Nakalipithecus

Nakalipithecus is an extinct genus of great ape known from the early Late Miocene of Kenya, representing a key taxon in understanding the divergence of extant African apes and humans. The type and only species, N. nakayamai, was described based on fossils recovered from the Nakali Formation, including a right mandibular fragment preserving the first to third molars (M₁–M₃) as the holotype (KNM-NA46400) and several isolated teeth comprising the hypodigm, such as an upper incisor, canine, premolars, and additional molars.^[1] These remains, housed at the National Museums of Kenya, date to 9.9–9.8 million years ago, within chronozone C5n.1n as determined by ⁴⁰Ar-³⁹Ar dating and magnetic polarity stratigraphy.^[1] The anatomy of Nakalipithecus nakayamai indicates a relatively large hominoid, with body size comparable to that of female gorillas or orangutans, based on dental dimensions.^[1] The mandible features a well-developed inferior transverse torus extending to the mid-M₁, with a thickness-to-height index of approximately 45% at M₁.^[1] Dentally, it exhibits thick enamel, voluminous cusps, and reduced but marked cingula; the lower molars are mesiodistally elongated, with M₃ larger than M₂, while the upper canine is low-crowned with a prominent lingual cusplet.^[1] These traits show similarities to the contemporaneous European ape Ouranopithecus macedoniensis (9.6–8.7 Ma) in overall size and some dental features, but N. nakayamai possesses less specialized characteristics, such as less inflated cusps and better-developed cingula, distinguishing it from earlier African hominoids like Proconsul and later forms like Samburupithecus.^[1] Phylogenetically, Nakalipithecus nakayamai is positioned near the last common ancestor of extant African great apes (gorillas and chimpanzees) and humans, suggesting that the African and Eurasian great ape lineages had diverged by the early Late Miocene.^[1] Its discovery underscores a higher diversity of large-bodied hominoids in East Africa during this interval, coinciding with the period of divergence between African apes and humans, and highlights the need for further fossil evidence to clarify intercontinental dispersals among Miocene apes.^[1] The Nakali site also yields other primates, contributing to a broader faunal assemblage that informs on paleoecology.^[1]

Discovery and Naming

Discovery

The fossils of Nakalipithecus were excavated in 2005 at the Nakali site in central Kenya's Baringo District during paleontological surveys conducted by a joint Japanese-Kenyan expedition team led by Yutaka Kunimatsu of Kyoto University.^[1] The discovery occurred within the context of ongoing fieldwork initiated in 2002 to explore early Late Miocene primate faunas in the region.^[2] The holotype specimen, cataloged as KNM-NA46400, comprises a right mandibular corpus preserving the first through third molars (M₁-M₃). Accompanying the holotype were 10 isolated teeth, including premolars, molars, and a deciduous tooth, all attributed to the same species based on shared morphological features.^[1] These remains represent the primary evidence for the genus and were initially recognized as belonging to a great ape due to their large size and dental characteristics.^[1] Geologically, the specimens derive from lahar (volcanic mudflow) deposits in the Upper Member of the Nakali Formation, a sequence of volcaniclastic sediments approximately 340 m thick divided into lower, middle, and upper units.^[1] The formation's age was established through magnetostratigraphy, correlating the hominoid-bearing horizon to polarity Chron C5n.1n, combined with biostratigraphic analysis of associated fauna, constraining the deposition to 9.9–9.8 million years ago.^[1] All Nakalipithecus fossils are housed and curated in the paleontology collections of the National Museums of Kenya in Nairobi, under the KNM (Kenya National Museums) designation.^[1]

Etymology

The genus name Nakalipithecus is derived from "Nakali," the locality in Kenya where the fossils were discovered, combined with the Greek word pithekos, meaning "ape," to reflect the site's significance in identifying this great ape taxon.^[1] The species name nakayamai honors the late Japanese geologist Katsuhiro Nakayama, who made key contributions to elucidating the geology of the Nakali Formation during the 2005 expedition.^[1] The taxon was formally named and described in 2007 by Kunimatsu et al. in a publication in the Proceedings of the National Academy of Sciences.^[1]

