharp, W.D., The utility of hominin lower molars for taxonomic and phylo- fi Quam, R.M., Falgueres, C., Pantoja-Perez, A., Bischoff, J., Poza-Rey, E., Pares, J.M., genetic purposes differs across the three molars. While rst molars Carretero, J.M., Demuro, M., Lorenzo, C., Sala, N., Martinon-Torres, M., García, N., show low variability among species but high classification ability, Alcazar de Velasco, A., Cuenca-Bescos, G., Gomez-Olivencia, A., Moreno, D., second and third molars show increased ranges of variation and Pablos, A., Shen, C.-C., Rodríguez, L., Ortega, A.I., García, R., Bonmatí, A., Ber- fi múdez de Castro, J.M., Carbonell, E., 2014. Neandertal roots: cranial and chro- decreased ability to correctly classify individuals. Lower rst molars nological evidence from Sima de los Huesos. Science 344, 1358e1363. are generally morphologically stable throughout the hominin fossil Bailey, S.E., 2002a. A closer look at Neanderthal postcanine dental morphology: I. record, but lower second and third molars show a clear trend to The mandibular dentition. New Anat. 269, 148e156. Bailey, S.E., 2002b. Neandertal dental morphology: implications for modern human distal reduction in later species that parallels distal reductions origins. Ph.D. Dissertation, Arizona State University. observed in upper and lower premolars and upper second and third Bailey, S.E., 2004. A morphometric analysis of maxillary molar crowns of Middle- molars. Although allometry is involved in the described morpho- Late Pleistocene hominins. J. Hum. Evol. 47, 183e198. logical changes, allometric variation in lower molars is substantially Bailey, S.E., Lynch, J.M., 2005. Diagnostic differences in mandibular P4 shape between Neandertals and anatomically modern humans. Am. J. Phys. Anthropol. weaker than in upper molars. This may be related to the higher 126, 268e277. stability of the lower dentition as compared with the upper denti- Bailey, S.E., Pilbrow, V.C., Wood, B.A., 2004. Interobserver error involved in inde- e tion. Although a parsimonious explanation for the pattern of molar pendent attempts to measure cusp base areas of Pan M1s. J. Anat. 205, 323 331. Bailey, S.E., Weaver, T.D., Hublin, J.-J., 2009. Who made the Aurignacian and other reduction observed in late Homo species is the inheritance from an early Upper Paleolithic industries? J. Hum. Evol. 57, 11e26. already reduced common ancestor, changes in developmental Bailey, S.E., Skinner, M.M., Hublin, J.-J., 2011. What lies beneath? An evaluation of timing can also be advocated to support a parallel reduction in lower molar trigonid crest patterns based on both dentine and enamel expression. Am. J. Phys. Anthropol. 145, 505e518. different lineages. Patterns of variation observed in this study, as Bailey, S.E., Benazzi, S., Hublin, J.-J., 2014. Allometry, merism, and tooth shape of the well as in previous papers in this series, evince some particularities upper deciduous M2 and permanent M1. Am. J. Phys. Anthropol. 154, 104e114. of European Pleistocene populations. Within their clear similarities Benazzi, S., Coquerelle, M., Fiorenza, L., Bookstein, F., Katina, S., Kullmer, O., 2011a. Comparison of dental measurement systems for taxonomic assignment of first with Neanderthals, the Sima de los Huesos hominins show a strong molars. Am. J. Phys. Anthropol. 