Alejandra Ortiz

Shara E Bailey

Miguel A Delgado

Clément Zanolli

Fabrice Demeter

Anne-Marie Bacon

Thi Diem

Tien Nguyen

Anh Duc Nguyen

Yingqi Zhang

Terry Harrison

Thi Mai

Huong Nguyen

Anh Tuan Nguyen

Jean-Jacques Hublin

Matthew M Skinner

A distinguishing feature of Pongo upper molars and its implications for the taxonomic identification of isolated hominid teeth from the Pleistocene of Asia

Keywords: Protoconule, mesial fovea, enamel-dentine junction, taxonomy, hominids Australopithecus, Paranthropus, Homo, Meganthropus, Sivapithecus, Pan, Gorilla, and Pongo, Australopithecus, Paranthropus, Homo, Pan, Gorilla, and Australopithecus, Paranthropus, Homo, Meganthropus, Sivapithecus, Pan, Gorilla, and Pongo, Australopithecus, Paranthropus, Homo, Pan, Gorilla, and Pongo

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Hominid biogeography in Asia during the Pleistocene is characterized by a complex scenario where at least four genera -Pongo, Homo, Gigantopithecus, and Meganthropus (coexisted in mainland and island Asia [START_REF] Ciochon | The mystery ape of Pleistocene Asia[END_REF][START_REF] Zanolli | Evidence for increased hominid diversity in the Early-Middle Pleistocene of Java, Indonesia[END_REF]. Although orangutans are today restricted to Borneo (Pongo pygmaeus) and northern Sumatra (Pongo abelii and Pongo tapanuliensis) [START_REF] Nater | Morphometric, behavioral, and genomic evidence for a new orangutan species[END_REF], they were widely distributed in China, Vietnam, Thailand, Laos, Cambodia, Peninsular Malaysia, Borneo, Sumatra, and Java during the This article is protected by copyright. All rights reserved.

The earliest definitive occurrence of H. erectus in Southeast Asia dates to ~1.6 Ma [START_REF] Antón | The natural history of Homo erectus[END_REF][START_REF] Falguères | Geochronology of early human settlements in Java: What is at stake?[END_REF][START_REF] Larick | Early Pleistocene 40 Ar/ 39 Ar ages for Bapang Formation hominins, Central Jawa, Indonesia[END_REF][START_REF] Swisher | Age of the earliest known hominids in Java, Indonesia[END_REF]. Fossil evidence for the penecontemporaneous occurrence of Homo in China comes from Yuanmou (~1.7 Ma; [START_REF] Zhu | Early evidence of the genus Homo in East Asia[END_REF], Gongwangling (~1.63 Ma; [START_REF] Zhu | New dating of the Homo erectus cranium from Lantian (Gongwangling), China[END_REF] and Shangshazui in the Nihewan Basin (~1.6-1.7 Ma; [START_REF] Ao | New evidence for early presence of hominids in north China[END_REF], and artefacts from the Paleolithic site of Shangchen suggest a hominin occupation of the Chinese Loess Plateau that dates to as early as ~2.1 Ma [START_REF] Zhu | Hominin occupation of the Chinese Loess Plateau since about 2.1 million years ago[END_REF]. Dentognathic remains from Longgupo Cave in Chongqing Municipality (~1.4-1.8 Ma; [START_REF] Han | Preliminary results of combined ESR/Useries dating of fossil teeth from Longgupo Cave, China[END_REF], once thought to represent the earliest record of hominins in China, are now believed to belong to an extinct ape [START_REF] Ciochon | The mystery ape of Pleistocene Asia[END_REF][START_REF] Ciochon | Divorcing hominins from the Stegodon-Ailuropoda fauna. New views on the antiquity of hominins in Asia[END_REF][START_REF] Etler | Longgupo: Early Homo colonizer or Late Pliocene Lufengpithecus survivor in South China?[END_REF][START_REF] Schwartz | Whose teeth?[END_REF]. Although they have received less attention than Longgupo, several additional Pleistocene sites in China and Vietnam have yielded hominoid fossils of uncertain or contentious taxonomic status (see [START_REF] Ciochon | Divorcing hominins from the Stegodon-Ailuropoda fauna. New views on the antiquity of hominins in Asia[END_REF] and these have been variously assigned to Australopithecus, H. erectus, Homo sp., cf. Ponginae, Pongo sp., Hemanthropus, Langsonia and Hominoidea (gen. et sp. indet.) [START_REF] Ciochon | Divorcing hominins from the Stegodon-Ailuropoda fauna. New views on the antiquity of hominins in Asia[END_REF]Harrison et al., 2014). In addition to specimens of known provenance discussed above, there are several isolated teeth from the von Koenigswald's Chinese Apothecary collection whose affinities have yielded varied opinions, but it appears that at least three hominoid groups are represented, including "Hemanthropus peii", Sinanthropus officinalis [= Homo erectus], and Pongo sp. (reviewed in [START_REF] Smith | Disentangling isolated dental remains of Asian Pleistocene hominins and pongines[END_REF].

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Many of the taxonomic uncertainties associated with Pleistocene hominoids in Asia stem from the problem of distinguishing between the molars of pongines and hominins, especially when they are worn, and this is compounded by the fact that many assemblages are composed of isolated teeth. Both pongines and hominins have relatively thick-enameled crenulated molars with a low occlusal topography (Fig. S1). Moreover, molars of these taxa overlap in crown formation times and daily secretion rates [START_REF] Grine | Fossil hominid teeth from the Sangiran Dome (Java, Indonesia)[END_REF][START_REF] Martin | Significance of enamel thickness in hominoid evolution[END_REF]Olejniczak et al., 2008a;[START_REF] Smith | Modern human molar enamel thickness and enamel-dentine junction shape[END_REF][START_REF] Smith | Dental tissue proportions in fossil orangutans from mainland Asia and Indonesia[END_REF][START_REF] Smith | Dental development in living and fossil orangutans[END_REF]; but see [START_REF] Smith | Disentangling isolated dental remains of Asian Pleistocene hominins and pongines[END_REF] for a recent comprehensive study on crown formation times in Pongo and Homo). It has also been suggested that the occlusal outline of Pongo upper molars is more oval than in African great apes and more similar in this regard to that of Homo [START_REF] Swindler | Comparative and evolutionary aspects of permanent dentition[END_REF]. Several experts have noted the remarkable tooth size and shape variation observed in fossil Pongo [START_REF] Harrison | Archaeological and ecological implications of the primate fauna from prehistoric sites in Borneo[END_REF]Harrison et al., 2014;[START_REF] Schwartz | A review of the Pleistocene hominoid fauna of the Socialist Republic of Vietnam (excluding Hylobatidae)[END_REF]. This observation is in line with the high degree of postcanine variation documented for extant Pongo in which there is no consistent dental morphological pattern within populations [START_REF] Uchida | Craniodental variation among the great apes[END_REF][START_REF] Uchida | Variation in tooth morphology of Pongo pygmaeus[END_REF]; but see [START_REF] Pilbrow | Dental diversity in Pongo as revealed by molar occlusal morphometrics[END_REF] and isolated teeth are difficult to separate at the species level (Harrison et al., 2014). Extinct and recent humans also exhibit a high degree of molar size variation, and both Pongo and Homo experienced similar overall trends of dental reduction over time [START_REF] Demeter | An archaic Homo molar from Northern Vietnam[END_REF][START_REF] Garn | Molar size sequences and fossil taxonomy[END_REF][START_REF] Goose | Dental measurement: an assessment of its value in anthropological studies[END_REF][START_REF] Grine | Fossil hominid teeth from the Sangiran Dome (Java, Indonesia)[END_REF]Harrison et al., 2014;Hooijer, 1948;[START_REF] Schuman | Metric and morphologic variations in the dentition of the Liberian chimpanzee: comparisons with anthropoid and human dentitions[END_REF][START_REF] Smith | Dental development in living and fossil orangutans[END_REF][START_REF] Tshen | Biogeographic distribution and metric dental variation of fossil and living orangutans (Pongo spp.)[END_REF]Weidenreich, 1937[START_REF] Weidenreich | Giant early man from Java and South China[END_REF]. This scenario becomes even more complex with the presence of a number of molars that, This article is protected by copyright. All rights reserved.

while falling closer in size to early Homo than to Pongo, exhibit a more ape-like morphology [START_REF] Ciochon | The mystery ape of Pleistocene Asia[END_REF][START_REF] Ciochon | Divorcing hominins from the Stegodon-Ailuropoda fauna. New views on the antiquity of hominins in Asia[END_REF].

