Lei Pan

Jean Dumoncel

Arnaud Mazurier

Clément Zanolli

Structural analysis of premolar roots in Middle Pleistocene hominins from China

Keywords: East Asian Middle Pleistocene Homo, permanent premolar, external root surface, crown-root tissue proportions

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Introduction

Hominin postcanine tooth root and canal anatomy is highly variable (e.g., [START_REF] Sert | Evaluation of the root canal configurations of the mandibular and maxillary permanent teeth by gender in the Turkish population[END_REF]Prado-Simón et al., 2012a;[START_REF] Kupczik | Mandibular molar root and pulp cavity morphology in Homo naledi and other Plio-Pleistocene hominins[END_REF]. Using direct observation of the external roots [START_REF] Turner | Root number determination in maxillary first premolars for modern human populations[END_REF][START_REF] Spencer | Tooth-root form and function in platyrrhine seed-eaters[END_REF][START_REF] Moggi-Cecchi | Early hominin dental remains from the Plio-Pleistocene site of Drimolen, South Africa[END_REF], 2D radiography [START_REF] Sperber | Morphology of the cheek teeth of early South African hominids[END_REF][START_REF] Abbott | A comparative study of tooth root morphology in the great apes, modern man and early hominids[END_REF]Wood et al., 1988;[START_REF] Wood | Analysis of the dental morphology of Plio-Pleistocene hominids. V. Maxillary postcanine tooth morphology[END_REF], and medical/micro-tomography [START_REF] Kupczik | Tooth root morphology and masticatory muscle force pattern in humans and nonhuman primates[END_REF][START_REF] Kupczik | Premolar root number variation in hominoids: genetic polymorphism vs. functional significance[END_REF][START_REF] Kupczik | Comparative observations on the tooth root morphology of Gigantopithecus blacki[END_REF][START_REF] Moore | Premolar root morphology and metric variation in Pan troglodytes verus[END_REF][START_REF] Moore | Premolar root and canal variation in extant non-human hominoidea[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF], studies have demonstrated that tooth roots preserve information about dietary adaptation, taxonomy and phylogeny [START_REF] Abbott | A comparative study of tooth root morphology in the great apes, modern man and early hominids[END_REF]Wood et al., 1988;[START_REF] Wood | Analysis of the dental morphology of Plio-Pleistocene hominids. V. Maxillary postcanine tooth morphology[END_REF][START_REF] White | Jaws and teeth of Australopithecus afarensis from Maka, Middle Awash, Ethiopia[END_REF][START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF]. In Australopithecus, Paranthropus and early Homo the plesiomorphic great-ape pattern is expressed by (1) three-rooted maxillary premolars with three canals, being supported by one lingual and two buccal roots, and (2) double-rooted mandibular premolars with three (P 3 ) or four (P 4 ) canals [START_REF] Abbott | A comparative study of tooth root morphology in the great apes, modern man and early hominids[END_REF]Wood et al., 1988;[START_REF] Emonet | Khoratpithecus et la radiation des hominoïdes en Asie du Sud-Est au Miocène[END_REF][START_REF] Moore | Premolar root morphology and metric variation in Pan troglodytes verus[END_REF][START_REF] Moore | Premolar root and canal variation in extant non-human hominoidea[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF][START_REF] Emonet | Subocclusal dental morphology of Sahelanthropus tchadensis and the evolution of teeth in hominins[END_REF]. The derived modern human pattern is a reduced premolar root number and simplified root form, predominately represented by single-rooted premolars, even if variants such as doublerooted maxillary premolars and Tomes'-rooted mandibular premolars (Tomes' root refers to some lower premolars being C-shaped in cross section, which resulted from the radicular or developmental grooves of different levels that are considered to be associated with the evolution from multiple roots to single root) also exist in modern human populations (e.g., [START_REF] Tomes | A manual of dental anatomy: human and comparative[END_REF][START_REF] Loh | Root morphology of the maxillary first premolar in Singaporeans[END_REF][START_REF] Sert | Evaluation of the root canal configurations of the mandibular and maxillary permanent teeth by gender in the Turkish population[END_REF][START_REF] Shields | Mandibular premolar and second molar root morphological variation in modern humans: what root number can tell us about tooth morphogenesis[END_REF][START_REF] Dou | Root anatomy and canal morphology of mandibular first premolars in a Chinese population[END_REF][START_REF] Scott | The Anthropology of Modern Human Teeth. Dental Morphology and its Variation in Recent and Fossil Homo sapiens[END_REF]. in addition to quantitative investigations involving standard metrics such as root length, root surface area, cervical area and root volume [START_REF] Kupczik | Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens[END_REF][START_REF] Cabec | Long anterior mandibular tooth roots in Neanderthals are not the result of their large jaws[END_REF], 2013;[START_REF] Moore | Premolar root morphology and metric variation in Pan troglodytes verus[END_REF][START_REF] Moore | Premolar root and canal variation in extant non-human hominoidea[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF][START_REF] Kupczik | Mandibular molar root and pulp cavity morphology in Homo naledi and other Plio-Pleistocene hominins[END_REF], advances in X-ray microtomographic (micro CT) techniques have helped to characterize premolar root phenotypes and reveal previously undocumented variation in fossil hominins (e.g., Prado-Simón et al., 2012b;[START_REF] Kaifu | Descriptions of the dental remains of Homo floresiensis[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF][START_REF] Liu | A mandible from the Middle Pleistocene Hexian site and its significance in relation to the variability of Asian Homo erectus[END_REF].. Studies on primate postcanine roots reveal that the proportion of crown and root surface areais a useful proxy for both functional and phylogenetic assessments. For example, among Gorilla gorilla, Pan troglodytes and P. pygmaeus, G. gorilla has the largest relative postcanine root surface area, resulting in a larger attachment area that provides a functional advantage as it consumes mechanically resistant foods, and closely related species have similar crown-root area proportions [START_REF] Kupczik | Premolar root number variation in hominoids: genetic polymorphism vs. functional significance[END_REF][START_REF] Kupczik | Molar crown and root size relationship in anthropoid primates[END_REF]; [START_REF] Kupczik | Comparative observations on the tooth root morphology of Gigantopithecus blacki[END_REF]. In contrast, landmarkbased, 3D geometric morphometric (GM) analyses of root and cervix shape [START_REF] Emonet | Threedimensional analysis of mandibular dental root morphology in hominoids[END_REF][START_REF] Moore | Premolar root morphology and metric variation in Pan troglodytes verus[END_REF][START_REF] Kupczik | On the relationship between maxillary molar root shape and jaw kinematics in Australopithecus africanus and Paranthropus robustus[END_REF] support a strong association between hominin taxonomy, phylogeny and root morphology. In spite of the fact that GM can quantify the 3D shape of the root better than standard linear and angular metrics, premolar root number and shape is highly variable, with few homologous landmarks that can be confidently placed [START_REF] Emonet | Threedimensional analysis of mandibular dental root morphology in hominoids[END_REF]. To overcome such obstacles and explore 3-D root morphology, the present study employs a recently developed landmark-free approach (Durrleman et al., 2012a;[START_REF] Dumoncel | Landmark-free 3D method for comparison of fossil hominins and hominids based on endocranium and EDJ shapes[END_REF][START_REF] Beaudet | Upper third molar internal structural organization and semicircular canal morphology in Plio-Pleistocene South African cercopithecoids[END_REF][START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF][START_REF] Braga | Efficacy of diffeomorphic surface matching and 3D geometric morphometrics for taxonomic discrimination of Early Pleistocene hominin mandibular molars[END_REF] to quantify the entire external shape of the root in fossil hominins.