144, 342e354. dental reduction, especially in the most distal molars. In fact, the SH Benazzi, S., Douka, K., Fornai, C., Bauer, C.C., Kullmer, O., Svoboda, J., Pap, I., hominins have more reduced second and third molars than classic Mallegni, F., Bayle, P., Coquerelle, M., Condemi, S., Ronchitelli, A., Harvati, K., Weber, G.W., 2011b. Early dispersal of modern humans in Europe and impli- Neanderthals. Homo antecessor hominins from TD6 in general have a cations for Neanderthal behaviour. Nature 479, 525e528. primitive lower dentition (with the exception of one extremely Bermúdez de Castro, J.M., 1989. Third molar agenesis in human prehistoric pop- reduced third molar) that contrasts with their more derived, ulations of the. Canary Islands. Am. J. Phys. Anthropol. 79, 207e215. Neanderthal-like upper teeth. These dental affinities, together with Bermúdez de Castro, J.M., Nicolas, M.E., 1995. Posterior dental size reduction in hominids: the Atapuerca evidence. Am. J. Phys. Anthropol. 96, 335e356. the information coming from non-dental evidence, support an early Bermúdez de Castro, J.M., Nicolas, M.E., 1996. Changes in the lower premolar-size model for the origin of Neanderthals. In order for this model to be sequence during hominid evolution. Phylogenetic implications. Hum. Evol. 11, e reconciled with the evidence coming from paleogenetic data, it may 205 215. Bermúdez de Castro, J.M., Arsuaga, J.L., Carbonell, E., Rosas, A., Martinez, I., be necessary to accept complex evolutionary dynamics and in- Mosquera, M., 1997. A hominid from the Lower Pleistocene of Atapuerca, Spain: teractions between late Lower and Middle Pleistocene populations. possible ancestor to Neandertals and modern humans. Science 276, 1392e1395. Bermúdez de Castro, J.M., Rosas, A., Carbonell, E., Nicolas, M.E., Rodríguez, J., Arsuaga, J.L., 1999a. A modern human pattern of dental development in Lower Acknowledgments Pleistocene hominids from Atapuerca-TD6 (Spain). Proc. Natl. Acad. Sci. 96, 4210e4213. We are grateful to members of the Atapuerca research team and Bermúdez de Castro, J.M., Rosas, A., Nicolas, M.E., 1999b. Dental remains from Atapuerca-TD6 (Gran Dolina site, Burgos, Spain). J. Hum. Evol. 37, 523e566. to researchers in the institutions where we have studied dental Bermúdez de Castro, J.M., Martinon-Torres, M., Sarmiento, S., Lozano, M., 2003a. fossils: O. Kullmer, B. Denkel, and F. Schrenk (Senckenberg Insti- Gran Dolina-TD6 versus Sima de los Huesos dental samples from Atapuerca: tute); D. Lordkipanidze, A. Vekua, and G. Kiladze (Georgian National evidence of discontinuity in the European Pleistocene population? J. Archaeol. Sci. 30, 1421e1428. Museum); H. de Lumley, M.A. de Lumley, and A. Vialet (Institut de Bermúdez de Castro, J.M., Martinon-Torres, M., Sarmiento, S., Lozano, M., Paleontologie Humaine); P. Mennecier (Musee de l'Homme); I. Arsuaga, J.L., Carbonell, E., 2003b. Rates of anterior tooth wear in Middle Tattersall, G. Sawyer, and G. García (American Museum of Natural Pleistocene hominins from Sima de los Huesos (Sierra de Atapuerca, Spain). e History); L. Jellema and Y. Haile-Selassie (Cleveland Museum of Proc. Natl. Acad. Sci. 100, 11992 11996. Bermúdez de Castro, J.M., Martinon-Torres, M., Carbonell, E., Sarmiento, S., Rosas, A., Natural History); and M. Botella (Laboratory of Physical Anthro- Van der Made, J., Lozano, M., 2004. The Atapuerca sites and their contribution to pology, University of Granada). We are also grateful to the Associate the knowledge of human evolution in Europe. Evol. Anthropol. 