How tooth wear in Papioninae offers insight into hominin evolution

Ian Towle^*

• Monash University, Melbourne, Australia

Old World monkeys are typically placed within a single family, Cercopithecidae, subdivided into Colobinae (leaf monkeys) and Cercopithecinae (cheek pouched monkeys). Molecular and fossil evidence supports a deep split between colobines and cercopithecines, occurring around the Oligocene-Miocene boundary, or slightly later in the early Miocene (~20 million years ago; e.g., Perelman et al., 2011;Finstermeier et al., 2013;Pozzi et al., 2014). This split is comparable in age to family level divergences in other anthropoid primate clades. For example, the common ancestor of living Cercopithecidae likely lived at a similar time as the common ancestor of living apes (Hominoidea) and New World monkeys (Platyrrhini), although there remains debate over which group has the deepest evolutionary roots (Finstermeier et al., 2013;Shao et al., 2023). Yet, in contrast to Cercopithecidae, these two other major extant anthropoid groups are subdivided into multiple families, three to five families are recognized among extant platyrrhines (Kay, 2015;Rylands et al., 2016;Silvestro et al., 2019), and two among apes (or more if the human lineage is retained at the family level; White et al., 2009;Tuttle, 2014;Schwartz, 2016). Combined with their pronounced differences in physiology, behaviour, and ecological specializations, this evidence strengthens the case for recognizing Colobidae and Cercopithecidae as distinct families within Cercopithecoidea. Such a two-family framework is not new, advocated by Hill (Hill, 1966) and later by Groves (Groves, 2000), and aligns with efforts to standardize higher level taxonomic ranks across primates (Goodman et al., 1998).If colobines and cercopithecines are elevated to family status, the major clades within cheek pouched monkeys must also be raised in rank, Papioninae (formerly Papionini) and Cercopithecinae (formerly Cercopithecini). This opinion piece focuses on tooth wear research on Papioninae, encompassing baboons (Papio), geladas (Theropithecus), macaques (Macaca), drills and mandrills (Mandrillus), and the paraphyletic mangabeys (Lophocebus, Rungwecebus, Cercocebus). I highlight recent advances positioning Papioninae as a key group for reconstructing hominin evolutionary history and ecology, focusing on tooth wear. The aim is to highlight recent developments and outline future directions, demonstrating how tooth wear in all Papioninae genera is useful for paleoanthropology. Geladas (Theropithecus)Geladas (Theropithecus gelada) are the sole representatives of a once widespread genus that ranged across Africa, Europe, and Asia. Fossil Theropithecus dispersed out of Africa during the same broad interval as hominins, reaching sites from Spain to India, which along with proposed ecological overlap with certain hominins have long encouraged comparative analyses (see Hughes et al., 2008). As such, both living and fossil Theropithecus play a central role in comparative studies of hominin dental wear and function (e.g., Jolly, 1970;Fashing et al., 2014). Geladas exhibit a distinctive diet among living primates, relying primarily on grass blades but also consuming underground storage organs, leaves, herbs, and occasional fruit, although increasing anthropogenic pressures have shifted many populations' habits and diet (Abu et al., 2018;Jarvey et al., 2018;Ahmed et al., 2023).Their dental morphology, large, relatively thick-enameled, high-crowned molars whose wear shows curved enamel crests, improves resistance to abrasion from silica rich and gritty foods while maintaining functional efficiency (Jolly, 1972;Jablonski, 1993;Shellis et al., 1998;Damuth and Janis, 2011;Kaiser et al., 2013;Pampush et al., 2013;Souron, 2018). Because geladas process substantial quantities of tough vegetation, they have become crucial for understanding how chewing mechanics and dental function are sustained through progressive wear, offering parallels to dietary adaptations in different hominin taxa (e.g., Jolly, 1970;Venkataraman et al., 2014;Souron, 2018).Microwear studies have found both population-level variation but also long-term relative dietary stability in Theropithecus, and they are commonly included in primate comparative studies due to their unique grass-based diet (Fashing et al., 2014;Shapiro et al., 2016;Merceron et al., 2021;Martínez et al., 2022;Louail et al., 2026). Additional research on dental topography and ontogenetic changes in mastication further underscores their value for linking living primate wear patterns to the fossil record (Venkataraman et al., 2014;Avià et al., 2022). Anterior tooth wear also provides key behavioral insights for not just Theropithecus but also primates more broadly (e.g., Walker, 1976;Ryan and Johanson, 1989;Ungar, 1990;Ungar, 1994;Delezene et al., 2016). Of note, taxon specific incisor striation patterns in geladas may resemble the large, directionally consistent scratches often interpreted in hominins as evidence for tool-assisted "stuff-and-cut" behavior. Their potential presence in geladas (Figure 1A) adds support for the need to re-evaluate if such striations necessarily imply specific types of unique tool use in fossil hominins (see Bax and Ungar, 1999). Further direct comparisons between hominin and non-human primates are therefore justified (e.g., assessing striations size, shape and orientations; see discussion in Towle et al., 2022a). Macaques are among the most widespread and ecologically adaptable primates, encompassing over 20 species across Asia and North Africa. Originating in Africa during the Miocene, they dispersed into Eurasia by the Late Miocene and reached eastern Asia by the Late Pliocene (Roos et al., 2019 and references therein). Their broad distribution, terrestrial habits, and dietary flexibility parallel aspects of hominin evolution, and both lineages likely expanded out of Africa under changing climatic regimes, albeit at different times. These parallels make macaques valuable models for understanding hominin dispersal, habitat flexibility, and dietary adaptation (Kato et al., 2014).Macaques are generalized omnivores, consuming fruits, seeds, leaves, and invertebrates, with substantial inter-and intraspecific variation (O'brien and Kinnaird, 1997;Hanya et al., 2011;Tsuji et al., 2015;Ruslin et al., 2019). Certain populations exploit specialized habitats, such as mangroves or coastal zones, while others increasingly rely on anthropogenic foods. Corresponding macrowear and microwear patterns reflect these dietary differences (Ungar, 1994(Ungar, , 2019;;Towle et al., 2022a;Ramírez-Pedraza et al., 2023). Japanese macaques on Koshima Island develop anterior beveling, root wear (noncarious cervical lesions), and large striations on anterior teeth, again resembling fossil Homo wear, caused by ingesting sand-laden foods and processing shellfish rather than tool-use behaviors (Towle et al., 2022a;Figure 1B,C). Long-term macaque study populations, such as Koshima, Tanaxpillo Island, and Cayo Santiago, therefore offer opportunities to assess different aspects of wear development (see Guatelli-Steinberg et al., 2022;Guatelli-Steinberg et al., 2025). Cultural behaviors that involve teeth, such as sand-washing of food, food stripping behaviour, or hair based dental flossing (Watanabe et al., 2007;Leca et al., 2010), likely leave measurable wear patterns, potentially similar to evidence from strepsirrhine primates where grooming can produce grooves on anterior teeth (Figure 1D). Such comparisons of behaviour and tooth wear would be very useful for understanding potential identifiers of culture in earlier hominins.Other types of tissue loss are important to consider, including the reassessment of processes such as dental caries, not just through comparison of caries patterns between hominins and extant primates in terms of diet (Towle et al., 2022b;Albrecht et al., 2024), but also potential additional comparisons, such as lesion manipulation in which large scratches are found in association with lesions (e.g., Figure 1E at least superficially looks similar to the example in Oxilia et al., 2015). Incorporating Papioninae examples into such archaeological studies may provide additional supporting evidence (presuming differences are found) or allow for further direct interpretation (if further similarities are found).Macaques exhibit relevant adaptive variation in enamel thickness and dental tissue proportions.Temperate species tend to have thicker molar enamel, likely reflecting hard or gritty seasonal foods (Kato et al., 2014). Tissue proportions also influence wear development in M. fascicularis (Selig et al., 2025), dietary variation shapes masticatory function (Kato et al., 2024), and variation in molar size, including disproportionately large M1s in cooler climates (Asahara and Nishioka, 2017). Integrating such morphological data with tooth wear is essential for disentangling overlapping selective pressures and other evolutionary processes/constraints, and can offer novel insights for hominin dental evolutionary studies. In particular, variation in tooth size across the dental arcade is often a key focus of hominin taxonomic studies. Consequently, comparisons with extant primates, especially widely distributed taxa such as macaques, are crucial for characterizing patterns of intra-and interspecific variation, as well as assessing how tooth wear, diet, and environmental factors contribute to dental variability.