A recent non-metric investigation [START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF] using 2D virtual sections showed that among Chinese Middle Pleistocene human dental remains, the expression of root/canal form could be roughly aligned in a chronological sequence --the earliest specimens show an archaic morphology shared with H. erectus in Asia while the mid-to late Middle Pleistocene (296-194 Ka BP) group assigned to "archaic H.

sapiens" shows a modern-like morphology (premolars exhibit symmetrical occlusal contour, simplified occlusal conformations, lack of cingulum, and gracile roots), while some archaic features are only weakly expressed [START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF][START_REF] Xing | Late Middle Pleistocene hominin teeth from Tongzi, southern China[END_REF]. East Asian mid-Middle Pleistocene H. erectus populations show closer affinity to other Early and Middle Pleistocene hominins in Eurasia than to East African early Homo [START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF]. The results of this previous study supported the hypothesis that at least some of the Early Pleistocene hominin groups in Eurasia may have contributed to modern human populations [START_REF] Kaifu | Taxonomic affinities and evolutionary history of the Early Pleistocene hominids of Java: dentognathic evidence[END_REF][START_REF] Martinón-Torres | Dental evidence on the hominin dispersals during the Pleistocene[END_REF]. However, despite the growing evidence from tooth structural organization-including aspects on the outer enamel surface (OES) and enamel dentine junction (EDJ) morphology (e.g., [START_REF] Xing | Hominin teeth from the Middle Pleistocene site of Yiyuan, Eastern China[END_REF][START_REF] Xing | The fossil teeth of the Peking Man[END_REF][START_REF] Liu | A mandible from the Middle Pleistocene Hexian site and its significance in relation to the variability of Asian Homo erectus[END_REF][START_REF] Martinón-Torres | A "source and sink" model for East Asia? Preliminary approach through the dental evidence[END_REF][START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF]-it is still difficult to satisfactorily link a number of late Middle Pleistocene dental specimens to known hominin lineages [START_REF] Chia | Notes on the human and some other mammalian remains from Changyang, Hupei[END_REF][START_REF] Bailey | A comparative dental metrical and morphological analysis of a Middle Pleistocene hominin maxilla from Chaoxian (Chaohu), China[END_REF][START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF]. To resolve this problem, morphological variation and evolutionary trends in the Chinese Middle Pleistocene fossil record needs to be further assessed.

By using micro-CT scanning and 3D image processing techniques, this study aims at (1) report premolar root metrics, quantify crown-root tissue proportions, and investigate root geometric morphometrics of Middle Pleistocene hominins in East Asia; (2) evaluate the taxonomic and phylogenetic utility of external root shape in hominins, and assess evolutionary trends of premolar root morphology; and (3) , compare the few specimens with no clear taxonomic attribution (previously categorized as "archaic H. sapiens") to early to mid-Middle Pleistocene East Asian H. erectus premolars, as well as with those of other Pleistocene and recent human groups from Africa and Eurasia, in order to shed light on their taxonomic status and eventually assess if multiple human lineages may have been living in East Asia during the Middle Pleistocene. Finally, this study will contribute to the discussion of the spatio-temporal distribution of the East Asian hominins.

Materials and Methods

Study sample

The Chinese Middle Pleistocene hominins consist of 11 premolars coming from six different localities and spanning the Middle Pleistocene (Table 1). This group is comprised of two samples. The first is Middle Pleistocene H. erectus (HEC, N=8) derived from Zhoukoudian and Chenjiawo sites and Xichuan county, with a chronological range from 800-282 Ka BP. Up until now, these specimens were attributed to H. erectus with no controversy [START_REF] Wu | Human evolution in China: a metric description of the fossils and a review of the sites[END_REF][START_REF] Xing | Morphological variation of Zhoukoudian H. erectus teeth[END_REF][START_REF] Xing | The fossil teeth of the Peking Man[END_REF][START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF]. We note here that the two specimens (PA 525 P 4 and PA 526 P 3 ) from Xichuan county, Hubei Province lack accurate stratigraphic context, but their morphological signature a priori places them close to Zhoukoudian H. erectus [START_REF] Wu | Human fossil teeth from Xichuan, Henan (In Chinese with English abstract)[END_REF]. Specifically, compared with H. sapiens, the Xichuan premolars display large crown size and robust roots, the foveae posterior are larger than the foveae anterior (see comparative studies by [START_REF] Wu | Human fossil teeth from Xichuan, Henan (In Chinese with English abstract)[END_REF] and [START_REF] Wu | Human evolution in China: a metric description of the fossils and a review of the sites[END_REF]). Later studies on the dental inner structure based on micro-CT technology further acknowledged a close affinity between the Xichuan specimens and Zhoukoudian H. erectus [START_REF] Xing | The fossil teeth of the Peking Man[END_REF]. The second sample is mid-to late Middle Pleistocene (296-194 Ka BP) hominin specimens with no clear taxonomic attribution (here referred as Homo sp. from China, HSPC, N=3)., Previous studies either allocated them to post-erectus Homo or to archaic H. sapiens [START_REF] Wu | Human evolution in China: a metric description of the fossils and a review of the sites[END_REF]. They overlap slightly in time with the HEC sample. The taxonomic status of HSPC sample needs to be further examined because a lack of diagnostic features prevents a confident taxonomic designation and their dental morphology has not been extensively studied in a comparative context (e.g., PA1578 P 3 from Panxian Dadong, see [START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF].

The comparative sample consists of 66 fossil and extant human premolars (Table 1). More precisely, we used microCT scans of Early Pleistocene H. erectus (HEJ, N=4) specimens from Sangiran, Java [START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF]; late Early Pleistocene Homo from North Africa (HNA, N=6) [START_REF] Zanolli | Endostructural characterization of the H. heidelbergensis dental remains from the early Middle Pleistocene site of Tighenif, Algeria[END_REF]; Neanderthals (NEA, N=27) derived from the sites Krapina, La Chaise (Abri Bourgeois-Delaunay) and Regourdou (NESPOS database, 2019); and recent modern humans (RMH, N=29) from Africa and Eurasia [START_REF] Pan | Intra-individual metameric variation expressed at the enamel-dentine junction of lower post-canine dentition of South African fossil hominins and modern humans[END_REF]. The dental position and label of each specimen were provided in Supplemental Online Material (SOM) Tables S1 andS2.