13, 25e41. Editor and the anonymous reviewers for their helpful and Bermúdez de Castro, J.M., Perez-Gonzalez, A., Martinon-Torres, M., Gomez- Robles, A., Rosell, J., Prado, L., Sarmiento, S., Carbonell, E., 2008. A new early constructive comments. This work was partially supported by Pleistocene hominin mandible from Atapuerca-TD6, Spain. J. Hum. Evol. 55, funding by the Spanish Ministry of Science and Innovation (Project 729e735. CGL2009-12703-C03-01-02-03). Fieldwork at Atapuerca is sup- Bermúdez de Castro, J.M., Martinon-Torres, M., Prado, L., Gomez-Robles, A., Rosell, J., Lopez-Polín, L., Arsuaga, J.L., Carbonell, E., 2010. New immature hominin fossil ported by the Consejería de Cultura y Turismo (Junta de Castilla y from European Lower Pleistocene shows the earliest evidence of a modern Leon) and the Fundacion Atapuerca. human dental development pattern. Proc. Natl. Acad. Sci. 107, 11739e11744. A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50 49

Bermúdez de Castro, J.M., Carretero, J.M., García-Gonzalez, R., Rodríguez-García, L., morphometrics: a practical approach through hominin molar morphology. Martinon-Torres, M., Rosell, J., Blasco, R., Martín-Frances, L., Modesto, M., Evolution 65, 1772e1790. Carbonell, E., 2012. Early Pleistocene human humeri from the Gran Dolina-TD6 Gomez-Robles, A., Bermúdez de Castro, J.M., Martinon-Torres, M., Prado-Simon, L., site (Sierra de Atapuerca, Spain). Am. J. Phys. Anthropol. 147, 604e617. Arsuaga, J.L., 2012. A geometric morphometric analysis of hominin upper sec- Bermúdez de Castro, J.M., Quam, R., Martinon-Torres, M., Martínez, I., Gracia- ond and third molars, with particular emphasis on European Pleistocene pop- Tellez, A., Arsuaga, J.L., Carbonell, E., 2015. The medial pterygoid tubercle in the ulations. J. Hum. Evol. 63, 512e526. Atapuerca Early and Middle Pleistocene mandibles: Evolutionary implications. Gomez-Robles, A., Bermúdez de Castro, J.M., Arsuaga, J.-L., Carbonell, E., Polly, P.D., Am. J. Phys. Anthropol. 156, 102e109. 2013. No known hominin species matches the expected dental morphology of Bischoff, J.L., Shamp, D.D., Aramburu, A., Arsuaga, J.L., Carbonell, E., Bermudez de the last common ancestor of Neanderthals and modern humans. Proc. Natl. Castro, J.M., 2003. The Sima de los Huesos hominids date to beyond U/Th Acad. Sci. 110, 18196e18201. equilibrium (>350 kyr) and perhaps to 400e500 kyr: new radiometric dates. Green, R.E., Krause, J., Briggs, A.W., Maricic, T., Stenzel, U., Kircher, M., Patterson, N., J. Archaeol. Sci. 30, 275e280. Li, H., Zhai, W., Fritz, M.H.-Y., Hansen, N.F., Durand, E.Y., Malaspinas, A.-S., Bischoff, J.L., Williams, R.W., Rosenbauer, R.J., Aramburu, A., Arsuaga, J.L., García, N., Jensen, J.D., Marques-Bonet, T., Alkan, C., Prüfer, K., Meyer, M., Burbano, H.A., Cuenca-Bescos, G., 2007. High-resolution U-series dates from the Sima de los Good, J.M., Schultz, R., Aximu-Petri, A., Butthof, A., Hober,€ B., Hoffner,€ B., Huesos hominids yields 600 kyrs: implications for the evolution of the early Siegemund, M., Weihmann, A., Nusbaum, C., Lander, E.S., Russ, C., Novod, N., Neanderthal lineage. J. Archaeol. Sci. 34, 763e770. Affourtit, J., Egholm, M., Verna, C., Rudan, P., Brajkovic, D., Kucan, Z., Gusic, I., Bookstein, F.L., 1991. Morphometric Tools for Landmark Data. Cambridge University Doronichev, V.B., Golovanova, L.V., Lalueza-Fox, C., de la Rasilla, M., Fortea, J., Press, Cambridge. Rosas, A., Schmitz, R.W., Johnson, P.L.F., Eichler, E.E., Falush, D., Birney, E., Carbonell, E., Bermúdez de Castro, J.M., Arsuaga, J.L., Allue, E., Bastir, M., Benito, A., Mullikin, J.C., Slatkin, M., Nielsen, R., Kelso, J., Lachmann, M., Reich, D., Pa€abo,€ S., Caceres, I., Canals, T., Díez, J.C., van der Made, J., Mosquera, M., Olle, A., Perez- 2010. A draft sequence of the Neandertal genome. Science 328, 710e722. Gonzalez, A., Rodríguez, J., Rodríguez, X.P., Rosas, A., Rosell, J., Sala, R., Grine, F.E., 1986. Dental evidence for dietary differences in Australopithecus and Vallverdú, J., Verges, J.M., 2005. An Early Pleistocene hominin mandible from Paranthropus: a quantitative analysis of permanent molar microwear. J. Hum. Atapuerca-TD6. Spain. Proc. Natl. Acad. Sci. 102, 5674e5678. Evol. 15, 783e822. Carretero, J.M., Arsuaga, J.L., Lorenzo, C., 1997. Clavicles, scapulae and humeri from Guatelli-Steinberg, D., Irish, J.D., 2005. Early hominin variability in first molar dental the Sima de los Huesos Site (Sierra de Atapuerca, Spain). J. Hum. Evol. 33, trait frequencies. Am. J. Phys. Anthropol. 128, 477e484. 357e408. Gunz, P., Mitteroecker, P., Bookstein, F., 2005. Semilandmarks in three dimensions. Carretero, J.M., Lorenzo, C., Arsuaga, J.L., 1999. Axial and appendicular skeleton of In: Slide, D.E. (Ed.), Modern Morphometrics in Physical Anthropology. Springer, Homo antecessor. J. Hum. Evol. 37, 459e499. New York, pp. 73e98. Caumul, R., Polly, P.D., 2005. Phylogenetic and environmental components of Hlusko, L.J., Maas, M.L., Mahaney, M.C., 2004. Statistical genetics of molar cusp morphological variation: skull, mandible, and molar shape in marmots (Mar- patterning in pedigreed baboons: implications for primate dental development mota, Rodentia). Evolution 59, 2460e2472. and evolution. J. Exp. Zool. 302B, 268e283. Dennell, R.W., Martinon-Torres, M.a., Bermúdez de Castro, J.M., 2011. Hominin Ho, S.Y.W., Phillips, M.J., Cooper, A., Drummond, A.J., 2005. Time dependency of variability, climatic instability and population demography in Middle Pleisto- molecular rate estimates and systematic overestimation of recent divergence cene Europe. Quatern. Sci. Rev. 30, 1511e1524. times. Mol. Biol. Evol. 22, 1561e1568. Emonet, E.-G., Tafforeau, P., Chaimanee, Y., Guy, F., de Bonis, L., Koufos, G., Jaeger, J.- Hublin, J.-J., 2009. The origins of Neandertals. Proc. Natl. Acad. Sci. 106, J., 2012. Three-dimensional analysis of mandibular dental root morphology in 16022e16027. hominoids. J. Hum. Evol. 62, 146e154. Irish, J.D., 1998. Ancestral dental traits in recent sub-Saharan Africans and the ori- Endicott, P., Ho, S.Y., Stringer, C., 2010. Using genetic evidence to evaluate four gins of modern humans. J. Hum. Evol. 34, 81e98. palaeoanthropological hypotheses for the timing of Neanderthal and modern Irish, J.D., Guatelli-Steinberg, D., 2003. Ancient teeth and modern human origins: an human origins. J. Hum. Evol. 59, 87e95. expanded comparison of African Plio-Pleistocene and recent world dental Freidline, S.E., Gunz, P., Harvati, K., Hublin, J.-J., 2013. Evaluating developmental samples. J. Hum. Evol. 45, 113e144. shape changes in Homo antecessor subadult facial morphology. J. Hum. Evol. 65, Jernvall, J., 2000. Linking development with generation of novelty in mammalian 404e423. teeth. Proc. Natl. Acad. Sci. 97, 2641e2645. García-Gonzalez, R., Carretero, J.M., Rodríguez, L., Gomez-Olivencia, A., Arsuaga, J.L., Kaifu, Y., Baba, H., Aziz, F., Indriati, E., Schrenk, F., Jacob, T., 2005. Taxonomic af- Bermúdez de Castro, J.M., Carbonell, E., Martínez, I., Lorenzo, C., 2009. Etude finities and evolutionary history of the early Pleistocene hominids of Java: analytique d'une clavicule complete de subadulte d'Homo antecessor (site de dentognathic evidence. Am. J. Phys. Anthropol. 