Taxonomy and Phylogeny

Classification

Nakalipithecus is classified within the domain Eukaryota, kingdom Animalia, phylum Chordata, class Mammalia, order Primates, suborder Haplorhini, infraorder Simiiformes, superfamily Hominoidea, and family Hominidae https://www.pnas.org/doi/10.1073/pnas.0706190104. Its subfamily placement remains incertae sedis, with ongoing debate regarding whether it represents a stem taxon to the Ponginae (orangutan clade) or Homininae (African ape and human clade) due to its mosaic of primitive and derived dental and cranial features https://www.pnas.org/doi/10.1073/pnas.0706190104. The genus Nakalipithecus is monotypic, containing only the single species N. nakayamai, with no recognized subspecies https://www.pnas.org/doi/10.1073/pnas.0706190104. This species was formally described in 2007 as a Late Miocene great ape based on a partial mandible and associated teeth from the Nakali Formation in Kenya https://www.pnas.org/doi/10.1073/pnas.0706190104. A 2017 morphometric analysis of the lower fourth deciduous premolar (dp₄) further supports Nakalipithecus' affinities with African hominoids, positioning it near the last common ancestor of gorillas, chimpanzees, and humans in morphospace and emphasizing its retention of primitive features shared with earlier Miocene African apes https://doi.org/10.1016/j.crpv.2016.10.004. This study highlights phenotypic continuity with later African hominoid lineages, reinforcing an African origin for the great ape radiation https://doi.org/10.1016/j.crpv.2016.10.004. The debated subfamily assignment stems from Nakalipithecus' combination of primitive traits, such as less inflated cusps and well-developed cingula on molars, alongside derived features suggesting proximity to the African great ape and human clade https://www.pnas.org/doi/10.1073/pnas.0706190104. These mosaic characteristics complicate precise phylogenetic placement, with some interpretations favoring a basal position within Hominidae rather than a clear affiliation to modern subfamilies https://doi.org/10.1016/j.crpv.2016.10.004.

Evolutionary Relationships

Nakalipithecus nakayamai is hypothesized to represent a stem great ape positioned close to the last common ancestor of the extant African great apes (gorillas, chimpanzees, and humans).^[1] Its dental morphology, including thick-enameled molars and low-crowned canines, aligns it with early members of the African ape-human clade (AAH), suggesting it predates and potentially contributes to the divergence of these lineages around 10–8 million years ago (Ma).^[3] Phylogenetic analyses place Nakalipithecus within the hominine clade, distinct from the pongine (orangutan) lineage, reinforcing its role in the early radiation of African hominoids.^[4] However, a 2023 phylogenetic analysis positions it as a stem hominid outside the crown Hominidae, highlighting the ongoing debate.^[5] The taxon exhibits notable similarities to Ouranopithecus macedoniensis from Greece (dated ~9.6–8.7 Ma), including comparable body size and certain craniodental features, leading to proposals of a sister taxon relationship or an ancestral-descendant link.^[1] However, Nakalipithecus retains more primitive traits, such as less inflated cusps and better-developed cingula on its teeth, indicating it may serve as an African precursor to this Eurasian form.^[6] A 2017 morphometric analysis of the lower fourth deciduous premolar (dp4) further supports closer affinities to African apes than to Eurasian taxa like Sivapithecus, with Nakalipithecus showing primitive morphology that bridges early Miocene forms to later hominines.^[6] Shared dental characteristics with Chororapithecus abyssinicus from Ethiopia (~10 Ma), such as large, thick-enameled molars, imply an early diversification of African great ape lineages, potentially including the gorilla-chimpanzee split.^[3] Both taxa highlight a previously underdocumented diversity of hominoids in Late Miocene Africa, challenging earlier models favoring Eurasian origins for the AAH clade.^[1] Nakalipithecus carries implications for "Out of Africa" dispersals among Miocene hominoids, potentially representing an African migrant that gave rise to Eurasian apes like Ouranopithecus via vicariance events driven by climate-induced habitat fragmentation between ~10–7 Ma.^[1] Its older age and basal features relative to European counterparts support an African source for these dispersals, coinciding with the expansion of open woodlands and savannas across Afro-Eurasia.^[3] This scenario underscores Nakalipithecus as a key node in the biogeographic history of great apes.^[4]