The high frequency of supernumerary teeth in Pongo further complicates the taxonomic attribution of isolated hominid teeth from the Asian Pleistocene. Between 7% and 20% of Pongo individuals possess one or more supernumerary teeth, most of which (~85%) are upper and lower M4s (Bergstrom et al., 2016;Hooijer, 1948;[START_REF] Mahler | Metric variation in the pongid dentition[END_REF][START_REF] Selenka | Menschenaffen (Anthropomorphae): Studien Über Entwickelung und Schädelbau; Zur Vergleichenden Keimesgeschichte der Primaten[END_REF]. When compared to their metameres, Pongo M4s are generally the smallest tooth in the molar row, with linear measurements falling well within the dimensions for Homo. While highly variable, M4s tend to retain the morphological features of their metameres and are not easily identified as supernumerary (Hooijer, 1948;[START_REF] Mahler | Metric variation in the pongid dentition[END_REF]. Thus, Pongo M4s could be easily confused with Homo molars, especially when only size variables are considered. This ambiguity has resulted in several isolated molar teeth from Pleistocene deposits being classified by different experts as either Pongo M4s or Homo sp./Homo erectus [START_REF] Harrison | Primate biogeography and ecology on the Sunda Shelf Islands: a paleontological and zooarchaeological perspective[END_REF][START_REF] Schwartz | A diverse hominoid fauna from the late middle Pleistocene breccia cave of Tham Khuyen, Socialist Republic of Vietnam (excluding Hylobatidae)[END_REF][START_REF] Schwartz | A review of the Pleistocene hominoid fauna of the Socialist Republic of Vietnam (excluding Hylobatidae)[END_REF][START_REF] Smith | Taxonomic assessment of the Trinil molars using non-destructive 3D structural and developmental analysis[END_REF].

Here we combine dental non-metric and geometric morphometric data to document a dental pattern on the upper molars that appears to be taxonomically diagnostic among Pleistocene and recent Pongo. We focused our analyses on the protoconule, a cuspule located on the mesial side of the protocone at the intersection of the preprotocrista and the hypoparacrista, as well as on shape differences of the mesial portion of the trigon between the protocone and paracone dentine horns. Specifically, we quantified patterns of intra and inter-group variation of This article is protected by copyright. All rights reserved.

these features in a large sample of upper molars encompassing a wide range of fossil and extant hominid taxa and examined how well they differentiate isolated teeth of Pongo from those of Homo and other hominids. To overcome the limitations introduced by dental wear, which can obliterate informative features at the level of the outer enamel surface (OES), data collection and analyses were performed at the enamel-dentine junction (EDJ) (Olejniczak et al., 2008a;[START_REF] Ortiz | Homology, homoplasy, and cusp variability at the enamel-dentine junction of hominoid molars[END_REF][START_REF] Skinner | Dental trait expression at the enamel-dentine junction of lower molars in extant and fossil hominoids[END_REF][START_REF] Smith | Disentangling isolated dental remains of Asian Pleistocene hominins and pongines[END_REF]. The EDJ is the interface between the enamel cap and dentine crown and preserves the end point of growth of the inner enamel epithelium (Butler, 1956; Schour & Massler, 1940). Unless they are the result of dental tissue mineralization, which occurs later in morphogenesis, cusps (including small cuspules, conules, and tubercles) and crests develop as folds of the inner enamel epithelium [START_REF] Butler | Homologies of molar cusps and crests, and their bearing on assessments of rodent phylogeny[END_REF]. Thus, the study of the EDJ not only allows to more clearly determine topological relationships among dental structures (compared to the OES where these relationships may be masked by enamel deposition), but also represents an important means to distinguish between hominid taxa, even at the infraspecific level (e.g., [START_REF] Hanegraef | Dentine morphology of Atapuerca-Sima de los Huesos lower molars: Evolutionary implications through three-dimensional geometric morphometric analysis[END_REF][START_REF] Macchiarelli | How Neanderthal molar teeth grew[END_REF]Machiarelli, Bondioli, & Mazurier, 2008;[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], 2008, 2013;[START_REF] Ortiz | Homology, homoplasy, and cusp variability at the enamel-dentine junction of hominoid molars[END_REF][START_REF] Skinner | Dental trait expression at the enamel-dentine junction of lower molars in extant and fossil hominoids[END_REF][START_REF] Skinner | Discrimination of extant Pan species and subspecies using the enamel-dentine junction morphology of lower molars[END_REF][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 | The Early Pleistocene deciduous hominid molar FS-72 from the Sangiran Dome of Java, Indonesia: A taxonomic reappraisal based on its comparative endostructural characterization[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].

The protoconule (alternatively referred to as the paraconule) is part of the basic mammalian tribosphenic molar (Bown & Kraus, 1979;Butler, 1978[START_REF] Butler | Tribosphenic molars in the Cretaceous[END_REF][START_REF] Crompton | The origin of the tribospenic molar[END_REF][START_REF] Szalay | Mixodectidae, Microsyopidae, and the insectivore-primate transition[END_REF][START_REF] Van Valen | Deltatheridia, a new order of mammals[END_REF] with a long and well-documented evolutionary history. It is This article is protected by copyright. All rights reserved.