[Figure 1 Here] 4 Baboons (Papio)Baboons (Papio) are large bodied, partly terrestrial monkeys distributed across much of sub-Saharan Africa. Their taxonomy has been repeatedly revised, with debate over whether the widespread regional forms represent a single species with multiple subspecies or several distinct species (P. anubis, P. papio, P. kindae, P. hamadryas, P. cynocephalus, and P. ursinus; Brasil et al., 2023;Sørensen et al., 2023). These taxa differ in morphology, behavior, and ecological niche (Zinner et al., 2021;Vilgalys et al., 2022;Sørensen et al., 2023;Brasil et al., 2025). Body size variation is often pronounced, with, for example, male Kinda baboons roughly half the size of chacma males (Jolly et al., 2011). Despite this, where these and all other baboon taxa meet, interbreeding produces extensive and complex hybrid zones.Hybridization, recent dispersal, and ecological terrestrial flexibility make baboons especially relevant for interpreting hominin evolution. Like hominins, Papio originated in Africa, are variably terrestrial, and exhibit many broadly comparable dietary behaviors (Codron et al., 2008;Hammond et al., 2025a,b). Hybrid zones are particularly informative, as admixture can modify dental phenotypes (Ackermann et al., 2014), offering a model for understanding variation in tooth wear in association with phylogenetic data, including among interbreeding hominin populations. Comparative studies across baboon populations show differences in wear rate and microwear patterns linked to both food mechanical properties and environmental abrasives such as grit (Nystrom et al., 2004;Galbany et al., 2011Galbany et al., , 2014)).Sex differences in dental wear are well documented, with male canine hypertrophy often associated with distinctive posterior wear patterns (Smith, 1969), a pattern of particular interest when comparing taxa with reduced canines, such as hominins. However, these differences are not always as clear cut as might be expected. In wild, non-provisioned baboons, overall tooth wear has been reported to be broadly similar between the sexes once tooth size is taken into account (Galbany et al., 2011), although some earlier studies documented higher wear rates in males (Bramblett, 1967). In contrast, recent studies of captive baboons have reported substantially greater wear in males than females that cannot be explained by differences in diet, age, or tooth size (Towle et al., 2024;Krueger et al., 2025). Other Papioninae primates similarly exhibit pronounced sex differences in wear patterns (e.g., Nass, 1981;Guatelli-Steinberg et al., 2022, 2025). This discrepancy presents a paradox, in contexts where diets are standardized (e.g., captive or provisioned settings), Papioninae primates show marked sex differences in dental wear, whereas in wild contexts these differences appear to be reduced. Sex differences in tooth morphology, and in both absolute and relative wear, therefore represent an important area for further research. Determining whether sexual dimorphic wear patterns occur in fossil hominins has important implications for reconstructing dietary and behavioral variation through time. In this regard, extant Papioninae, particularly baboons, provide a valuable comparative framework for interpreting hominin dental wear patterns.Comparison of wild baboon tooth wear with controlled feeding studies in captive individuals demonstrate predictable patterns of occlusal wear progression despite substantial differences in diet and in overall tissue loss (Towle et al., 2024;Krueger et al., 2025). These findings support the idea that wear patterning may represent a phylogenetically informative trait. In particular, such studies, by comparing tooth wear patterns within the same taxa across different environments and contexts, can inform debates surrounding distinctive posterior occlusal wear patterns observed in specific hominin taxa, such as the flat wear characteristic of Paranthropus and the distinctive dentine exposure patterns often found in Homo. Distinguishing what aspects of wear are more purely diet-related from those that may, at least partly, reflect predictable wear patterning (i.e., largely independent of the actual foods consumed) can therefore be assessed using living primate proxies, especially those like baboons living in a range of relevant environmental settings. Baboons also offer key insights into atypical or unusual wear observed in hominins, as they can help rule out alternative causes such as unusual diets, grit mastication, or malocclusion (Figure 1F, G), factors that are particularly difficult to evaluate in isolated fossil samples without such contextualization.Microwear analyses indicate that some baboon populations display features associated with hard-object feeding, likely arising from incidental grit ingestion rather than dietary items (Daegling and