As our study requires a complete external root surface model, only premolars with intact roots were selected for the comparative sample. For three Chinese H. erectus teeth (M3887, PA525 and PA526), minor reconstructions were conducted when the roots are lightly damaged as can be seen in Figure S1. For the entire study sample the occlusal wear ranges from stage 1 to 3 following [START_REF] Molnar | Human tooth wear, tooth function and cultural variability[END_REF].

Micro-tomographic scanning

Ten of the eleven East Asian specimens were imaged using a 225kV μ-CT scanner at the Institute of Vertebrate Paleontology and Paleoanthropology of the CAS, Beijing, with the following parameters: 120-140 kV voltage, 100-120 mA current, angular step of 0.5° over a scan angle of 360°. The final volumes were reconstructed with an isotropic voxel size ranging from 40 to 70 µm. The fossil premolar PMU M3887 housed at Uppsala University was scanned at the Multidisciplinary Laboratory of the ICTP, Trieste [START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF]. Information on the crown outer and inner morphology were available for some specimens in previous publications (see Table 1). Root and pulp canal form of these specimens was evaluated using crosssectional analyses [START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF].

A semi-automatic threshold-based segmentation was conducted using Avizo 8.0 (FEI Visualization Sciences Group,), following an adaptation of the half-maximum height method [START_REF] Fajardo | Assessing the accuracy of high-resolution X-ray computed tomography of primate trabecular bone by comparisons with histological sections[END_REF][START_REF] Spoor | The bony labyrinth of Neanderthals[END_REF][START_REF] Coleman | CT thresholding protocols for taking measurements on threedimensional models[END_REF]. Cementum deposits (if any) were separated from the root dentine surface and were not included in the GM analysis. Considering that cementum helps maintain the integrity of the root grows throughout life [START_REF] Selvig | The fine structure of human cementum[END_REF][START_REF] Bosshardt | Dental cementum: the dynamic tissue covering of the root[END_REF][START_REF] Yamamoto | Histology of human cementum: Its structure, function, and development[END_REF], and is probably correlated with severe attrition and stress (e.g., [START_REF] Trinkaus | Brief communication: Paleopathology of the Kiik-Koba 1 Neandertal[END_REF], as distinct from dentine tissue, the removal of cementum deposit is warrented. After the segmentation, a surface model was generated using the "unconstrained smoothing" parameter in Aviso. The 3D models of the right teeth were flipped so as to for comparison with the rest of the sample.

Root metrics and statistical tests

Using Avizo 8.0, premolar root metrics were assessed using published methods [START_REF] Kupczik | Comparative observations on the tooth root morphology of Gigantopithecus blacki[END_REF][START_REF] Kupczik | Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens[END_REF][START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF][START_REF] Moore | Premolar root morphology and metric variation in Pan troglodytes verus[END_REF][START_REF] Kupczik | Mandibular molar root and pulp cavity morphology in Homo naledi and other Plio-Pleistocene hominins[END_REF]. Seven variables were collected (Fig. 1 andTable 2): root length (RL, in mm, the maximum length was taken in multi-rooted teeth), root diameter (the mesio-distal as well as the bucco-lingual diameters, in mm), lateral enamel surface area (LEA, mm 2 ), root surface area (RA, mm 2 ), root volume (RV, mm 3 ), root pulp cavity volume (Vrpc, mm 3 ) and cementum volume when present (Vce, mm 3 ). Two indices were derived from the collected variables: the crown-root ratio (CRR=LEA/RA*100) and the ratio between bucco-lingual and mesio-distal root diameters (D(b-l)/D(m-d)).

For separating root(s) from the crown, we defined 10-20 landmarks along the cervical line (Fig. 1) and used the "slice-points to fit" option in Aviso to generate a cervical plane (i.e., the best-fit plane of the cervical line): the crown and root(s) were then sectioned through the cervical plane (plane A). Root length is the (maximum) distance between root apex to plane A, root diameters were measured based on plane A. Root volume and the volume of the root pulp chamber were also measured based on the cervical plane (see Fig. 1 and Table 2 for detailed definition of parameters). Plane A was then moved gradually to the occlusal side of the tooth, until the most apical plane (plane B) showing the occlusal enamel was located. The enamel area between planes A and B is the lateral enamel surface area (LEA, mm 2 ), the root surface area from plane A to the root apex is the root area (RA, mm 2 ). Differences in the distribution of LEA, RA and CRR between groups were tested using the rank-based Kruskal-Wallis test with Conover's post hoc comparisons (see Table S3 for details).

Geometric morphometric analyses

We applied a recently developed geometric morphometric (GM) landmark-free approach to compare the root external surface shape (see previous applications on primate teeth in [START_REF] Beaudet | Upper third molar internal structural organization and semicircular canal morphology in Plio-Pleistocene South African cercopithecoids[END_REF][START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF]. This method relies on the construction of group-average surface models and their deformation to the investigated surfaces (Durrleman et al., 2012a, b;[START_REF] Dumoncel | Landmark-free 3D method for comparison of fossil hominins and hominids based on endocranium and EDJ shapes[END_REF]. The surfaces are represented by a set of oriented faces and the comparison does not assume a point-to-point correspondence between samples, as in classical landmark-based GM analyses. First, the unscaled root surfaces of each tooth class were aligned together using the "Align Surface" module in Avizo 8.0. Then deformations between surfaces were mathematically modeled as a diffeomorphism, i.e., a one-to-one deformation of the 3D space that is smooth, invertible, and with a smooth inverse, which is particularly appropriate for comparing overall shapes and local orientation in the field of computational anatomy [START_REF] Glaunes | Template estimation form unlabeled point set data and surfaces for computational anatomy[END_REF][START_REF] Durrleman | Morphometry of anatomical shape complexes with dense deformations and sparse parameters[END_REF].

From a set of surfaces, together with a set of initial control points located near its most variable parts, a mean shape and a set of momenta parameterizing the deformations of the mean shape to each individual were estimated (Durrleman et al., 2012a, b). Using the packages ade4 1.7-13 [START_REF] Dray | The ade4 package: implementing the duality diagram for ecologists[END_REF] and Morpho v.2.6 [START_REF] Schlager | Retrodeformation of fossil specimens based on 3D bilateral semi-landmarks: Implementation in the R package "Morpho[END_REF]) for R v.3.5.1 (R Development Core Team, 2018), a between-group principal component analysis (bgPCA) (Mitteroecker and Bookstein, 2011) was performed. We first conducted principal component analysis (PCA) using deformation momenta to reduce the dimensions of our dataset, and computed the bgPCA from a subset of PCA scores (8~10) which accumulates around 90% of the overall shape variation. The three Chinese Homo sp. specimens were projected a posteriori in the bgPCA plot. The residual of allometric signals were tested using the coefficient of determination (R 2 ) of a multiple regression [START_REF] Bookstein | Morphometric Tools for Landmark Data: Geometry and Biology[END_REF] in which the explicative variable is the root area as proxy for size and the dependent variables are the bgPC scores.