Anatomy

Craniodental Morphology

The mandibular corpus of Nakalipithecus nakayamai is robust but less so than that of most Eurasian Miocene hominoids, such as Ouranopithecus and Sivapithecus, with a height of approximately 44 mm and thickness of 19.8 mm at the level of M₁, yielding a thickness/height index of about 45%.^[1] The symphysis exhibits moderate eversion, a well-developed inferior transverse torus that extends to the mid-M₁, and moderate symphyseal torsion.^[1] A superior transverse torus is moderately developed, and the lateral aspect of the corpus is vertical below M₁, transitioning to a more oblique orientation posteriorly.^[1] The canines are small and low-crowned, with subtle sexual dimorphism inferred from size variation; for example, the upper canine (KNM-NA 47594) measures 10.7 mm mesiodistally by 10.5 mm buccolingually.^[1] It features a prominent lingual cingulum and a large lingual cusplet, with light wear indicating functionality in a frugivorous context.^[1] Lower canines, though not preserved, are expected to show similar low-crown morphology based on overall dental proportions.^[1] The premolars display elongated forms with distinct features; the P₃s (e.g., KNM-NA 46423: 12.4 mm MD × 8.2 mm BL; KNM-NA 46431: 9.3 mm MD × 11.1 mm BL) are semitriangular in occlusal outline, exhibiting weak bilateral compression, a wide distal fovea, and a prominent lingual cingulum.^[1] The P₄s (e.g., KNM-NA 46424: 10.4 mm MD × 14.0 mm BL; KNM-NA 46430: 9.5 mm MD × 11.9 mm BL) are broader, with voluminous cusps positioned peripherally, large distal foveae, and oblique wear facets suggesting shearing capabilities.^[1] Both premolar types show thin enamel relative to molars and moderate development of accessory structures.^[1] The molars are quadrate and relatively flat, with low, massive cusps, thick enamel, a flat dentine/enamel junction, and reduced but marked cingula; the M₁s (e.g., KNM-NA 46400: 15.6 mm MD × 14.0 mm BL; KNM-NA 47591: 12.4 mm MD × 13.4 mm BL) are slightly smaller than M₂ (KNM-NA 46400: 16.2 mm MD × 15.8 mm BL), while M₃s are the largest and mesiodistally elongated (e.g., KNM-NA 46400: 19.5 mm MD × 15.1 mm BL).^[1] Wear patterns on the holotype molars indicate heavy attrition on buccal cusps of M₁–M₂, with preserved lingual relief, and spacious occlusal basins.^[1] The deciduous lower fourth premolar (dp₄, KNM-NA 46435: 11.4 mm MD × 9.4 mm BL) retains primitive features, including massive low cusps, a broad distal fovea, and a weakly developed buccal cingulum with small depressions at the protoconid base.^[1] The overall dental arcade is parabolic, with postcanine teeth arranged in a gently diverging row, representing an intermediate morphology between more V-shaped prosimian patterns and the U-shaped arcs of modern hominoids.^[1] These craniodental traits, including thick molar enamel and reduced cingula, align Nakalipithecus with basal great apes.^[1]