128, 709e726. Gran Dolina, Sierra d'Atapuerca, Burgos, Espagne). L'Anthropologie 113. Klingenberg, C.P., Gidaszewski, N.A., 2010. Testing and quantifying phylogenetic Gharaibeh, W.S., 2005. Correcting for the effect of orientation in geometric mor- signals and homoplasy in morphometric data. Syst. Biol. 59, 245e261. phometrics studies of side-view images of human heads. In: Slice, D.E. (Ed.), Klingenberg, C.P., 2011. MorphoJ: an integrated software package for geometric Modern Morphometrics in Physical Anthropology. Kluwer Academic/Plenum morphometrics. Mol. Ecol. Resour. 11, 353e357. Publishers, New York, pp. 117e143. Klingenberg, C.P., Marugan-Lob on, J., 2013. Evolutionary covariation in geometric Gomez-Olivencia, A., 2009. Estudios paleobiologicos sobre la columna vertebral y morphometric data: analyzing integration, modularity, and allometry in a la caja toracica de los humanos fosiles del pleistoceno, con especial referencia phylogenetic context. Syst. Biol. 62, 591e610. alosfosiles de la Sierra de Atapuerca. Ph.D. Dissertation, Universidad de Kondo, S., Yamada, H., 2003. Cusp size variability of the maxillary molariform teeth. Burgos. Anthropol. Sci. 111, 255e263. Gomez-Olivencia, A., Carretero, J.M., Arsuaga, J.L., Rodríguez-García, L., García- Krings, M., Stone, A., Schmitz, R.W., Krainitzki, H., Stoneking, M., Pa€abo,€ S., 1997. Gonzalez, R., Martínez, I., 2007. Metric and morphological study of the upper Neandertal DNA sequences and the origin of modern humans. Cell 90, 19e30. cervical spine from the Sima de los Huesos site (Sierra de Atapuerca, Burgos, Krings, M., Geisert, H., Schmitz, R.W., Krainitzki, H., Pa€abo,€ S., 1999. DNA sequence of Spain). J. Hum. Evol. 53, 6e25. the mitochondrial hypervariable region II from the Neandertal type specimen. Gomez-Olivencia, A., Carretero, J.M., Lorenzo, C., Arsuaga, J.L., Bermúdez de Proc. Natl. Acad. Sci. 96, 5581e5585. Castro, J.M., Carbonell, E., 2010. The costal skeleton of Homo antecessor: pre- Kupczik, K., Hublin, J.-J., 2010. Mandibular molar root morphology in Neanderthals liminary results. J. Hum. Evol. 59, 620e640. and Late Pleistocene and recent Homo sapiens. J. Hum. Evol. 59, 525e541. Gomez-Robles, A., 2010. Analisis de la forma dental en la filogenia humana. Ten- Lacruz, R.S., de Castro, J.M.B., Martinon-Torres, M., O'Higgins, P., Paine, M.L., dencias y modelos evolutivos basados en metodos de morfometría geometrica. Carbonell, E., Arsuaga, J.L., Bromage, T.G., 2013. Facial morphogenesis of the Ph.D. Dissertation, Universidad de Granada. earliest Europeans. PLoS ONE 8, e65199. Gomez-Robles, A., Polly, P.D., 2012. Morphological integration in the hominin Lordkipanidze, D., Ponce de Leon, M.S., Margvelashvili, A., Rak, Y., Rightmire, G.P., dentition: evolutionary, developmental, and functional factors. Evolution 66, Vekua, A., Zollikofer, C.P.E., 2013. A complete skull from Dmanisi, Georgia, and 1024e1043. the evolutionary biology of early Homo. Science 342, 326e331. Gomez-Robles, A., Martinon-Torres, M., Bermúdez de Castro, J.M., Lorenzo, C., 2007. Evolucion de la mano de los homínidos. Analisis morfologico de Margvelashvili, A., Bastir, M., Arsuaga, J.L., Perez-P erez, A., Estebaranz, F., los fosiles de la Sierra de Atapuerca. Ph.D. Dissertation, Universidad Complu- Martínez, L.M., 2007. A geometric morphometric analysis of hominin upper first tense de