generally present in early primates from the Paleocene and Eocene [START_REF] Szalay | Evolutionary history of the primates[END_REF], and is variably retained in extant strepsirrhines [START_REF] Schwartz | Evolutionary relationships of living lemurs and lorises (Mammalia, Primates) and their potential affinities with European Eocene Adapidae[END_REF] and platyrrhines [START_REF] Kay | Platyrrhine origins: a reappraisal of the dental evidence[END_REF][START_REF] Marivaux | Neotropics provides insights into the emergence of New World monkeys: New dental evidence from the late Oligocene of Peruvian Amazonia[END_REF]. A discernable protoconule is present in most stem catarrhines (i.e., propliopithecids, pliopithecoids, and dendropithecids) [START_REF] Alba | A new species of Pliopithecus Gervais, 1849 (Primates: Pliopithecidae) from the Middle Miocene (MN 8) of Abocador de Can Mata (els Hostalets de Pierola, Catalonia, Spain)[END_REF][START_REF] Alba | The oldest pliopithecid record in the Iberian Peninsula based on new material from the Vallès-Penedès Basin[END_REF][START_REF] Cote | A detailed assessment of the maxillary morphology of Limnopithecus evansi with implications for the taxonomy of the genus[END_REF][START_REF] Delson | Evolution and interrelationships of the catarrhine primates[END_REF][START_REF] Harrison | Small-bodied apes from the Miocene of East Africa[END_REF][START_REF] Harrison | A taxonomic revision of the small catarrhine primates from the early Miocene of East Africa[END_REF][START_REF] Harrison | Taxonomy and phylogenetic relationships of early Miocene catarrhines from Sihong, China[END_REF][START_REF] Harrison | A new species of Pliopithecus from the middle Miocene of China and its implications for early catarrhine zoogeography[END_REF][START_REF] Harrison | A new middle Miocene pliopithecid from Sant Quirze, northern Spain[END_REF][START_REF] Rossie | A new pliopithecoid genus from the early Miocene of Uganda[END_REF][START_REF] Szalay | Evolutionary history of the primates[END_REF][START_REF] Zhang | A new middle Miocene pliopithecid from Inner Mongolia, China[END_REF]although Godinot, 1994 contends that the protoconule is lacking in propliopithecids) and stem hominoids (i.e., proconsulids and afropithecids) [START_REF] Andrews | A revision of the Miocene Hominoidea of East Africa[END_REF][START_REF] Harrison | New fossil anthropoids from the middle Miocene of East Africa and their bearing on the origin of the Oreopithecidae[END_REF][START_REF] Harrison | Late Oligocene to middle Miocene catarrhines from Afro-Arabia[END_REF][START_REF] Harrison | Dendropithecoidea, Proconsuloidea, and Hominoidea[END_REF][START_REF] Harrison | The anatomy and systematic position of the early Miocene proconsulid from Meswa Bridge, Kenya[END_REF][START_REF] Gros | The Miocene Hominoidea of East Africa. Fossil Mammals of Africa[END_REF][START_REF] Pilbeam | Tertiary Pongidae of East Africa: Evolutionary relationships and taxonomy[END_REF]. A variably developed protoconule is also ubiquitous in fossil hominids from the Miocene of Africa and Eurasia (see Begun, 1992;Ishida & Pickford, 1997;[START_REF] Kordos | A new reconstruction of RUD 77, a partial cranium of Dryopithecus brancoi from Rudabánya, Hungary[END_REF][START_REF] Kunimatsu | Maxillae and associated gnathodental specimens of Nacholapithecus kerioi, a large-bodied hominoid from Nachola, northern Kenya[END_REF][START_REF] Pickford | A new look at Kenyapithecus based on recent discoveries in western Kenya[END_REF]Perez de los Rios, 2015;[START_REF] Pilbeam | Miocene hominoids from Pakistan[END_REF][START_REF] Suwa | A new species of great ape from the late Miocene epoch in Ethiopia[END_REF][START_REF] Von Koenigswald | Gigantopithecus blacki von Koenigswald, a giant fossil hominoid from the Pleistocene of Southern China[END_REF][START_REF] Zhang | Gigantopithecus blacki: a giant ape from the Pleistocene of Asia revisited[END_REF]. The presence of a protoconule can be inferred, therefore, to be a primitive primate feature that has been retained as part of the ancestral catarrhine and hominoid morphotypes [START_REF] Delson | Evolution and interrelationships of the catarrhine primates[END_REF][START_REF] Harrison | Small-bodied apes from the Miocene of East Africa[END_REF][START_REF] Harrison | The phylogenetic relationships of the early catarrhine primates: A review of the current evidence[END_REF]. However, the frequency of occurrence and the degree of development of the protoconule among fossil catarrhines has not been documented.

Studies of the protoconule among extant hominoids date back to the late 1800s, with [START_REF] Selenka | Menschenaffen (Anthropomorphae): Studien Über Entwickelung und Schädelbau; Zur Vergleichenden Keimesgeschichte der Primaten[END_REF] comprehensive anatomical descriptions of Pongo. [START_REF] Selenka | Menschenaffen (Anthropomorphae): Studien Über Entwickelung und Schädelbau; Zur Vergleichenden Keimesgeschichte der Primaten[END_REF]

reported a

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high incidence (76%-94% for females and 90%-100% for males, depending on serial position) of the protoconule in Pongo upper molars (see also Hooijer, 1948). Subsequent studies have documented the presence (in different frequencies and degrees of expression) of the protoconule in extant African apes [START_REF] Korenhof | Morphogenetical aspects of the human upper molar[END_REF][START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF][START_REF] Remane | Zahne und Gebiss. Primatologia III[END_REF][START_REF] Swindler | Dentition of living primates[END_REF][START_REF] Swindler | Comparative and evolutionary aspects of permanent dentition[END_REF]. [START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF] reported that 15-37% of Pan paniscus and 20-66% of Pan troglodytes upper molars have distinct protoconules. The cuspule has also been reported in humans, with an incidence ranging between ~5% and 45% depending on the population examined [START_REF] Kanazawa | A quantitative investigation of irregular cuspules in human maxillary permanent molars[END_REF]. There are no published accounts of the occurrence of the protoconule in hylobatids, but anecdotal reports have indicated that the cuspule is either small in size or absent [START_REF] Delson | Are there any anthropoid primate living fossils[END_REF][START_REF] Szalay | Evolutionary history of the primates[END_REF]. A poorly expressed protoconule would be expected in hylobatids, given the characteristically low and rounded cusps and crests on the upper molars. A survey by TH of a sample (n=61) of Hylobates spp. found that a small but discernable protoconule was observable in 44% of M1s, 77% of M2s and 25% of M3s (unpublished data). The presence of a protoconule was also reported in a collection of fossil upper molars of Nomascus from the Pleistocene of southern China [START_REF] Zhang | Fossil gibbons (Mammalia, Hylobatidae) from the Pleistocene of Chongzuo, Guangxi, China[END_REF]. The lack of a more comprehensive comparative analysis of the protoconule using a large and diverse sample of hominoids has limited the implications of Selenka's (1898) initial observations on the high incidence of this cuspule in Pongo and its potential usefulness in hominoid systematics.

Materials and methods

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Sample

Our study sample consists of 469 upper molars of eight hominid genera, including Australopithecus, Paranthropus, Homo, Pan, Gorilla, Pongo, Sivapithecus, and Meganthropus.

Micro-computed tomography was used to image the EDJ of the upper molars (Table 1). All data were derived from original specimens and include the following taxa: Australopithecus provided in Table S1. We made no attempt to record the sex of the specimens given that our sample mainly comprises isolated teeth and that the sex is unknown for most fossils. However, previous studies have found that discrete traits do not significantly vary between sexes in great apes and hominins [START_REF] Uchida | Craniodental variation among the great apes[END_REF][START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF][START_REF] Ortiz | Homology, homoplasy, and cusp variability at the enamel-dentine junction of hominoid molars[END_REF]. Although some specimens in our sample exhibit moderate wear (up to [START_REF] Molnar | Human tooth wear, tooth function and cultural variability[END_REF] stage 5), this did not affect the accurate assessment of protoconule expression at the EDJ. However, specimens were not included in the geometric morphometric analysis if the tips of the protocone and/or paracone were worn or damaged.

The fossil Pongo material from China was recovered from Pleistocene cave deposits in Guangxi (Harrison et al., 2014) and was scanned with a TX225-Actis microCT system using the following parameters: 130 kV, 0.2-0.24 mA, and a 0. [START_REF] Wollny | MIA -a free and open source software for gray scale medical image analysis[END_REF]. Filtered image stacks were segmented into enamel and dentine tissues using Avizo/Amira 6.3 (FEI Visualization Sciences Group). Digital surface models (.ply format) of the EDJ were produced in Avizo using the surface generation module.

