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Gigantopithecus blacki was a giant hominid that inhabited densely forested environments of Southeast Asia during the Pleistocene epoch1. Its evolutionary relationships to other great ape species, and the divergence of these species during the Middle and Late Miocene epoch (16-5.3 million years ago), remain unclear2,3. Hypotheses regarding the relationships between Gigantopithecus and extinct and extant hominids are wide ranging but difficult to substantiate because of its highly derived dentognathic morphology, the absence of cranial and post-cranial remains1,3-6, and the lack of independent molecular validation. We retrieved dental enamel proteome sequences from a 1.9-million-year-old G. blacki molar found in Chuifeng Cave, China7,8. The thermal age of these protein sequences is approximately five times greater than that of any previously published mammalian proteome or genome. We demonstrate that Gigantopithecus is a sister clade to orangutans (genus Pongo) with a common ancestor about 12-10 million years ago, implying that the divergence of Gigantopithecus from Pongo forms part of the Miocene radiation of great apes. In addition, we hypothesize that the expression of alpha-2-HS-glycoprotein, which has not been previously observed in enamel proteomes, had a role in the biomineralization of the thick enamel crowns that characterize the large molars in Gigantopithecus9,10. The survival of an Early Pleistocene dental enamel proteome in the subtropics further expands the scope of palaeoproteomic analysis into geographical areas and time periods previously considered incompatible with the preservation of substantial amounts of genetic information. E.C. and F.W. are supported by VILLUM FONDEN (17649) and by the European Commission through a Marie Skłodowska-Curie (MSCA) Individual Fellowship (795569). T.M.-B. is supported by BFU2017-86471-P (MINECO/FEDER, UE), NIHM grant U01 MH106874, Howard Hughes International Early Career grant, Obra Social ‘La Caixa’ and Secretaria d’Universitats i Recerca and CERCA Programme del Departament d’Economia i Coneixement de la Generalitat de Catalunya (GRC 2017 SGR 880). E.C., J.C., J.V.O., D.S. and P.G. are supported by the Marie Skłodowska-Curie European Training Network (ETN) TEMPERA, a project funded by the European Union’s EU Framework Program for Research and Innovation Horizon 2020 under grant agreement no. 722606. M.J.C. and M.M. are supported by the Danish National Research Foundation award PROTEIOS (DNRF128). Work at the Novo Nordisk Foundation Center for Protein Research is funded in part by a donation from the Novo Nordisk Foundation (NNF14CC0001). Research at Chuifeng Cave is made possible by support from the National Natural Science Foundation of China (41572023) and by a grant from the Bagui Scholar of Guangxi. M.K. was supported by a Deutsche Forschungsgemeinschaft (DFG) fellowship (KU 3467/1-1) and the Postdoctoral Junior Leader Fellowship Programme from ‘la Caixa’ Banking Foundation (LCF/BQ/PR19/11700002). M.E.A. is supported by the Independent Research Fund Denmark (7027-00147B). We thank E. Willerslev for critical reading of the manuscript, scientific support and guidance.

Orangutans (genus Pongo) are the largest arboreal mammals, but Bornean orangutans (P. pygmaeus spp.) also spend time on the ground. Here, we investigate ground use among orangutans using 32,000 hr of direct focal animal observations from a well-habituated wild population of Bornean orangutans (P. p. wurmbii) living in a closed-canopy swamp forest at Tuanan, Central Kalimantan, Indonesia. Ground use did not change with increasing observation time of well-habituated individuals, suggesting it was not an artifact of observer presence. Flanged males spent the most time on the ground (ca. 5% of active time), weaned immatures the least (around 1%). Females and immatures descended mainly to feed, especially on termites, whereas flanged males traveled more while on the ground. Flanged males may travel more inconspicuously, and perhaps also faster, when moving on the ground. In addition, orangutans engaged in ground-specific behavior, including drinking from and bathing in swamp pools. Supplementary records from 20 ground-level camera traps, totaling 3986 trap days, confirmed the observed age-sex biases in ground use at Tuanan. We conclude that ground use is a natural part of the Bornean orangutan behavioral repertoire, however it remains unclear to what extent food scarcity and canopy structure explain population differences in ground use.