Results

Comparisons of external root morphology

Distal and lingual views of each East Asian fossil maxillary and mandibular premolar are shown in Figures 2 and3, together with representatives of the comparative sample. Most of the East Asian Middle Pleistocene upper premolars show relatively derived features compared with East African early Homo (Wood et al., 1988). Whereas the latter usually appear to be double or triple-rooted, the East Asian Middle Pleistocene sample shows incompletely separated or fused buccal and lingual branches with bifid apex (Fig. 2A, C andI). This is similar to Early Pleistocene H. erectus from Sangiran (Fig. 2D andJ) and clearly is different from bifid or single-rooted premolars generally seen in later Homo (recent modern humans and Neanderthals, Fig. 2E-H, K andL). Only one H. erectus specimen (PA 831 from Hexian) displays an early Homo-like condition with three well-separated root branches that spread in different directions, covered with thick cementum (Fig. 2B). The Homo sp. specimen PA 76 from Changyang has two nearly parallel fused branches (with thick cementum at the apex) indented by a large and deep distal groove (Fig. 2C). This P 3 configuration is similar to Indonesian H. erectus (Fig. 2D) and Neanderthals (Fig. 2E,F), and can be found in modern humans, but the root of PA 76 differs considerably from Chinese H. erectus premolars whose lingual root branch is markedly diverging (Fig. 2A andB). For the P 4 s, the only H. erectus from China (PA 68, Zhoukoudian site) shows sub-parallel branches linked together with short free apices (Fig. 2I), but a broad generalization cannot be made on the basis of a single tooth. The pattern expressed for PA 68 (H. erectus from Zhoukoudian site) is not found in any other specimens. Indonesian H. erectus shows diverging branches at mid-root (Fig. 2J), while Neanderthals and modern humans tend to have fused roots until the apex with a deep distal groove delineating the branches (Fig. 2K,L).

With regard to lower premolars, in contrast to the North African late Early Pleistocene hominins from Tighenif that show a radical groove with two root apices (Fig. 3D, M andN), the East Asian hominins exhibit a single root with one apex (Fig. 3A-C, I-L), even if some of them also express a slight degree of Tomes' root. The East Asian fossil hominins mostly differ from later Homo by their more vertically oriented and pillar-like root axis, as well as a considerable amount of cementum deposit which makes them more robust than modern humans (Fig. 3D,L), while the root apex tends to be leaning more distally in the latter groups (Fig. 3E-H, O-S). It is also important to note that East Asian H. erectus generally show stout and large root branches (also reported by [START_REF] Xing | The fossil teeth of the Peking Man[END_REF][START_REF] Zanolli | Inner tooth morphology of Homo erectus from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden[END_REF]. As for the late Middle Pleistocene Homo sp., the slender root with sharp apical-third of the P 3 PA 1578 (site Panxian Dadong) (Fig. 3C) resembles the morphology found in modern humans, while the P 4 specimen PA 81 (site Changyang) (Fig. 3L) has three fused root branches, as the cementum covers 3/4 of the root body, the overall morphology of PA 81 root approximates H. erectus condition.

Root dimensions and volumetric measurements

HEC and HNA display the largest root volume, followed by Neanderthals and modern humans.

Sangiran H. erectus premolars exhibit a low total root volume in P 3 that is comparable to modern humans (Fig. 4A,B; Tables S1 andS2), but Sangiran P 4 s have a larger root volume, falling within the distribution of Neanderthals. The late Middle Pleistocene Homo sp. PA 76 P 3 (Changyang site) and PA 1578 P 3 (Panxian Dadong site) show a root volume that is in the range of both Chinese H. erectus and Neanderthals (Fig. 4A), , but PA 81 P 4 from Changyang exhibits a large root volume that exceeds all other lower premolars examined here (Fig. 4B).

Chinese H. erectus and North African Homo have the largest root pulp chamber volume, followed by Neanderthals. Modern humans display the smallest pulp volume among groups (Figs. 4C,D). PA 1578 P 3 and PA 81 P 4 exhibit large pulp volume, reaching (or even exceeding) the range of earlier groups like HNA and HEC (Figs. 4C,D). Root pulp chamber volume of the only Sangiran H. erectus specimen that we could segment falls within the range of Neanderthals (Fig. 4C), as does that of PA 76 P 3 from Changyang.

We were able to clearly segment the cementum layers in six of our eleven East Asian Middle Pleistocene specimens (Table S2). A large amount of cementum is observed in East Asian and North African fossil hominins and some of the Neanderthal specimens (mainly from La Chaise-Abri-Bourgeois-Delaunay and Regourdou). These specimens are clearly distinct from modern humans both in the presence and volume of cementum, as easily seen on the root surface (Figs. 2 and3). Le [START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF] reported an extensive degree of hypercementosis in Krapina Neanderthals. Unfortunately, the (possible) presence of a cementum layer is difficult to identify on the Krapina premolar examined here from the micro-CT scans. Due to strong fossilization, cementum deposits cannot be identified in Javanese H. erectus either. Further quantification of the volume as well as the 3D distribution of cementum in these specimens will require higher-quality data.

Crown-root surface area proportions

Nonparametric tests reveal significant differences in lateral enamel area (LEA) and root area (RA) among groups, but no significant differences among groups in crown root ratio (CRR). Conover's post hoc tests demonstrate distinctions between fossil and modern taxa (Table S3), but it should be pointed out that our sample size is small, statistical inferences should be addressed with caution.

For the P 3 , the lowest CRR values are found in the Chinese H. erectus specimens PA 67 (from Zhoukoudian) and PA 832 (from Hexian) (Table 3 and Fig. 4E), suggesting a relatively large root surface area, though these are not statistically significant. It overlaps with the lower range of Neanderthals, but not with the values of Sangiran 4 and recent modern humans (Fig. 4E). The CRR of PA 76 (from Changyang) is near the average for modern humans and overlaps the Neanderthal range (Fig. 4E). Modern humans have significantly smaller absolute enamel and root surface area than Sangiran 4, early to mid-Middle Pleistocene East Asian H. erectus and Neanderthals (Table S3), but do not differ from the estimates of the P 3 PA 76 from Changyang. For the P 4 , the Zhoukoudian H. erectus specimen PA 68 shows a similar signal to the P 3 s, with low CRR values. With regard to mandibular premolars, marked overlap of CRR is seen between the groups (Table 3 and Fig. 4F). Modern humans have significantly smaller root area than all the fossil groups (Table 3 andTable S3), resulting in higher crown-root proportions (Fig. 4F). One late Middle Pleistocene Chinese Homo sp. specimen (P 3 PA 1578 from Panxian Dadong) shows a high CRR value, even exceeding the high estimates of Neanderthals and modern humans (Fig. 4F). In contrast, the CRR ratio of the P 4 PA 81 from Changyang is considerably lower than the other groups, including the early to mid-Middle Pleistocene Chinese H. erectus specimens (Table 3, Fig. 4F and Table S2).