Protoconule expression

The protoconule is a cuspule mesiobuccal to the protocone, located on the preprotocrista or at the intersection of the preprotocrista and the hypoparacrista [START_REF] Butler | Homologies of molar cusps and crests, and their bearing on assessments of rodent phylogeny[END_REF][START_REF] Remane | Zahne und Gebiss. Primatologia III[END_REF][START_REF] Szalay | Mixodectidae, Microsyopidae, and the insectivore-primate transition[END_REF]. Protoconule expression at the EDJ was assessed visually by AO using the following scoring system (Fig. 1): Grade 0, protoconule is absent; Grade 1, faint furrows and other irregularities, slight bumps or weak pointed elevations are present; Grade 2, a moderate protoconule is present, and roughly <50% of the height of the protocone from the same tooth.

Grade 3: a large protoconule is present, and ≥50% of the height of the protocone from the same tooth as seen from the midpoint of the mesial marginal ridge. Grade 1 is considered the This article is protected by copyright. All rights reserved.

"suspected" category as per [START_REF] Turner | Scoring procedures for key morphological traits of the permanent dentition: the Arizona State University Dental Anthropology System[END_REF] and [START_REF] Skinner | The presence of accessory cusps in chimpanzee lower molars is consistent with a patterning cascade model of development[END_REF] to include those cases where it was unclear whether or not a poorly developed protoconule was present. Following [START_REF] Skinner | The presence of accessory cusps in chimpanzee lower molars is consistent with a patterning cascade model of development[END_REF], most of the "suspected" cases may represent protoconules whose growth initiated but did not progress substantially due to mineralization. This category was further subdivided into grade 1A (bumps and other barely discernible irregularities) and grade 1B (slight pointed elevations that may mark the presence of a dentine horn). The significance of the observed patterns was tested via bootstrapping (1,000 iterations) performed in R 3.4.3 (R Development Core Team, 2017).

The lowest score was chosen when protoconule expression fell at the boundary of two grades. Moreover, when two or more cuspules were present on the preprotocrista, the lingualmost cuspule was scored as the protoconule. To further standardize data collection procedures and minimize error introduced by differences in orientation along the axes and colormap settings, protoconule expression was collected using the following protocol. Each tooth was oriented in occlusal view (i.e., occlusal surface parallel to the xy-plane of the Cartesian coordinate system), with the mesial side of the crown placed at the bottom of the screen. The mesial portion of the longitudinal (mesiodistal) groove was then aligned with the y-axis, and the main buccolingual groove with the x-axis. Once oriented, each tooth was rotated -90 degrees around the local x-axis using the transform editor in Avizo/Amira 6.3 so that the entire mesial aspect of each tooth crown was perpendicular to the xy-plane. A grid was added and color settings of the EDJ models were set to color mode "boundary ids" for consistency. Protoconule expression was collected This article is protected by copyright. All rights reserved.

exclusively on that view, without further manipulation of the virtual models. To test for error in protoconule assessment all teeth were scored twice by AO, with scoring sessions separated by ~1.5 months. The percentage of disagreement between the two sessions was 4.1%, and in all cases, disagreements were never greater than one grade of expression.

While an account of the developmental homology of the protoconule is beyond the scope of our study, we note that assessments of protoconule expression may vary depending on both tooth orientation and dental tissue analyzed. For example, we found cases in which the protoconule were expressed as an expansion of the mesiolingual corner of the tooth along the occlusal surface with minimal height involvement (Fig. S2). Similarly, enamel deposition and the confounding effect of a marked cingulum on the mesiolingual aspect of teeth may lead to score the protoconule as present at the OES (Fig. S3a,b). We also found cases of protoconule presence derived entirely from enamel deposition (i.e., the protoconule is absent at the EDJ, but present at the OES), especially among early hominins (Fig. S3c). It is important to note, however, that there are limitations to the size of structures that can be imaged based on scan resolution, scan parameters and post-processing steps (i.e., one must consider that very small dentine horns could be present but not detectable on the final EDJ model). Even more common are small irregularities and faint bumps on the preprotocrista as observed at the EDJ, with no clear protoconule presence or absence at the OES (Fig. S3d,e). Orientation and the angle from which a tooth is viewed may also lead to ambiguities regarding protoconule presence (Fig. S3f).

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Therefore, to minimize potential ambiguities, our study focused exclusively on the EDJ using standardized views, as described above, to record protoconule expression.

Geometric morphometric analysis

We performed a geometric morphometric analysis in order to quantify and visualize group shape differences in the mesial portion of the upper molars between the protocone and paracone that may be associated with protoconule expression (and/or that of other conules). To do so, 3D landmarks and semi-landmarks were digitized on the virtual models of the EDJ using Avizo/Amira 6.3. As illustrated in Fig. 2, landmarks were placed on the tip of the protocone, the midpoint of the mesial marginal ridge and tip of the paracone, and a set of 23 semi-landmarks connecting the three main landmarks were digitized to capture the entire configuration of the mesial portion of the trigon and mesial marginal ridge (hereafter mesial fovea; [START_REF] Harrison | Taxonomy and phylogenetic relationships of early Miocene catarrhines from Sihong, China[END_REF]. Landmark and semi-landmark digitizing was conducted by AO.

Intra-observer error was assessed on 25 randomly selected teeth in which all landmarks and semi-landmarks were digitized twice, with sessions separated by three months. The systematic and random error was tested using repeated measures ANOVA (Anova-RM) and intra-class correlation coefficients (ICC), respectively. Anova-RM tests the hypothesis of no differences between the means of repeated observations under the hypothesis of nonindependence, while ICC measures the relationship between the intergroup and intragroup variance. The results of the Anova-RM show that there are non-significant differences between This article is protected by copyright. All rights reserved.

digitizing sessions (Table S2). Similarly, ICC values reveal a highly significant correlation between sessions (Table S3). Both analyses indicate that intra-observer error is negligible.

Landmark and semi-landmark data were imported to R 3.4.3 using Nat (v.1.8.9; Jefferis and Manton, 2017) and Arothron [START_REF] Profico | Arothron: R functions for geometric morphometric analyses[END_REF] packages. The R packages Geomorph (Adams et al., 2015) and Morpho [START_REF] Schlager | Morpho: Calculations and visualisations related to geometric morphometrics[END_REF], as well as MorphoJ [START_REF] Klingenberg | MorphoJ: an integrated software package for geometric morphometrics[END_REF] were used for data processing and analyses. Semi-landmarks were slid along their curve by minimizing the bending energy of the thin-plate spline deformation between each specimen and the Procrustes average for the sample [START_REF] Gunz | Semilandmarks in three dimensions[END_REF]. Because we were investigating bilateral structures (i.e., right and left upper molars), a reflection procedure was implemented in order to discard non-relevant right-left differences. We randomly chose to work on right molars, such that all left landmark/semilandmark configurations were mirror-imaged. After sliding, landmark and semi-landmark configurations were superimposed using a generalized Procrustes analysis to convert them into shape variables. Centroid size calculated from landmark/semi-landmark data was analyzed independently to test for mesial fovea differences attributed exclusively to size. To that end, we conducted Kruskal-Wallis one-way analysis of variance among samples, for each molar type analyzed independently and combined (Table S4). Analyses were performed in Statistica StatSoft 10.0.

(CVA). When M1-M3 were combined, we also included those molars of uncertain serial position. Due to restrictions in the CVA computation (the number of variables must be smaller than the number of specimens), we reduced the dimensionality of our data by using a subset of principal components (ranging from 15 to 18) explaining 95% of the total shape variation to compute the CVAs. The PCAs and CVAs were conducted in MorphoJ [START_REF] Klingenberg | MorphoJ: an integrated software package for geometric morphometrics[END_REF] and PAST [START_REF] Hammer | PAST: Paleontological statistics software package for education and data analysis[END_REF]. Finally, we used pairwise discriminant function analysis (DFA), as implemented in MorphoJ [START_REF] Klingenberg | MorphoJ: an integrated software package for geometric morphometrics[END_REF], to investigate mesial fovea shape differences between different group pairs. As it deals with pairwise comparisons, this test differs from the classic CVA in that it allows the calculation and visualization of sample mean differences between pairs. Mahalanobis distances were used to assess the magnitude of the morphological differences between sample means, with significance set at p < 0.01.