Madrid. molar shape. J. Hum. Evol. 53, 272e285. Lorenzo, C., Arsuaga, J.L., Carretero, J.M., 1999. Hand and foot remains from the Gran Gomez-Robles, A., Martinon-Torres, M., Bermúdez de Castro, J.M., Prado, L., Dolina Early Pleistocene site (Sierra de Atapuerca, Spain). J. Hum. Evol. 37, 501e522. Sarmiento, S., Arsuaga, J.L., 2008. Geometric morphometric analysis of the Lozano, M., Bermúdez de Castro, J.M., Carbonell, E., Arsuaga, J.L., 2008. Non- crown morphology of the lower first premolar of hominins, with special masticatory uses of anterior teeth of Sima de los Huesos individuals (Sierra de attention to Pleistocene Homo. J. Hum. Evol. 55, 627e638. Atapuerca, Spain). J. Hum. Evol. 55, 713e728. Gomez-Robles, A., Bermúdez de Castro, J.M., Martinon-Torres, M., Prado-Simon, L., Macaluso, P.J., 2010. Variation in dental remains from Dmanisi, Georgia. Anthropol. 2011a. Crown size and cusp proportions in Homo antecessor upper first molars. Sci. 118, 31e40. A comment on Quam et al. 2009. J. Anat. 218, 258e262. Macho, G.A., Moggi-Cecchi, J., 1992. Reduction of maxillary molars in Homo sapiens Gomez-Robles, A., Martinon-Torres, M., Bermúdez de Castro, J.M., Prado-Simon, L., sapiens: a different perspective. Am. J. Phys. Anthropol. 87, 151e159. Arsuaga, J.L., 2011b. A geometric morphometric analysis of hominin upper Maddux, S.D., Franciscus, R.G., 2009. Allometric scaling of infraorbital surface premolars. Shape variation and morphological integration. J. Hum. Evol. 61, topography in Homo. J. Hum. Evol. 56, 161e174. 688e702. Manzi, G., Mallegni, F., Ascenzi, A., 2001. A cranium for the earliest Europeans: Gomez-Robles, A., Olejniczak, A.J., Martinon-Torres, M., Prado-Simon, L., Bermúdez phylogenetic position of the hominid from Ceprano, Italy. Proc. Natl. Acad. Sci. de Castro, J.M., 2011c. Evolutionary novelties and losses in geometric 98, 10011e10016. 50 A. Gomez-Robles et al. / Journal of Human Evolution 82 (2015) 34e50

Margvelashvili, A., 2006. The morphological description of the dental remains from Perez, S.I., Bernal, V., Gonzalez, P.N., 2006. Differences between sliding semi- the early Paleolithic site of Dmanisi (Georgia). M.Sc. Dissertation, Instituto landmark methods in geometric morphometrics, with an application to hu- Politecnico de Tomar-Universidade de Tras-os-Montes, Portugal, and Uni- man craniofacial and dental variation. J. Anat. 208, 769e784. versitat Rovira i Virgili, Spain. Rak, Y., Kimbel, W.H., Hovers, E., 1994. A Neandertal infant from Amud Cave, Israel. Martínez de Pinillos, M., Martinon-Torres, M., Skinner, M.M., Arsuaga, J.L., Gracia- J. Hum. Evol. 26, 313e324. Tellez, A., Martínez, I., Martín-Frances, L., Bermúdez de Castro, J.M., 2014. Reich, D., Green, R.E., Kircher, M., Krause, J., Patterson, N., Durand, E.Y., Viola, B., Trigonid crests expression in Atapuerca-Sima de los Huesos lower molars: in- Briggs, A.W., Stenzel, U., Johnson, P.L., Maricic, T., Good, J.M., Marques-Bonet, T., ternal and external morphological expression and evolutionary inferences. C. R. Alkan, C., Fu, Q., Mallick, S., Li, H., Meyer, M., Eichler, E.E., Stoneking, M., Palevol 13, 205e221. Richards, M., Talamo, S., Shunkov, M.V., Derevianko, A.P., Hublin, J.