Geometric morphometric analyses

GM analysis of root shape shows substantial morphological distinctions among groups (Fig. 5). There is also an allometric signal observed along bgPC1 and/or bgPC2 for tooth positions P 3 , P 4 and P 4 (Table S4), mostly distinguishing the late Early/early Middle Pleistocene specimens from North Africa and Indonesia from the Eurasian groups fossil and extant groups. H. erectus specimens from Sangiran and the late Early Pleistocene North African Homo specimens from Tighenif are distinguished morphologically from modern humans and Neanderthals along bgPC1, the former showing separated root branches while the latter mostly exhibit fused roots/single root. Regarding P 3 s, recent modern humans are placed at the upper part of bgPC2 by showing separated root branches (Fig. 5A), while Neanderthals show incompletely separated roots. Two of the East Asian specimens, the Zhoukoudian H. erectus specimen PA 67 and the late Middle Pleistocene Homo sp. specimen PA 76 from Changyang site show similarities with Neanderthals, while the Hexian H. erectus tooth PA 832 exhibits a primitive condition with three separated root branches (Fig. 5A). The Chinese H. erectus specimen PA 68 falls between the ranges of of later the Homo groups (Fig. 5B), showing single root with bifid apex. Fused root branches separate younger hominin groups from Sangiran H. erectus.

Regarding lower premolars, hominins from Tighenif are at the lower end of bgPC1 (Figs. 5C andD), showing strongly expressed radical grooves at the distal or distolingual face of the root, and a tendency for separated root tips at the apex. The H. erectus P 3 s from China (PA 110 Zhoukoudian, PA 526 Xichuan) are located at the lower half of bgPC2, indicating that they tend to have stout root body and blunt apex (Fig. 5C), in accordance with gross observation (Fig. 3E,F). In contrast, the root shape of Homo sp. specimen PA 1578 from Panxian Dadong falls within the modern human range and is quite distant from Neanderthals (Fig. 5C).

For the lower P 4 s, the Homo sp. specimen PA 81 from Changyang is closer to the Chinese H. erectus range, showing an intermediate morphology between the more archaic figure represented by the Tighenif premolars and the Neanderthal root shape (Fig. 5D).

Discussion

Premolar root anatomy in great apes and hominins has long been recognized to reflect taxonomy, phylogeny, and function (e.g., Wood et al., 1988;[START_REF] Kupczik | Tooth root morphology and masticatory muscle force pattern in humans and nonhuman primates[END_REF]Prado-Simón et al., 2012a;[START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF]. Several studies have discovered a mosaic of primitive and derived dental traits among the socalled archaic H. sapiens or post-erectus Homo from the East Asian mid-to late Middle Pleistocene (e.g., specimens from Panxian Dadong and Changyang) [START_REF] Chia | Notes on the human and some other mammalian remains from Changyang, Hupei[END_REF][START_REF] Wu | Human evolution in China: a metric description of the fossils and a review of the sites[END_REF][START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF], but these traits are not diagnostic enough to link these specimens to any particular Homo species, and therefore the taxonomic assignment of these specimens remains inconclusive [START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF].

Recent observations on the root-canal configuration [START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF] as well as the qualitative and quantitative comparisons of external root morphology presented here reveal a modern-like trend of root fusion and root number reduction for the East Asian mid-to late Middle Pleistocene Homo sp. specimens.

They differ from Chinese H. erectus in that the latter taxon generally shows stout premolar root(s) with thick cementum [START_REF] Xing | The fossil teeth of the Peking Man[END_REF][START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF]. Moreover, when multiple root branches exist, the upper premolar roots of East Asian H. erectus often spread in different directions (including H. erectus from Yiyuan site reported by [START_REF] Xing | Hominin teeth from the Middle Pleistocene site of Yiyuan, Eastern China[END_REF]. These features of in East Asian H. erectus could be interpreted as a functional response to withstand high biomechanical forces and better sustain high occlusal loads [START_REF] Macho | Effects of loading on the biochemical behavior of molars of Homo, Pan, and Pongo[END_REF][START_REF] Kupczik | Premolar root number variation in hominoids: genetic polymorphism vs. functional significance[END_REF][START_REF] Kupczik | On the relationship between maxillary molar root shape and jaw kinematics in Australopithecus africanus and Paranthropus robustus[END_REF][START_REF] Xing | The fossil teeth of the Peking Man[END_REF]. In any case, the stout, pillar-like morphology of Chinese H. erectus premolars represent an autapomorphic feature characterizing this fossil human group [START_REF] Weidenreich | The dentition of Sinanthropus pekinensis: a comparative odontography of the hominids[END_REF][START_REF] Liu | A mandible from the Middle Pleistocene Hexian site and its significance in relation to the variability of Asian Homo erectus[END_REF][START_REF] Xing | The fossil teeth of the Peking Man[END_REF]. The analyses presented here of root and cementum volume further confirms that larger roots and considerable cementum deposits are commonly found in East Asian H. erectus, late Early Pleistocene Homo from Tighenif and those mid-to late Middle Pleistocene hominins found in China.

Implications of root metrics and, crown-root tissue proportions

Compared with modern humans, Neanderthals have larger root and pulp volume in both the anterior [START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF] and molar dentition [START_REF] Kupczik | Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens[END_REF]. Our results show that Neanderthals display larger premolar roots as well, although our sample size is small. Compared with modern humans, East Asian Middle Pleistocene hominins show larger root, pulp chamber and cementum volume. An increased root volume and pulp chamber volume may be due to a high frequency of occlusal loads. In addition, a large pulp chamber could allow for deposition of tertiary dentine (Le [START_REF] Cabec | Long anterior mandibular tooth roots in Neanderthals are not the result of their large jaws[END_REF][START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF].