Visualization of mesial fovea shape variation

We followed two different approaches to visualize mesial fovea shape variation in hominoids. First, we used a method for warping 3D surfaces following Bookstein's (1989) thin plate spline algorithm based on a reference and a target configuration. We used a 3D surface of the mesial fovea of a recent H. sapiens as a template to visualize shape deformations along PC and CV axes in all taxa analyzed. The 3D shape deformation analysis was done in R 3.4.3 (R Development Core Team, 2017) using the package Morpho [START_REF] Schlager | Morpho: Calculations and visualisations related to geometric morphometrics[END_REF]. Second, This article is protected by copyright. All rights reserved.

wireframe graphs were performed in MorphoJ [START_REF] Klingenberg | MorphoJ: an integrated software package for geometric morphometrics[END_REF] using the pairwise DFA tool.

The superimposed wireframes representing mean landmark configurations were depicted using a combination of the 3 axes (1 vs. 2, 1 vs. 3, 2 vs. 3; only the most informative combination -1 vs.

2 -presented here). Scatterplots were employed to visualize the data point distribution in the multivariate shape space.

Tooth identification of Pleistocene Pongo from China and Vietnam

Tooth type identification of fossil Pongo from China was performed by Reiko Kono based on observations of known characteristics for each tooth type, published descriptions of the Pongo dentition, and presence of inter-proximal facets (Kono, personal communication). Pongo teeth from Vietnam of unknown or ambiguous serial position were assigned to a molar type following a cross-validation approach in which each specimen was considered "unknown" and then classified based on the Pongo sample with secure molar identification (M1-3s; unfortunately no microCT scans of extant Pongo individuals with M4s were available to include in this analysis). To this end, all 53 upper molars of extant Pongo and fossil Pongo from China and Vietnam with known serial position were combined in a single sample. For each molar type, we conducted a CVA using the set of principal components that explained 95% of variation in the PCA. Classification accuracy of mesial fovea shape was 82.7% (Fig. S4). Given the high classification accuracy, we used posterior probabilities derived from the CVA to provide a statistically-based classification for the fossil Pongo upper molars from Vietnam (Table S5).

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Results

The frequency and expression of the protoconule at the EDJ per molar type and group at different taxonomic levels are summarized in Table S6 andFigs. 3 andS5 For all molars combined, our dental non-metric analyses reveal that a moderate or large protoconule (i.e., grades 2 or 3) is present in 80% and 86.7% of the Pleistocene and extant Pongo individuals, respectively. This is also the case for Sivapithecus, as the three specimens examined here presented a grade 2 protoconule. In contrast, moderate to pronounced expressions of the protoconule in hominins, Meganthropus, and African great apes occur in low frequencies (0%-14.1% when data are pooled at the genus level, with the greatest frequency seen in Homo). In these latter groups, the protoconule is most frequently either absent or weakly expressed (for grades 0 and 1A/1B combined: 95.9% in Australopithecus, 95.6% in Paranthropus, 85.3% in Homo, 100% in Meganthropus, 100% in Pan, and 100% in Gorilla). While sample sizes for early Homo and H. erectus are small, when the protoconule in Homo is examined at the species level, moderate to pronounced expressions were most commonly observed among Pleistocene H. sapiens (22.2%) and H. neanderthalensis (19.3%).. As illustrated in Figs. 7-8 and S7-8, however, even the greatest degrees of protoconule expression in humans are generally characterized by a small and low but pointed cuspule located more mesially on the preprotocrista. Conversely, the protoconule in Pongo tends to be located more lingually than in This article is protected by copyright. All rights reserved.

humans, closer to the protocone, and its morphology is characterized by either a tall and pointed or broad and blunt tubercle (Figs. 456). The bootstrapping analysis (95% confidence) indicates that frequencies of moderate to pronounced protoconules in Pongo are significantly different from hominins and African great apes. Differences among species within the Homininae, on the other hand, are nearly always non-significant (Table S7).

Similar results were obtained when each molar type was analyzed independently (rather than combined). Frequencies of moderate to pronounced protoconules (grades 2 and 3) in Pongo (including Pleistocene and extant samples) are between 78.4% (M1) and 93.8% (M2). The largest protoconules (grade 3), however, are more frequently found in M1. Grade 3 protoconules were never observed in our Homo sample, but moderate or medium-sized (grade 2) expressions were found in 8.6% (M2) to 25% (M1) of the molars. Compared to its metameres, the protoconule is more common and/or strongly expressed in M1 in later Homo (including MPEH, H. neanderthalensis, and H. sapiens); this trend is particularly clear in Pleistocene H. sapiens. In contrast, none of the H. erectus specimens in our sample exhibited moderate (grade 2) to large (grade 3) protoconule expressions, but an accessory tubercle was observed on the mesial marginal ridge of Sangiran 4 M3 (Fig. 7d). A similar accessory tubercle was also found on both Trinil 11620 and Trinil 11621 (Fig. 9). Grades 2 or 3 of protoconule in the African great apes examined were also rare or absent.

Although only the protoconule was the focus of this study, it is worth noting the high frequency of accessory cuspules and/or tubercles on the mesial marginal ridge and/or This article is protected by copyright. All rights reserved. preparacrista of human molars (Fig. S12), especially in Homo sapiens (83.3% and 71.4% of the Pleistocene and recent H. sapiens examined, respectively), followed by H. neanderthalensis (52.6%) and H. erectus (28.6%). Pongo also exhibits a relatively high frequency of accessory cuspules and/or tubercles (extant Pongo: 30%, Pleistocene Pongo from China: 48%, Pleistocene Pongo from Vietnam: 54%) in addition to the protoconule, whereas they occur in low frequencies in early hominins (0%-6.7%) and African great apes (0%-3.5 %). where the overlap among Pongo, Australopithecus, and Paranthropus takes place. It is also worth noting the high degree of overlap in mesial fovea shape between the M1 of Paranthropus and all species of Homo (except for H. erectus) (Fig. S17). This degree of overlap, however, does not hold true for M2 and M3. Overall, mesial fovea shape possesses a high classification accuracy, reaching ~72%-85% when all teeth are analyzed together and between ~91%-95% when each molar type is analyzed independently (Table 2). Interestingly, the variation in mesial fovea for M3 appears to be particularly high among members of the genus Homo (Fig. S19).

When multivariate techniques are performed exclusively on Homo and Pongo, PCA and CVA results for all molars combined also show a clear separation between the two genera . While all species of Homo cluster together and occupy the negative side along the first canonical axis, our three samples of Pongo are located on the positive side of this first axis. As summarized in Table S8, classification accuracy for the species included in the two genera reaches ~88% (all molar types included). Although correct taxonomic classification is slightly higher when each molar type is analyzed independently (between 88%-92%), the position of early Homo along the first canonical axis relative to other hominoid taxa is slightly more ambiguous. That is, M1 mesial fovea shape of H. erectus s.l. This article is protected by copyright. All rights reserved.