Grine, 1999). Baboons have also informed understanding of molar enamel thickness variation in hominins, suggesting thick enamel may reflect body size changes rather than solely dietary pressures (Grine et al., 2005). As teeth wear, they undergo predictable morphological changes, each stage of which is subject to selection, since these wear related morphological transformations contribute to maintaining functional efficiency throughout the tooth's lifespan (Kay, 1975;Ungar, 2015;Knight-Sadler and Fiorenza, 2017). Integrating tooth wear with dental structural traits provides a robust framework for investigating the evolutionary, ecological, and functional significance of hominin dentitions, and nonhuman primate comparisons are key to this (e.g., Figure 1H illustrates links between tooth wear and tertiary dentine formation; Towle, 2019).Mandrillus comprises two species, the drill (Mandrillus leucophaeus) and the mandrill (Mandrillus sphinx), among the largest and most sexually dimorphic Old-World monkeys. Their morphology reflects a semi-terrestrial foraging niche, including robust crania and enlarged premolars suited for cracking hard seeds and nuts, combined with frequent foraging on the rainforest floor in search of fallen fruits, seeds, fungi, and invertebrates (Lahm, 1986;Fleagle and McGraw, 1999;Daegling and McGraw, 2007;Hongo et al., 2018;Guatelli-Steinberg et al., 2024). They are opportunistic omnivores, while fruit often dominates the diet, seeds, tubers, arthropods, small vertebrates, and eggs are consumed when available. Mandrillus societies are highly structured and can fuse into massive congregations with several hundred individuals (Abernethy et al., 2002). Such demography offers opportunities to study population level variation in tooth wear, with direct relevance for interpreting variability within large hominin assemblages.Dental studies show accelerated wear in older mandrills, producing notably flatter molars, an outcome interpreted as an adaptation for processing hard foods (Galbany et al., 2014). This has important implications for hominin research since steep versus flat occlusal wear is frequently used to infer both diet but also phylogenetic interpretations. However, further research is needed, as alternative/additional considerations, such as mechanical stabilization of the dentition or the development of secondary shearing crests, may also account for changes in occlusal inclination with advancing wear (Smith, 1984;Berthaume et al., 2010;Lucas et al., 2013;Fiorenza et al., 2018;Ungar et al., 2018).Buccal microwear on mandibular molars in a large free-ranging mandrill population demonstrates clear seasonal dietary shifts (Percher et al., 2017). Because mandrills inhabit dense tropical forests, these findings contribute to broader discussions of hominin ecology, since forested habitats likely played important roles in different hominin groups (e.g., Ben Arous et al., 2025). Mangabeys provide a valuable framework for understanding Papioninae and hominin dental evolution, as their diversity reflects convergent adaptations. Early classifications placed all mangabeys in a single genus, Cercocebus (see Groves, 1978), but morphological and behavioral evidence later distinguished two forms, the more terrestrial Cercocebus and the more arboreal Lophocebus (Nakatsukasa, 1996;McGraw and Bshary, 2002). Molecular studies have found that these genera are polyphyletic relative to other Papioninae (Disotell, 1996;Fleagle and McGraw, 1999). A third taxon, described in 2005 and initially placed in Lophocebus, was shown to be closer to baboons, leading to the recognition of a new genus, Rungwecebus kipunji (Davenport et al., 2006). Across all mangabeys, diets are dominated by fruits, seeds, and nuts (Poulsen et al., 2001;Wieczkowski, 2009;Avià et al., 2022). Several species, notably the sooty mangabey (Cercocebus atys), consume hard seeds year-round, exhibiting thick enamel, enlarged premolars, and robust jaws adapted for durophagy (Daegling et al., 2011;McGraw et al., 2012;Guatelli-Steinberg et al., 2024). Therefore, they are key taxa for comparison with hominins that are sometimes considered hard-food object specialists. Mangabey's relatively small body size evolved independently within Papioninae, potentially also paralleling size reductions in certain hominin lineages and therefore may also provide a comparative basis for studying the relationship between body size and dental changes.Mangabeys are also informative for dental morphology more broadly. Premolar enlargement and molarization correlate with dietary differences (Guatelli-Steinberg et al., 2024), while variation in enamel thickness and its relationship to wear offers insights into functional adaptation (Guatelli-Steinberg et al., 2023). Tooth wear studies show