In hominids, root surface area is linked to masticatory function, with larger root surface area suggested to be adaptively associated with higher occlusal loads and higher mechanical resistance of food items [START_REF] Kupczik | Tooth root morphology and masticatory muscle force pattern in humans and nonhuman primates[END_REF][START_REF] Kupczik | Premolar root number variation in hominoids: genetic polymorphism vs. functional significance[END_REF][START_REF] Kupczik | On the relationship between maxillary molar root shape and jaw kinematics in Australopithecus africanus and Paranthropus robustus[END_REF][START_REF] Spencer | Tooth-root form and function in platyrrhine seed-eaters[END_REF][START_REF] Kupczik | Comparative observations on the tooth root morphology of Gigantopithecus blacki[END_REF]. It has been suggested that the larger molar root area in Neanderthals as compared to recent and Pleistocene H. sapiens could indicate that Neanderthals were able to generate larger biting forces [START_REF] Kupczik | Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens[END_REF]. However, a study based on finite element analyses showed that the taurodont morphology of the Neanderthal post-canine dentition did not improve the functional biomechanics of the tooth [START_REF] Benazzi | Exploring the biomechanics of taurodontism[END_REF]. Thus it is still uncertain if the enlargement of premolar root area observed in Chinese H. erectus, together with their robust and stout premolar roots [START_REF] Xing | The fossil teeth of the Peking Man[END_REF], reflects an adaptation to mechanically resistant diet, or to other evolutionary processes. Considering that their average premolar root area approximates that of Neanderthals, we hypothesize that it more likely represents an adaptation to a high attrition diet or is a by-product of pleiotropy or genetic drift [START_REF] Benazzi | Exploring the biomechanics of taurodontism[END_REF]. Future analyses based on finite element analyses will help to resolve this question.

Taxonomic and phylogenetic value of external root shape

External root shape carries a taxonomic signal, but due to the lack of homologous landmarks, 3D morphometric analysis is often limited to an broad approximation of the real root morphology (e.g., [START_REF] Emonet | Khoratpithecus et la radiation des hominoïdes en Asie du Sud-Est au Miocène[END_REF][START_REF] Emonet | Threedimensional analysis of mandibular dental root morphology in hominoids[END_REF][START_REF] Kupczik | On the relationship between maxillary molar root shape and jaw kinematics in Australopithecus africanus and Paranthropus robustus[END_REF]. (Semi)landmark-based GM is effective in depicting root morphology of the anterior dentition, as the surface relief of incisors and canines is relatively simple with few bifurcations or grooves [START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF]. In contrast to the above-mentioned studies, this analysis uses deformationbased GM that for the first time compares the complete premolar root surface morphology in fossil and modern humans, and explore the reliability of the radicular conformation for assessing taxonomy. Overall, there is a clear distinction between the Early Pleistocene and Late Pleistocene to Holocene human groups.

The different root morphology of Neanderthals and modern humans are also clearly identified in these GM analyses. However, Chinese early Middle Pleistocene H. erectus premolar roots (PA 67 P 3 and PA 68 P 4 , Zhoukoudian site, 800-770 Ka) tend to show a morphology that either approximates the Neanderthal condition, or that is intermediate between Neanderthals and modern humans. The late Middle Pleistocene hominin from Changyang (PA 76 P 3 ) is closely placed with one East Asian H. erectus specimen (PA 67), and they both fall close to the range of Neanderthals. Whether this result indicates a phylogenetic link between Chinese H. erectus and Neanderthals (or with the still underrepresented Denisovans which shows Tomes' root in P 3 , a trait that is commonly seen in fossil hominins but is also seen in modern humans [START_REF] Chen | A late Middle Pleistocene Denisovan mandible from the Tibetan Plateau[END_REF]) or reflects homoplasy will need to be assessed in the future, integrating evidence from the crown morphology and notably from the enamel-dentine junction, which is recognized as offering significant phylogenetic information (e.g., [START_REF] Macchiarelli | How Neanderthal molar teeth grew[END_REF][START_REF] Macchiarelli | From outer to inner structural morphology in dental anthropology: integration of the third dimension in the visualization and quantitative analysis of fossil remains[END_REF]Skinner et al., 2008aSkinner et al., , b, 2009;;[START_REF] Zanolli | The late Early Pleistocene human dental remains from Uadi Aalad and Mulhuli-Amo (Buia), Eritrean Danakil: macromorphology and microstructure[END_REF][START_REF] Zanolli | Brief communication: molar crown inner structural organization in Javanese Homo erectus[END_REF][START_REF] Pan | Further morphological evidence on South African earliest Homo lower postcanine dentition: enamel thickness and enamel dentine junction[END_REF][START_REF] Pan | Intra-individual metameric variation expressed at the enamel-dentine junction of lower post-canine dentition of South African fossil hominins and modern humans[END_REF].

Considering that the fossil and modern human groups included here can be clearly distinguished by premolar morphology, we wanted to evaluate where the mid-to late Middle Pleistocene hominin specimens from China would fit. As previously mentioned, the P 3 specimen dated to the late Middle Pleistocene period from Changyang falls next to the range of Neanderthals (Fig. 5A) and shows a close affinity with one early Middle Pleistocene H. erectus (PA 67 from Zhoukoudian site); the mid-Middle Pleistocene P 3 (PA 1578 from Panxian Dadong), however, is clearly situated within the modern human distribution (Fig. 5C). The P 4 specimen from Changyang (PA 81) shares a similar shape with older Chinese H. erectus and is distinguished from the Early Pleistocene specimens from Tighenif, as well as from Neanderthals and modern humans (Fig. 5D). The root of PA 81 is quite robust, three-grooved and exhibits a thick cementum deposit [START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF]. The absolutely large enamel and root area, large root and pulp chamber volume also point toward a H. erectus-like signal. We thus interpret the root structural signature of this specimen as retaining primitive features, compatible with the hypothesis that a H. erectus-like lineage survived in the late Middle Pleistocene.

However, it should be noted that although the P 3 PA 76 from Changyang has similar root shape to Chinese H. erectus, it has smaller pulp chamber volume and relative root surface area within the range of modern humans instead of H. erectus. Further analysis on other morphological aspects of this specimen is needed before a taxonomic assessment can be made with any confidence. On the other hand, the late Middle Pleistocene hominin PA 1578 from Panxian Dadong is clearly distinct from any of the Pleistocene samples and falls within the variability of modern humans. In fact, previous study on crown morphology of the four hominin teeth found at Panxian Dadong (I 1 , C 1 , P 3 , and P 3 ) suggested that these teeth show a combination of archaic and modern-like features that is in line with Middle and Upper Pleistocene populations from East and West Asia [START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF]. Our observations on the root morphology further recognize the primitive morphology of Panxian Dadong P 3 (e.g., a relatively stout root compared with modern humans, and considerable cementum covering 2/3 part or the root, Fig. 3L). On the other hand, small root and pulp chamber volume and a small relative root surface area place PA 1578 close to modern humans. We concur that this specimen likely belonged to an ancient population that is close to modern humans. Based on crown and root morphology, these "archaic H. sapiens" or "non-erectus" hominins like Panxian Dadong [START_REF] Liu | Late Middle Pleistocene hominin teeth from Panxian Dadong, South China[END_REF], and Tongzi [START_REF] Xing | Late Middle Pleistocene hominin teeth from Tongzi, southern China[END_REF] from the mid-to late Middle Pleistocene and the Xujiayao hominin from the early Late Pleistocene [START_REF] Xing | Hominin teeth from the early Late Pleistocene site of Xujiayao, Northern China[END_REF] express remarkable derived features that is typically found in later Homo lineages. The dispersal of these "non-H. erectus" hominins might be further clarified following a comparative study on the dentition of the Denisovan specimen from the Xiahe site [START_REF] Chen | A late Middle Pleistocene Denisovan mandible from the Tibetan Plateau[END_REF].