The results of the pairwise DFAs, performed to compare group means in mesial fovea shape, are illustrated in Fig. 12. Our results show that extant and fossil Pongo have a similar mesial fovea shape and only subtle differences can be observed, such as the slightly sharper, more pronounced and buccally oriented protoconule in extant Pongo. The shape of the mesial marginal ridge of both Pleistocene and extant Pongo is similar, although it is slightly more expansive mesially in the fossil group. When Pongo and humans (with H. erectus s.l. analyzed both independently and as part of the genus Homo) are compared, differences in mesial fovea shape become evident. The protocone is more peripherally positioned in Pongo and a pointed and strongly developed protoconule tubercle is present on the preprotocrista. Both the mesial marginal ridge and the protoconule also tend to be more mesially positioned in Pongo compared to Homo. The mean shape of the mesiolingual aspect of human upper molars reveals no traces of a protoconule. Human molars, however, exhibit a mesiobuccal expansion of the buccal portion of the mesial marginal ridge and an overall increase in the surface area of the paracone relative to the protocone. There are no major differences between Pongo and humans in the position of the paracone cusp apex along the mesiodistal and buccolingual planes. However, height differentials between the protocone and paracone apices are more pronounced in Pongo teeth, which show a notably lower protocone relative to the paracone. Overall, humans possess higher-cusped crowns and a concomitantly relatively deep and broad mesial fovea.

Discussion

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More than 100 years ago, Selenka (1898) identified the common presence of the protoconule among extant Pongo, including both males and females. Reported frequencies were 90% (males) and 76% (females) for M1, 100% (males) and 94% (females) for M2, and 100% (males) and 93% (females) for M3, suggesting that the protoconule is slightly more common in males (vs. females) and in M2 and M3 (vs. M1). Hooijer (1948), in his comprehensive study of orangutan dental remains from Quaternary deposits in Sumatra, reported similar results for fossil Pongo, where frequencies of protoconule expression range between 88% (M1 and M3) and 90% (M2). He also noted a high incidence (85%-100%) of the protoconule in his comparative sample of recent Pongo. Our results of the presence of the protoconule at the EDJ of Pongo teeth are consistent with these earlier observations. We found that the M2 is the most common tooth of the upper molar row exhibiting the protoconule. When present, however, the protoconule appears to be more strongly expressed in M1.

While neither [START_REF] Selenka | Menschenaffen (Anthropomorphae): Studien Über Entwickelung und Schädelbau; Zur Vergleichenden Keimesgeschichte der Primaten[END_REF] nor Hooijer (1948) examined patterns of protoconule expression in other great apes, subsequent observations [START_REF] Delson | Evolution and interrelationships of the catarrhine primates[END_REF][START_REF] Korenhof | Morphogenetical aspects of the human upper molar[END_REF][START_REF] Swindler | Dentition of living primates[END_REF][START_REF] Swindler | Comparative and evolutionary aspects of permanent dentition[END_REF][START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF] revealed that this cuspule occurs much less frequently in Pan and Gorilla. This appears to be particularly true in Pan, as the protoconule in this genus is generally "barely distinguished" according to Swindler & Olshan (1988: 277). Gorilla occupies an intermediate position between Pan and Pongo in protoconule incidence and degree of expression. Our data support these claims, but the results differ from a large-scale study by [START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF]

on the African great ape dentition. Contrary to our results

This article is protected by copyright. All rights reserved. and those reported elsewhere [START_REF] Korenhof | Morphogenetical aspects of the human upper molar[END_REF][START_REF] Swindler | Dentition of living primates[END_REF][START_REF] Swindler | Comparative and evolutionary aspects of permanent dentition[END_REF], [START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF] identified relatively high frequencies of protoconule expression in Pan (M1: 33%-66%, M2: 33%-66%, M3: 15%-45%, depending on the species and subspecies sampled).

No frequency data for the protoconule in Gorilla were reported but given that [START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF] sample size was much larger than that used here or elsewhere, it is likely that these inconsistencies are the product of sampling error and the different dental tissues examined.

Overall, however, even using [START_REF] Pilbrow | Dental variation in African apes with implications for understanding patterns of variation in fossil apes[END_REF] data, Pan has a rarer occurrence of the protoconule relative to Pongo.

The protoconule is a commonly used character in reconstructions of the evolutionary relationships of catarrhine primates (e.g., Alba et al., 2015;[START_REF] Delson | Evolution and interrelationships of the catarrhine primates[END_REF][START_REF] Harrison | The phylogenetic relationships of the early catarrhine primates: A review of the current evidence[END_REF][START_REF] Nengo | New infant cranium from the African Miocene sheds light on ape evolution[END_REF][START_REF] Rossie | A new pliopithecoid genus from the early Miocene of Uganda[END_REF][START_REF] Stevens | Palaeontological evidence for an Oligocene divergence between Old World monkeys and apes[END_REF]. As noted above, the ancestral catarrhine and hominoid morphotypes would have included the presence of a protoconule as a common feature of the upper molars [START_REF] Delson | Evolution and interrelationships of the catarrhine primates[END_REF][START_REF] Harrison | The phylogenetic relationships of the early catarrhine primates: A review of the current evidence[END_REF]. Our results show that Pongo exhibits the ubiquitous presence of a large protoconule, while this cuspule in Pan and Gorilla occurs much less frequently and is more weakly expressed.

Although our Sivapithecus sample is small, the morphology and frequency of the protoconule present in this extinct taxon closely approximates those of Pongo, which further strengthen the long-held view regarding the close relationship between the two taxa. In hominins the protoconule is either lost or reduced; in the latter case, the protoconule is generally expressed as a small but pointed cuspule. Homo neanderthalensis and H. sapiens also exhibit a multiplication This article is protected by copyright. All rights reserved.

of conules along the preprotocrista and the mesial marginal ridge. Given these findings, Pongo can be inferred to have retained the primitive catarrhine and hominoid condition, while hominines (i.e., African apes and hominins) are derived in having protoconules that occur less frequent and are relatively less well-developed.

Our multivariate analyses also revealed the distinctive mesial fovea shape of Pongo upper molars compared to those of Homo and other hominids. While these analyses suggest that shape variation of the mesial fovea is higher in Homo than in Pongo, we caution that this could be an artifact of the larger number of individuals from the genus Homo used in our study. Compared to H. erectus s.l. and the genus Homo as a whole, both Pleistocene and recent Pongo molars exhibit, on average, a marginalized protocone with a marked protoconule on the preprotocrista and greater height differentials between a relatively low protocone and tall paracone. This pattern in Pongo is consistent across a temporo-spatially diverse sample of individuals, despite the high degree of variation in other dental morphological features documented among members of this genus [START_REF] Harrison | Archaeological and ecological implications of the primate fauna from prehistoric sites in Borneo[END_REF]Harrison et al., 2014;[START_REF] Schwartz | A review of the Pleistocene hominoid fauna of the Socialist Republic of Vietnam (excluding Hylobatidae)[END_REF][START_REF] Uchida | Craniodental variation among the great apes[END_REF][START_REF] Uchida | Variation in tooth morphology of Pongo pygmaeus[END_REF][START_REF] Tshen | Biogeographic distribution and metric dental variation of fossil and living orangutans (Pongo spp.)[END_REF]. However, an interesting characteristic of Pleistocene Pongo from Vietnam is the regular presence of a small tubercle on the preparacrista in addition to the protoconule; this was rarely observed in fossil Pongo from China and our sample of extant orangutans. Using a large dataset, [START_REF] Smith | Dental tissue proportions in fossil orangutans from mainland Asia and Indonesia[END_REF][START_REF] Smith | Dental development in living and fossil orangutans[END_REF] also reported that Pleistocene Pongo has slightly higher average enamel thickness, especially in upper and lower M1, but that cuspal enamel thickness does not differ between Pleistocene and extant Pongo. They also noted that while both groups This article is protected by copyright. All rights reserved. exhibit similar long-period line periodicity values, fossil Pongo teeth have a higher number of lines and thus slightly lower extension rates.