differences in where maximal wear occurs (Morse et al., 2013), and direct comparisons with hominins illustrate the relevance of mangabeys for interpreting wear related function (Daegling et al., 2011). Convergent traits such as thick enamel can also allow insight on fallback food strategies (McGraw et al., 2012(McGraw et al., , 2014)), that has often been seen as critical in the dental evolution of certain hominin taxa, and recent analyses of dental mechanical properties and fracture patterns (Guatelli-Steinberg et al., 2023;Towle et al., 2021Towle et al., , 2023) ) provide a foundation for integrating tooth wear, structure, and function in mangabeys and hominin comparisons. Tooth wear in Papioninae offers a unique perspective on hominin evolution. These often terrestrial primates are widely distributed across Africa and Eurasia and exhibit ecological flexibility, rapid dispersal, and hybridization patterns that parallel aspects of hominin evolution. Comparisons across Papioninae, linking ecology, behavior, and specific evolutionary processes, therefore provide powerful analogues for interpreting dietary adaptation, functional morphology, and phylogeny. Such comparisons are not new, as the examples above illustrate, but there is lots more that can be done in comparisons for hominin research with this highly relevant group for tooth wear a key opportunity.Wear analyses are particularly informative when combined with studies of dental morphology, allowing direct tests of hypotheses about function and adaptation. Tooth wear is often overlooked in studies of primate dental evolution, including those focused on hominin interpretations, where analyses often emphasize correlations in tooth size or developmental 'rules' or 'constraints' in which worn specimens are typically excluded (e.g., Schroer and Wood, 2015;Carter and Worthington, 2016;Evans et al., 2016). Recent critiques have highlighted some of the limitations of these approaches/conclusions (Roseman and Delezene, 2019;Boughner et al., 2021). An additional critique could relate to the lack of consideration of tooth wear, since it is rarely discussed or considered in these contexts. Other evolutionary changes, such as enamel thickness variation, have often been treated similarly. Because tooth wear provides a direct indicator of dental function, its omission from discussions of tooth size and proportions, enamel thickness change, and related morphological traits represent an opportunity for further research. In certain studies removing worn teeth is absolutely necessary (e.g., in morphological comparison that largely depend on specific cusp morphology/traits). But evolutionary interpretations of overall size or characteristics could strongly benefit from including teeth at different stages of wear, and Papioninae may be a great place to look for such comparisons.Compounding this issue is the near complete lack of non-human primate comparisons in hominin focused atypical wear research, with studies rarely incorporating non-human primate comparative samples (e.g., Molnar, 2011;Margvelashvili et al., 2013;Sun et al., 2014;Estalrrich et al., 2017;Frayer et al., 2017;Lozano et al., 2017;Xing et al., 2017). In contrast, for normal physiological wear there is a wide range of research conducted on these groups directly in comparison with hominins, as discussed in the genera sections above, but also in other broader primate comparative works (e.g., Ryan and Johanson, 1989;Ungar et al., 2010;Daegling et al., 2011;Estebaranz et al., 2012;Towle et al., 2017;Ungar and Berger, 2018).Papioninae are particularly valuable for paleoanthropological research, in some respects even more so than closely related great apes. This is due to their extensive radiation across diverse habitats, their comparatively rich and informative fossil record, and their wide range of diets and behaviors, including increased terrestriality and omnivory. In addition, Papioninae exhibit broad ecological and dietary diversity, well documented and finely varying dental morphologies, and numerous functional convergences that are directly relevant to interpreting hominin adaptations.Emerging technologies, such as portable 3D scanning, enable non-invasive, longitudinal monitoring, and advances in microwear analysis have expanded interpretive power and sample sizes in recent years. Together, these tools can benefit both conservation and evolutionary research. Overall, tooth wear in Papioninae can provide a robust framework for understanding primate and hominin dental adaptation and emphasizing the importance of comparative primate models for paleoanthropology, either through comparisons with data from the literature, or through new field, experimental and osteological studies. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.