Moreover, it is important to underscore that there is root shape variation in the East Asian H. erectus sample: for instance, robust and fully separated root branches are found in PA 832 P 3 , a mid-Middle Pleistocene H. erectus from Hexian site, while the older PA 67 P 3 from Zhoukoudian shows incompletely separated double roots (also see [START_REF] Pan | Comparative observations on the premolar root and pulp canal configurations of Middle Pleistocene Homo in China[END_REF]. This may indicate a late survival of archaic lineages, which is in line with previous studies on crown morphology [START_REF] Xing | Middle Pleistocene hominin teeth from Longtan Cave, Hexian, China[END_REF][START_REF] Liu | A mandible from the Middle Pleistocene Hexian site and its significance in relation to the variability of Asian Homo erectus[END_REF].

Previous investigations of hominin dental crown morphology using GM techniques [START_REF] Martinón-Torres | Hominin lower second premolar morphology: evolutionary inferences through geometric morphometric analysis[END_REF][START_REF] Gómez-Robles | A geometric morphometric analysis of hominin upper first molar shape[END_REF], 2008) have observed different evolutionary trends in different dental positionswithin the same tooth class. For example, Neanderthals show a primitive P 4 morphology but a derived P 3 . Furthermore, morphogenesis of the upper and lower dentition are under the control of different genetic programs [START_REF] Thomas | Role of Dlx-1 and Dlx-2 genes in patterning of the murine dentition[END_REF][START_REF] Mccollum | Developmental genetics and early hominid craniodental evolution[END_REF]. Given this, we emphasize that any taxonomic interpretation of our results must be approached with caution and placed within a larger contextespecially with the incorporation of other dental morphological evidence. Nonetheless, our preliminary results show that in contrast to non-metric configurations (e.g., root number and form [START_REF] Abbott | A comparative study of tooth root morphology in the great apes, modern man and early hominids[END_REF]Wood et al., 1988;[START_REF] Moore | Premolar root and canal variation in extant non-human hominoidea[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF]) and standard metrics (e.g., root length and width [START_REF] Kupczik | Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens[END_REF]Prado-Simón et al., 2012b;[START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF]), GM analysis of premolar root surface is an useful indicator for taxonomy. On the other hand, we caution that other than root length and some particular metrics that distinguish the roots of modern humans vs. Neanderthals (e.g., [START_REF] Cabec | Long anterior mandibular tooth roots in Neanderthals are not the result of their large jaws[END_REF], 2013), root shape and structure are highly phenotypically plastic and quite variable within groups.

Recent studies have demonstrated that there is considerable variation in premolar root number within a single species, and even in the same individual [START_REF] Kupczik | Premolar root number variation in hominoids: genetic polymorphism vs. functional significance[END_REF][START_REF] Shields | Mandibular premolar and second molar root morphological variation in modern humans: what root number can tell us about tooth morphogenesis[END_REF][START_REF] Moore | Premolar root and canal variation in extant non-human hominoidea[END_REF][START_REF] Moore | Premolar root and canal variation in South African Plio-Pleistocene specimens attributed to Australopithecus africanus and Paranthropus robustus[END_REF].

However, as shown in our GM analyses, despite differences in root shape, it is still possible to quantify differences between recent humans and Pleistocene human groups/taxa and separate them.

Conclusion

This study first provides a quantitative comparison of permanent maxillary and mandibular premolar root morphologies in Chinese Middle Pleistocene hominins. Stout root shape, relatively large root area, large root and pulp chamber volume and a considerable amount of cementum deposit are commonly observed in East Asian Middle Pleistocene H. erectus [START_REF] Xing | The fossil teeth of the Peking Man[END_REF]this study). Together with previous landmarkbased GM studies on hominoid molars [START_REF] Emonet | Threedimensional analysis of mandibular dental root morphology in hominoids[END_REF], the present work suggests that external root surface morphology is taxonomically informative. The 3D landmark-free approach to quantify dental root surface morphology in fossil and modern humans, and used here is able to discriminate among several hominin groups, and enabled evaluation of the taxonomic affinities of a number of East Asian Middle Pleistocene hominin specimens. Integration of a larger sample and other dental evidence from more diverse time periods localities should help to better characterize the spatial-temporal dispersal of multiple Homo lineages in East Asia. In addition, as molar root form is more conservative in its phenotypic expression, further study on molar root surface morphology may also provide useful information on the evolutionary relationships in fossil hominins.

Crown-root metrics (lateral enamel area, root area and crown-root ratio) together with external root GM analyses suggest that among the three Chinese mid-to late Middle Pleistocene Homo sp. specimens investigated here, the two premolars from Changyang (the P 3 PA 76 and the P 4 PA 81) retain some primitive features in their root structure and are possibly late representatives of a H. erectus-like lineage. Conversely, the third specimen (P 3 PA 1578 from Panxian Dadong) exhibits unambiguous modern-like features and may represent evidence for an early presence of modern humans in Asia. In addition, recent study on the 240-172 Ka dental remains from Tongzi site, China hints at the existence of multiple paleodemes in East Asian during mid-to late Middle Pleistocene period [START_REF] Xing | Late Middle Pleistocene hominin teeth from Tongzi, southern China[END_REF]. Even if the available dating ranges of these specimens do not fully overlap, the penecontemporaneity of the Changyang and Panxian Dadong hominins is reasonable, suggesting that our results are potentially consistent with the possible coexistence of multiple distance between the root apex and the center of the cervical plane, when more than one root branches exist, the maximum length was taken.

mm Table S1 and Figure

S2

Bucco-lingual root diameter D(b-l) At the cervical plane, the largest diameter of the root along the bucco-lingual direction.

mm Table S1 Mesio-distal root diameter D(m-d) At the cervical plane, the largest diameter of the root along the mesio-distal direction.

mm Table S1 Root area RA The total root surface area from the cervical plane to the apex. mm 2 Table 3 and Table S2 Lateral enamel surface area LEA The outer enamel surface area between the cervical plane and the most apical plane (i.e., Plane B) showing the occlusal enamel that is parallel to the cervical mm 2 Table 3 and Table S2 plane.