The results of this study suggest that the protoconule (and overall morphology of the mesial fovea) is a potentially useful feature for the taxonomic identification of isolated hominid teeth found in Pleistocene deposits of Asia, especially when dental wear has obliterated diagnostic features at the external enamel surface. We caution, however, that, because of the degree of morphological overlap and the fact that this variant is not unique to Pongo, this feature cannot be used in isolation. Instead, the mesial fovea shape and presence of a well-developed protoconule should be used in combination with other diagnostic features for accurate taxonomic assessments. Previous studies of tooth microstructure and dental tissue proportions have identified additional dental features that differ between pongines and hominins, such as the equal reduction of enamel and dentine in Pongo, but preferential loss of dentine in Homo during the process of tooth size reduction in these two groups [START_REF] Grine | Fossil hominid teeth from the Sangiran Dome (Java, Indonesia)[END_REF][START_REF] Smith | Dental tissue proportions in fossil orangutans from mainland Asia and Indonesia[END_REF] with the exception of Neanderthals, see Olejniczak et al., 2008b). Pongo molars also differ from those of humans in having lower dentine horns and an overall different shape of the EDJ with broader crowns, broad and shallow dentinal intercuspal furrow pattern and less medially placed lingual dentine tips [START_REF] Olejniczak | Quantification of dentine shape in anthropoid primates[END_REF]Olejniczak et al., 2008a;[START_REF] Smith | Disentangling isolated dental remains of Asian Pleistocene hominins and pongines[END_REF][START_REF] Zanolli | Evidence for increased hominid diversity in the Early-Middle Pleistocene of Java, Indonesia[END_REF]. In addition, when only fossil individuals were considered, [START_REF] Smith | Reassessing enigmatic Asian hominoid dental remains[END_REF] found that enamel has a more uniform distribution across the enamel cap in fossil Pongo than in H. erectus, although relative enamel thickness values overlap substantially in the two taxa This article is protected by copyright. All rights reserved. [START_REF] Smith | Disentangling isolated dental remains of Asian Pleistocene hominins and pongines[END_REF]. However, when only extant members of Pongo and Homo were considered and analyzed in 3D (i.e., the entire crown) rather than 2D cross sections, [START_REF] Kono | Molar enamel thickness and distribution patterns in extant great apes and humans: new insights based on a 3-dimensional whole crown perspective[END_REF] noted that while Pongo exhibited relatively thicker occlusal but thinner basal enamel, molar enamel in Homo was relatively thicker throughout the crown. Long-period line periodicity is also significantly higher in Pleistocene Pongo than in Homo, resulting in longer crown formation times in the former group [START_REF] Rong | Periodicity of Retzius lines in fossil Pongo from South China[END_REF][START_REF] Smith | Reassessing enigmatic Asian hominoid dental remains[END_REF][START_REF] Smith | Dental development in living and fossil orangutans[END_REF][START_REF] Smith | Disentangling isolated dental remains of Asian Pleistocene hominins and pongines[END_REF]. The feasibility to collect some of these data, however, is highly contingent on dental wear. Given the growing access to micro-computed tomographic facilities, reduced dependency on the need of unworn teeth, and ubiquitous presence of the protoconule in Pongo molars irrespective of their position in the molar row, we highlight the discriminatory power carried by this feature for differentiating Pongo from Homo. Thus, our results provide a morphological feature that may prove useful in helping to resolve ambiguities in cases where the taxonomic identification of Asian hominoid dental remains has proven problematic. This is an issue of paramount importance, because in order to document the spatial and temporal distribution of hominins and Pongo in the Pleistocene of Asia it is crucial to have a high degree of confidence in taxonomic attributions. The morphological features discussed here can be used as part of a suite of diagnostic dental traits to help determine the taxonomic identity of isolated This article is protected by copyright. All rights reserved. 93LT40;c. HH53;d. 77.TO.o.10.A.20;e. 77.TO.o.13.21;f. HH38;g. 77.TO.13.XX; h. 93LT7; i. 93LT38.

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  anamensis (n=3), Australopithecus afarensis (n=7), A. africanus (n=39), Paranthropus robustus (n=41), P. boisei (n=4), early Homo (n=8), H. erectus sensu lato (n=7), Middle Pleistocene European hominins (MPEH, n=3), Homo neanderthalensis (n=57), Homo sapiens (n=95), Pan troglodytes (n=58), Pan paniscus (n=9), Gorilla sp. (n=28),Pongo sp. (n=105), Sivapithecus sp. (n=3), and Meganthropus sp. (n=2) . Samples with less than seven specimens were only analyzed at the generic level, except for H. erectus s.l. as this species is of primary interest for this study. The early Homo sample includes hominins classified as Homo habilis, Homo rudolfensis, and individuals from the Early Pleistocene whose affinities have been only identified as Homo sp. The H. erectus s.l. sample includes specimens from both Africa and Asia (KNM-ER 1808h M2, Sangiran 4 M1-3, Sangiran 7-3b M1, Sangiran 7-3c M2, and Sangiran 11 UM).The Pongo sample comprises specimens from the Pleistocene of China (n=25) and Vietnam (n=50), as well as extant Pongo (P. abelii=12, P. pygmaeus=8, and Pongo sp.=10). Furthermore, it should be noted that our Meganthropus sample is represented by Trinil 11620 and Trinil 11621, specimens with controversial affinities and that have been previously attributed to other taxa (see above, and Zanolli et al., 2019). Additional details of the fossil specimens used in this study are This article is protected by copyright. All rights reserved.

  2 or 0.5 mm copper filter. The Pleistocene Pongo specimens from Vietnam come from Hang Hum, Lang Trang, Tham Khuyen and Tham Om Caves and were scanned with a v|tome|x L 240-180 instrument (90-100 kV, 400 µA, 0.1 mm copper) at the AST-RX platform of the MNHN Paris. The two Trinil molars (11620 and 11621) were scanned by synchrotron radiation-based μCT on beamline ID 19 of the European Synchrotron Radiation Facility at Grenoble using an absorption mode with an isotropic voxel size of 31.12 μm 3 at an energy of 60 keV10. The microCT datasets of these two specimens were downloaded from at the European Synchrotron Radiation Facility Paleontological Microtomographic Database (http://paleo.esrf.eu). The three Sivapithecus molars were scanned by X-ray computed microtomography at the Multidisciplinary Laboratory of the ICTP, Trieste This article is protected by copyright. All rights reserved.

(

  [START_REF] Tuniz | The ICTP-Elettra X-ray laboratory for cultural heritage and archaeology. A facility for training and education in the developing world[END_REF], according to the following parameters: 110 kV voltage, 90 mA current, angular step of 0.25° over a scan angle of 360°. The final volumes were reconstructed with an isotropic voxel size of 20.6 µm All other specimens were scanned with either a BIR ACTIS 225/300 (130 kV, 100 μA, 0.25 brass filter) or a Skyscan 1172 (100 kV, 94 µA, 2.0 mm aluminum and copper) scanner. Pixel dimensions and slice spacing of the resultant images ranged between 10 and 60 μm. The complete image stack of each tooth was filtered to improve tissue gray-scale homogeneity

  . Examples of each degree of protoconule expression observed in each taxon are provided in Figs. 4-9 and S6-11.