-J., Kelso, J., Martínez, I., Arsuaga, J.L., 1997. The temporal bones from Sima de los Huesos site Slatkin, M., Pa€abo,€ S., 2010. Genetic history of an archaic hominin group from (1984e1994). J. Hum. Evol. 33, 283e318. Denisova Cave in Siberia. Nature 468, 1053e1060. Martinon-Torres, M., 2006. Evolucion del aparato dental en homínidos: estudio de Rightmire, G.P., 2001. Patterns of hominid evolution and dispersal in the Middle los dientes humanos del Pleistoceno de la Sierra de Atapuerca (Burgos). Ph.D. Pleistocene. Quatern. Int. 75, 77e84. Dissertation, Universidad de Santiago de Compostela. Rohlf, F.J., 1998. TpsRelw. TpsSeries. Department of Ecology and Evolution, SUNY, Martinon-Torres, M., Bastir, M., Bermúdez de Castro, J.M., Gomez, A., Sarmiento, S., Stony Brook, New York. Muela, A., Arsuaga, J.L., 2006. Hominin lower second premolar morphology: Rohlf, F.J., 2005. TpsDig2. TpsSeries. Department of Ecology and Evolution, SUNY, evolutionary inferences through geometric morphometric analysis. J. Hum. Stony Brook, New York. Evol. 50, 523e533. Rohlf, F.J., Slice, D., 1990. Extensions of the Procrustes method for the optimal su- Martinon-Torres, M., Bermúdez de Castro, J.M., Gomez-Robles, A., Arsuaga, J.L., perimposition of landmarks. Syst. Zool. 39, 40e59. Carbonell, E., Lordkipanidze, D., Manzi, G., Margvelashvili, A., 2007a. Dental Rosas, A., Bermúdez de Castro, J.M., 1999. The ATD6-5 mandibular specimen from evidence on the hominin dispersals during the Pleistocene. Proc. Natl. Acad. Sci. Gran Dolina (Atapuerca, Spain). Morphological study and phylogenetic impli- 104, 13279e13282. cations. J. Hum. Evol. 37, 567e590. Martinon-Torres, M., Bermúdez de Castro, J.M., Gomez-Robles, A., Bastir, M., Scott, G.R., Turner, C.G., 1997. The Anthropology of Modern Human Teeth: Dental Sarmiento, S., Muela, A., Arsuaga, J.L., 2007b. Gran Dolina-TD6 and Sima de los Morphology and its Variation in Recent Human Populations. Cambridge Uni- Huesos dental samples: preliminary approach to some dental characters of versity Press, Cambridge. interest for phylogenetic studies. In: Bailey, S.E., Hublin, J.-J. (Eds.), Dental Schwartz, J.H., Tattersall, I., 2005. The Human Fossil Record, Craniodental Perspectives on Human Evolution. Springer, Berlin, pp. 65e79. Morphology of Early Hominids (Genera Australopithecus, Paranthropus, Orrorin), Martinon-Torres, M., Bermúdez de Castro, J.M., Gomez-Robles, A., and Overview. Wiley-Liss, New York. Margvelashvili, A., Prado, L., Lordkipanidze, D., Vekua, A., 2008. Dental remains Skinner, M.M., Gunz, P., Wood, B.A., Hublin, J.-J., 2008. Enamel-dentine junction from Dmanisi (Republic of Georgia): morphological analysis and comparative (EDJ) morphology distinguishes the lower molars of Australopithecus africanus study. J. Hum. Evol. 55, 249e273. and Paranthropus robustus. J. Hum. Evol. 55, 979e988. Martinon-Torres, M., Bermúdez de Castro, J.M., Gomez-Robles, A., Prado-Simon, L., Stringer, C., 2012. The status of Homo heidelbergensis (Schoetensack 1908). Evol. Arsuaga, J.L., 2012. Morphological description and comparison of the dental Anthropol. 21, 101e107. remains from Atapuerca-Sima de los Huesos site (Spain). J. Hum. Evol. 62, Suwa, G., Wood, B.A., White, T.D., 1994. Further analysis of mandibular molar crown 7e58. and cusp areas in Pliocene and early Pleistocene hominids. Am. J. Phys. Martinon-Torres, M., Spev ackova, P., Gracia-Tellez, A., Martínez, I., Bruner, E., Anthropol. 93, 407e426. Arsuaga, J.L., Bermúdez de Castro, J.M., 2013. Morphometric analysis of molars Suwa, G., White, T.D., Howell, F.C., 1996. Mandibular postcanine dentition from the in a Middle Pleistocene population shows a mosaic of ‘modern’ and Neander- Shungura Formation, Ethiopia: crown morphology, taxonomic allocations, and thal features. J. Anat. 223, 353e363. Plio-Pleistocene hominid evolution. Am. J. Phys. Anthropol. 