Root volume RV Volume of the dentine, pulp chamber and cementum in the root, between the cervical plane and the root apex. S2 Cementum volume Vce The total volume of the cementum. mm 3 Table S2 D(b-l)/D(m-d) Ratio between bucco-lingual and mesiodistal root diameters.

Dimensionless Table S1 The crown-root ratio CRR= LEA/RA*100

Ratio between lateral enamel area and root area.

Dimensionless Figure 4, Table 3 and

15a

  Age : (A)[START_REF] An | New magnetostratigraphic dates of Lantian Homo erectus[END_REF]; (B)[START_REF] Grün | ESR analysis of teeth from the palaeoanthropological site of Zhoukoudian, China[END_REF]; (C)[START_REF] Huang | ESR dating of tooth enamel: comparison with U-Series, FT and TL dating at the Peking Man Site[END_REF]; (D) Shen et al. (2001); (E) Grün et al. (1998); (F) Yuan et al. (1986); (G) Karkanas et al. (2008); (H) Huffman (2001); (I) Sahnouni and Van der Made (2009); (J) Rink et al. (1995); (K) Macchiarelli et al. (2006) ; (L) Turq et al. (2008) b Citations: (1) Woo (1964); (2) Xing (2012); (3) Xing et al. (2018); (4) Black et al. (1933); (5) Zanolli et al. (2018); (6) Xing et al. (2014); (7) Liu et al. (2017); (8) Bailey and Liu (2010); (9) Chia (1957); (10) Liu et al. (2013); (11) Zanolli and Mazurier (2013); (12) Radovčić et al. (1988); (13) NESPOS database (2019); (14) Macchiarelli et al. (2006) ; (15) Pan et al. (2017) c Partly damaged root, restored. For the restoration see Figure S1.

Figure

  Figure 4 andTable S2

Figure 1 .

 1 Figure 1. Lingual view of a lower premolar, illustrating the separation of the crown and root along the cervical plane, and the extraction of lateral enamel (A). Linear and volumetric measurements were taken on root(s) (B), cementum (C) and root pulp cavity (D).

Figure 2 .

 2 Figure 2. Comparisons of external root morphology in upper premolars (cementum shown as transparent). HEC: early to mid-Middle Pleistocene H. erectus from China; HSPC: late Middle Pleistocene Homo sp. from China; HEJ: Javanese H. erectus; NEA: Neanderthals; RMH: recent modern humans; D: distal view; L: lingual view. Neanderthal specimen IDs were detailed in Figures S1 and S2.

Figure 3 .

 3 Figure 3. Comparisons of external root morphology in lower premolars (cementum shown in transparent). HEC: early to mid-Middle Pleistocene H. erectus from China; HSPC: late Middle Pleistocene Homo sp. from China. HNA: Early Pleistocene Homo from North Africa; NEA: Neanderthals; RMH: recent modern humans; D: distal view; L: lingual view. Neanderthal specimen IDs were detailed in Figures S1 and S2.

Figure 4 .

 4 Figure 4. Standard box and whisker plot of the root volume (A-B) and root pulp volume (C-D) and crownroot area ratio (E-F) in each group with regard to dental position, mid-to late Middle Pleistocene Homo sp. specimens were highlighted. A, C and E: upper premolars; B, D and F: lower premolars. The interquartile range (25 th -75 th percentiles: boxes), 1.5 interquartile ranges (whiskers) and the median values (black line) are shown. Outliers more than 1.5 interquartile ranges from the box are signified with circles, extremes more than 3 interquartile ranges from the box are signified with asterisks. Abbreviations: RV: root volume; Vrpc: root pulp chamber volume; CRR: crown-root ratio; HEJ: Javanese H. erectus; HEC: H. erectus from China; HSPC: Homo sp. from China; HNA: Early Pleistocene Homo from North Africa; NEA: Neanderthals; RMH: recent modern humans. Due to strong fossilization, the Vrpc of 3 HEJ specimens cannot be segmented.

Figure 5 .

 5 Figure 5. Between-group PCA of the deformation-based shape comparisons of premolar roots. Distal (left and upper ones at the X and Y axes, respectively) and lingual side (right and lower ones at the X and Y axes) of the extreme conformation of root shape are presented. Comparative groups are H. erectus from China (HEC), early H. erectus from Java (HEJ), Early Pleistocene Homo from North Africa (HNA), Neanderthals (NEA) and recent modern human (RMH).

Table 1 .

 1 Composition of the study sample

							Chronological	Citations
	Specimen	P 3	P 4	P 3	P 4	Provenance
							range a	b
	Middle Pleistocene H.
	erectus from China
	(HEC)
	PA 102				1	Chenjiawo	650-500 Ka (A)	1
							320-282 Ka (B,
							C)
	PA 110			1		Layer 3	2, 3
							500 Ka (D)
						Zhoukoudian
						Loc. 1
	PMU M3887 c				1	Layer 5	550-500 Ka (E)	4, 5
	PA 67	1
						Layer 11	800-770 Ka (F)	2, 3
	PA 68		1
	PA 832	1				Hexian	437-387 Ka (E)	6, 7
	PA 525 c				1
						Xichuan	Unknown	8
	PA 526 c			1

Late Middle Pleistocene Homo sp. from China (HSPC)

	Tighenif 2			1	1
		3	7	4	5	Level 8	Krapina	140-120 Ka (J)	12, 13
						Abri
	Neanderthals (NEA)		1	2	1	Bourgeois-	La Chaise	146-106 Ka (K)	13, 14
						Delaunay
				2	2		Regourdou	70 Ka (L)	13
						Asian/
	Recent Modern Humans					European/
		5	7	8	9
	(RMH)					South
						African
	PA 76	1
						Changyang	196-194 Ka (F)	9
	PA 81				1
	PA 1578			1		Panxian Dadong	296-233 Ka (G)	10
	Early Pleistocene H.
	erectus (HEJ)
						Pucangan
	Sangiran 4	2	2				Sangiran	1.8-1.6 Ma (H)	5
						Fm.
	Early Pleistocene Homo
	from North Africa (HNA)

Table 2 .

 2 Definition of metrics taken for the study. Protocols proposed by[START_REF] Kupczik | Comparative observations on the tooth root morphology of Gigantopithecus blacki[END_REF];Prado- Simón et al. (2012); Le[START_REF] Cabec | Anterior tooth root morphology and size in Neanderthals: Taxonomic and functional implications[END_REF].

	Metrics	Acronym	Complete definition	Dimension	Descriptive statistics
	Root length	RL	Along the long axis of the root, linear

Table S2 Table 3 .

 S23 Average values for the lateral enamel area (LEA), root area (RA) and crown-root ratio (CRR) measured in the later Middle Pleistocene Homo sp. from China, together with comparative samples.

	Figure captions:
	Dental positions	Groups	LEA (mm 2 )	RA (mm 2 )	CRR (%)
		HEC
	P 3

2 Tighenif ~1.0 Ma (I)