  Figs. S13-16 for PCA). In most cases, Pongo is also clearly differentiated from all members of

  occupies an intermediate position between Pongo and the other Homo samples analyzed. Although our early Homo sample overlaps with H. sapiens in M1 mesial fovea shape, that of M2 and M3 makes early Homo cluster closely with Pongo. The M2 and M3 of H. erectus s.l., however, show closer affinities with later Homo in mesial fovea shape (Figs. S24-26).

  and worn hominoid upper molars from the Pleistocene of Asia. Begun, D.R. (1992). Dryopithecus crusafonti sp. nov., a new Miocene hominoid species from Can Ponsic (northeastern Spain). American Journal of Physical Anthropology, 87, 291-309. Bergstrom, K., Jenkins, K., Levy, J., Velasquez, R.J., Lewis, P.J., & Campbell, T.L. (2016, April). Computed tomography survey of supernumerary molars in extant orangutans with implications for studies of the primate fossil record. Paper presented at the Paleoanthropology Society 2016 Meeting, Atlanta. GA. PaleoAnthropology Abstracts, A4. Bookstein, F.L. (1989). Principal Warps: Thin-plate splines and the decomposition of deformations. IEEE Transactions on pattern analysis and machine intelligence, 11, 567-585. Bown, T.M., & Kraus, M.J. (1979). Origin of the tribosphenic molar and the metatherian and eutherian dental formulae. In J.A. Lillegraven, Z. Kielan-Jaworowska, & W.A. Clemens (Eds.), Mesozoic mammals: The first two-thirds of mammalian history (pp. 172-181). Berkeley, University of California Press. Butler, P. M. (1956). The ontogeny of molar pattern. Biological Reviews of the Cambridge Philosophical Society, 31, 30-69. Butler, P.M. (1978). Molar cusp nomenclature and homology. In P.M. Butler and K.A. Joysey (Eds.), Development Function and Evolution of Teeth (pp. 439-453). New York, NY: Academic Press.

Fig. 1 .

 1 Fig. 1. Scoring system of protoconule expression at the EDJ of hominoid molars. (a) Grade 0:

Fig. 2 .

 2 Fig. 2. Landmarks and semi-landmarks used in the mesial fovea shape analysis. Big and small

Fig. 3 .

 3 Fig. 3. Frequencies of protoconule expression in hominid upper molars. (a) Comparisons at the

Fig. 4 .

 4 Fig. 4. Upper molars of Pleistocene Pongo from China with different degrees of protoconule

Fig. 5 .

 5 Fig. 5. Upper molars of Pleistocene Pongo from Vietnam with different degrees of protoconule

Fig. 6 .

 6 Fig. 6. Upper molars of extant Pongo with different degrees of protoconule expression. (a-b)

Fig. 7 .

 7 Fig. 7. Upper molars of H. erectus s.l. with different degrees of protoconule expression. (a) grade

Fig. 8 .

 8 Fig. 8. Upper molars of H. sapiens with different degrees of protoconule expression. (a) grade 0;

Fig. 9 .

 9 Fig. 9. Upper molars of Sivapithecus (a-c) and Meganthropus (d-e) with different degrees of

Fig. 10 .

 10 Fig. 10. Plot of the first two discriminant functions of mesial fovea shape in M1-M3 combined.

Fig. 11 .

 11 Fig. 11. Plot of the first two discriminant functions of mesial fovea shape in M1-M3 combined.

Fig. 12 .

 12 Fig. 12. Comparisons of mesial fovea variation between (A) extant and fossil Pongo, (B) H.

Table 2 .

 2 Discriminant model accuracy results for mesial fovea shape variation and percent of the variation accounted for by each axis. All species of fossil and extant hominids included.

	CVA Hominoidea	M1-M3*	M1-M3**	M1	M2	M3
	Total	364	359	122	132	101
	Model accuracy (without resampling)	71.68%	84.96%	94.60% 93.18% 91.09%
	Model accuracy (with resampling)	60.85%	71.36%	68.03% 66.67% 52.48%
	Axis 1 -% Variation	41.17	33.13	36.54	36.98	38.41
	Axis 1 -Eigenvalue	3.041	3.065	4.786	5.315	4.51
	Axis 2 -% Variation	26.94	23.49	20.84	23.88	18.25
	Axis 2 -Eigenvalue	1.990	2.173	2.73	3.433	2.142
	Axis 3 -% Variation	11.17	14.32	13.01	14.85	13.85
	Axis 3 -Eigenvalue	0.825	1.325	1.704	2.135	1.627
	Axis 4 -% Variation	7.83	10.66	8.85	7.829	12.76
	Axis 4 -Eigenvalue	0.578	0.986	1.159	1.124	1.499

Table 2 .

 2 Discriminant model accuracy results for mesial fovea shape variation and percent of the variation accounted for by each axis. All species of fossil and extant hominids included.

	CVA Hominoidea	M1-M3*	M1-M3**	M1	M2	M3
	Total	364	359	122	132	101
	Model accuracy (without resampling)	71.68%	84.96%	94.60% 93.18% 91.09%
	Model accuracy (with resampling)	60.85%	71.36%	68.03% 66.67% 52.48%
	Axis 1 -% Variation	41.17	33.13	36.54	36.98	38.41
	Axis 1 -Eigenvalue	3.041	3.065	4.786	5.315	4.51
	Axis 2 -% Variation	26.94	23.49	20.84	23.88	18.25
	Axis 2 -Eigenvalue	1.990	2.173	2.73	3.433	2.142
	Axis 3 -% Variation	11.17	14.32	13.01	14.85	13.85
	Axis 3 -Eigenvalue	0.825	1.325	1.704	2.135	1.627
	Axis 4 -% Variation	7.83	10.66	8.85	7.829	12.76
	Axis 4 -Eigenvalue	0.578	0.986	1.159	1.124	1.499

Laboratoire BABEL, FRE 2029 CNRS, Université Paris Descartes, Faculté de chirurgie dentaire, Montrouge, France

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Acknowledgments

This article is protected by copyright. All rights reserved.

For access to specimens, we thank the following institutions: Croatian Museum of Natural History, Croatia; Ditsong National Museum of Natural History, South Africa; Francisc Rainer Anthropology Institute, Romania; National Museums of Kenya, Kenya; Musée d'Art et d'Archéologie du Périgord, Framce; Musée d'Archéologie Nationale de Saint-Germain-en-Laye, France; Musée d'Angoulême, France; Musée National de Préhistoire des Eyzies, France; Musée de l'Homme, France; Museo Nacional de Ciencias Naturales, Spain; Max Planck Institute for Evolutionary Anthropology, Germany; Royal Museum for Central Africa, Belgium; Museum für Vor und Frühgeschichte, Germany; National Museum of Ethiopia, Museum National d'Histoire Naturelle, France; American Museum of Natural History, USA; Rockefeller Museum, Israel; Royal Belgian Institute of Natural Sciences, Belgium; Senckenberg Research Institute, Germany; Sackler School of Medicine, Israel; Smithsonian National Museum of Natural History, USA; Center for the Study of Human Origins at New York University, USA; Tel Aviv University, Israeil; Leipzig University, Germany; University of Witwatersrand, South Africa; Direction du Patrimoine Culturel and Musée Archéologique de Rabat, Morocco. We also thank Colin Menter, Bill Kimbel, Jay Kelley, Friedemann Schrenck, and Ottmar Kullmer for access to material. We thank Heiko Temming, David Plotzki, Lukas Westphal, Kornelius Kupczik, Ellis Locke, and Susan Antón for help and comments. For technical help with the microCT scanning of the Sivapithecus specimens, we thank Federico Bernardini and Claudio Tuniz. This research was supported by the National Science Foundation (award number: 1341148), the Wenner-Gren Foundation, the Leakey Foundation, and the NYU GSAS James Arthur Fellowship (grants to This article is protected by copyright. All rights reserved.

Data availability statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to previous agreements with participating institutions.

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