101, 247e282. Meyer, M., Kircher, M., Gansauge, M.-T., Li, H., Racimo, F., Mallick, S., Schraiber, J.G., Takahashi, M., Kondo, S., Townsend, G.C., Kanazawa, E., 2007. Variability in cusp size Jay, F., Prüfer, K., de Filippo, C., Sudmant, P.H., Alkan, C., Fu, Q., Do, R., of human maxillary molars, with particular reference to the hypocone. Arch. Rohland, N., Tandon, A., Siebauer, M., Green, R.E., Bryc, K., Briggs, A.W., Oral Biol. 52, 1146e1154. Stenzel, U., Dabney, J., Shendure, J., Kitzman, J., Hammer, M.F., Shunkov, M.V., Tobias, P.V., 1991. Olduvai Gorge. The Skulls Endocasts and Teeth of Homo habilis. Derevianko, A.P., Patterson, N., Andres, A.M., Eichler, E.E., Slatkin, M., Reich, D., Cambridge University Press, Cambridge. Kelso, J., Pa€abo,€ S., 2012. A high-coverage genome sequence from an archaic Vialet, A., 2005. La face superieure et moyenne des hominides fossiles depuis le Denisovan individual. Science 338, 222e226. Pleistoc ene inferieur recent. Ph.D. Dissertation, Museum National d'Histoire Meyer, M., Fu, Q., Aximu-Petri, A., Glocke, I., Nickel, B., Arsuaga, J.-L., Martinez, I., Naturelle, Paris. Gracia, A., de Castro, J.M.B., Carbonell, E., Pa€abo,€ S., 2014. A mitochondrial Vialet, A., 2009. L'evolution de l'homme en Eurasie: nouveaux fossiles, nouveaux genome sequence of a hominin from Sima de los Huesos. Nature 505, outils d'analyse. L'Anthropologie 113, 245e254. 403e406. Weaver, T.D., 2009. The meaning of Neandertal skeletal morphology. Proc. Natl. Moggi-Cecchi, J., Boccone, S., 2007. Maxillary molars cusp morphology of South Acad. Sci. 106, 16028e16033. African australopithecines. In: Bailey, S.E., Hublin, J.-J. (Eds.), Dental Perspec- Weaver, T.D., Roseman, C.C., Stringer, C.B., 2007. Were Neandertal and modern tives on Human Evolution. Springer-Verlag, Berlin, pp. 53e64. human cranial differences produced by natural selection or genetic drift? Moggi-Cecchi, J., Grine, F.E., Tobias, P.V., 2006. Early hominid dental remains from J. Hum. Evol. 53, 135e145. members 4 and 5 of the Sterkfontein Formation (1966e1996 excavations): Williams, B.A., Corruccini, R.S., 2007. The relationship between crown size and catalogue, individual associations, morphological descriptions and initial complexity in two collections. Dent. Anthropol. 20, 29e32. metrical analysis. J. Hum. Evol. 50, 239e328. Wood, B.A., Abbott, S.A., 1983. Analysis of the dental morphology of Plio-Pleistocene Molnar, S., 1971. Human tooth wear, tooth function and cultural variability. Am. J. hominids I. Mandibular molars: crown area measurements and morphological Phys. Anthropol. 34, 175e190. traits. J. Anat. 136, 197e219. Muttoni, G., Scardia, G., Kent, D.V., Swisher, C.C., Manzi, G., 2009. Pleistocene Wood, B., Abbott, S.A., Graham, S.H., 1983. Analysis of the dental morphology of magnetochronology of early hominin sites at Ceprano and Fontana Ranuccio, Plio-Pleistocene hominids II. Mandibular molars e study of cusp areas, ﬁssure Italy. Earth Planet. Sci. Lett. 286, 255e268. pattern and cross sectional shape of the crown. J. Anat. 137, 287e314. Ovchinnikov, I.V., Gotherstrom, A., Romanova, G.P., Kharitonov, V.M., Liden, K., Zanolli, C., 2013. Additional evidence for morpho-dimensional tooth crown varia- Goodwin, W., 2000. Molecular analysis of Neanderthal DNA from the northern tion in a new Indonesian H. erectus sample from the Sangiran Dome (Central Caucasus. Nature 404, 490e493. Java). PLoS ONE 8, e67233.