Body Size and Sexual Dimorphism

The body size of Nakalipithecus nakayamai has been estimated solely from dental and mandibular dimensions, as no postcranial skeletal elements are available for analysis. The holotype mandible (KNM-NA 46400), presumed to represent a female based on the morphology of an associated upper canine, is similar in dental size to female gorillas and orangutans.^[1] These assessments derive from mandibular robusticity metrics, such as a body height of approximately 44 mm and thickness of 19.8 mm at the M₁ level (yielding a thickness/height index of ≈45%), and tooth dimensions.^[1] Dental metrics further indicate moderate levels of sexual dimorphism in N. nakayamai. For instance, variation in third molar (M₃) crown areas—where isolated specimens are ≈70% the size of the holotype M₃—implies a size difference of around 30% between presumed sexes, serving as a proxy for overall dimorphism that is less extreme than the ≈50% canine size disparity observed in modern gorillas (Gorilla gorilla).^[1] Canine dimensions support this pattern, with the low-crowned, buccolingually broad upper canine (10.7 mm mesiodistal length, 10.5 mm buccolingual breadth) of the holotype consistent with a female, and inferred male canines showing a 20–30% size increase based on comparable ratios in related Miocene hominoids.^[1]^[2] This estimated size positions N. nakayamai as a medium- to large-bodied great ape, likely adapted for a mix of arboreal and possibly more terrestrial behaviors akin to extant pongines and hominines, though direct locomotor evidence is lacking.^[1]

Paleobiology

Habitat and Paleoecology

Nakalipithecus nakayamai inhabited the Upper Nakali Beds of the Nakali Formation in north-central Kenya, located along the eastern margin of the Gregory Rift Valley. These deposits, dated to approximately 9.9–9.8 million years ago, consist primarily of volcanic mudflow sediments indicative of lakeside and riverine environments influenced by rift-related tectonic activity. This tectonic setting, part of the broader East African Rift System, promoted rapid sediment accumulation and local hydrological variations, shaping a dynamic landscape during the early Late Miocene.^[1] The paleoenvironment at Nakali featured seasonal sclerophyllous evergreen woodlands dominated by C3 vegetation, reflecting a warm and humid climate typical of the Late Miocene before a broader transition to drier conditions across East Africa between approximately 10 and 7 million years ago. Stable isotope analyses of herbivore tooth enamel confirm the prevalence of C3 plants, suggesting a more forested setting compared to contemporaneous sites like the Samburu Hills, where C4 grasslands were emerging. This woodland habitat supported browsers such as early black rhinoceroses (Diceros spp.), highlighting a mosaic of closed-canopy forests interspersed with open areas.^[1]^[7]^[8] The associated fauna from the Upper Nakali Beds includes a diverse mammalian assemblage, featuring suids such as Nyanzachoerus nakaliensis, hippopotamids like Kenyapotamus coryndonae, proboscideans, and various rodents, with no evidence of direct ecological competitors to Nakalipithecus among large primates. This community coexisted with early bovids, indicating a mixed forest-savanna mosaic where Nakalipithecus likely occupied the niche of a mid-level browser, foraging in the understory of sclerophyllous woodlands. The faunal composition underscores a wetter, resource-rich ecosystem transitional within the Pikermian Biome, influenced by regional rift dynamics.^[1]^[9]^[10]

Diet and Behavior

Nakalipithecus nakayamai exhibited a mixed diet inferred from its dental morphology, featuring thick enamel and low, voluminous cusps on the molars that supported consumption of both soft vegetation, such as fruits and leaves, and harder items like nuts and seeds.^[1] This configuration indicates primarily folivorous habits with a supplementary hard-object feeding component, though less specialized for abrasive foods compared to contemporaneous Eurasian hominoids like Ouranopithecus.^[1] Locomotion in Nakalipithecus is inferred to have involved arboreal quadrupedalism, with potential terrestrial elements, based on its estimated body mass comparable to that of female gorillas or male orangutans and the robusticity of the mandible, consistent with the generalized great ape bauplan; however, no postcranial fossils are available to confirm these traits directly.^[1] Social behavior likely included unimale polygynous grouping akin to modern gorillas, as suggested by high intraspecific variation in third molar crown size (approximately 70% relative to the holotype), indicative of sexual dimorphism that correlates with polygynous mating systems in extant apes.^[1] Territorial displays were probably mediated by canine honing, evidenced by the mesial honing facet on the upper canine, a feature shared with other Miocene hominoids for intraspecific competition.^[1]