The fossil record of the hylobatids (the family of hominoids comprising the extant gibbons and siamang) is poorly known, being largely restricted to fossil and subfossil remains from the Pleistocene and Holocene of China and Southeast Asia (dating back to ∼2 Ma; Jablonski and Chaplin, 2009; Harrison, 2016; Zhang et al., 2018; Ortiz et al., 2019). All these specimens can be referred to extant genera (i.e., Hylobates, Hoolock, Nomascus, and Symphalangus) or to recently extinct crown members (i.e., Bunopithecus and Junzi; Ortiz et al., 2015, 2019; Harrison, 2016; Zhang et al., 2018; Turvey et al., 2018). Previously, several fossil primate taxa from Miocene localities in Europe, Asia, and East Africa, dating back to as old as 20 Ma, have been recognized as hylobatids, based on their small size and primitive craniodental features (e.g., Le Gros Clark and Leakey, 1951; Hürzeler, 1954; Zapfe, 1961; Simons, 1972; Simons and Fleagle, 1973; Andrews and Simons, 1977; Andrews, 1978; Fleagle, 1984). However, all these have subsequently been shown to be stem catarrhines rather than hylobatids, or are too inadequately known to determine their relationships (see Harrison, 2016). Recently, Kapi ramnagarensis from the Middle Miocene of India has been recognized as an early hylobatid (Gilbert et al., 2020a) based on an isolated M3, but given the scant evidence, its phylogenetic status remains questionable. Consequently, definitive fossil evidence of the evolutionary history of the hylobatids is unknown before the Early Pleistocene. The lack of a fossil record in deep time is perplexing, given that molecular evidence indicates that hylobatids diverged from other hominoids (i.e., the hominids, comprising the great apes and humans) during the Early Miocene at 17–22 Ma, with crown hylobatids originating at about 8 Ma (Raaum et al., 2005; Bininda-Emonds et al., 2007; Chatterjee et al., 2009; Fabre et al., 2009; Chan et al., 2010, Chan et al., 2012; Matsudaira and Ishida, 2010, 2021; Thinh et al., 2010; Israfil et al., 2011; Perelman et al., 2011; Matsui and Hasegawa, 2012; Springer et al., 2012; Finstermeier et al., 2013; Schrago and Voloch, 2013; Carbone et al., 2014; Pozzi et al., 2014). Thus, the molecular evidence implies that there is a lengthy ghost lineage in the fossil record spanning more than 10 million years.

In 2006, Yuanmoupithecus xiaoyuan, a new species of fossil catarrhine from the Late Miocene (8.2–7.1 Ma) of Yuanmou in Yunnan Province, southwest China was introduced (Pan, 2006), and it was suggested that the taxon might be closely related to proconsuloids or dendropithecoids from the Early Miocene of East Africa. However, preliminary reports raised the possibility that Yuanmoupithecus might represent a fossil hylobatid (Harrison et al., 2002, 2008; Harrison, 2010a, 2016; Ortiz et al., 2015; Gilbert et al., 2020a). We present here several newly discovered specimens and the first detailed description and comparison of Yuanmoupithecus, and provide crucial evidence to help determine its phylogenetic relationships. Although the fossil material from Yuanmou consists primarily of isolated teeth, the material exhibits a suite of synapomorphies that provides compelling support for a phylogenetic link between Yuanmoupithecus and extant hylobatids. The evidence indicates that Yuanmoupithecus represents the primitive sister taxon of crown hylobatids. As such, Y. xiaoyuan represents the only species predating the Pleistocene that can be definitively identified as a member of the Hylobatidae, and it extends the fossil record of the clade back to 7–8 Ma. The identification of Yuanmoupithecus as an early hylobatid fills a critical gap in the evolutionary history of hominoids that has until now remained elusive.

The Yuanmoupithecus specimens were recovered from the Late Miocene Xiaohe Formation in the Yuanmou Basin, located 112 km northwest of Kunming, the capital of Yunnan Province in southwest China (Fig. 1). The fossil localities are situated near the villages of Xiaohe and Leilao. Xiaohe is located 30 km northwest of Yuanmou, the county capital, whereas Leilao is situated 21 km northwest of Yuanmou, and 9 km southwest of Xiaohe. The specimens from Xiaohe (YV 0126 and YV 2024) were recovered in 1986–1990 during excavation at Fangbeiliangzi, a low hill just to the southwest of the village on the southern side of the Xiaohe River, by a joint team from the Yunnan Provincial Museum, Chuxiong Prefectural Museum and the Yuanmou Man Museum (formerly the Yuanmou Man Exhibition Hall). The remaining 20 specimens are from Dashuqingliangzi, a ridge overlooking the eastern side of the village of Leilao. Five of the Leilao specimens (PDYV 1944–1947 and YV 1726) were recovered as part of excavations carried out in 1999 (Pan, 2006), and two specimens (YV 95002 and YV 95003) were recovered during excavations in 2006–2007 directed by Liang Zheng from the Yunnan Institute of Cultural Relics and Archaeology in Kunming (Harrison et al., 2008). The remaining specimens from Leilao, including the lower face of a juvenile individual (KIZ-P000001) and the holotype (YML 234), were collected by local villagers from the Xiaohe Formation.

The granitic and metamorphic basement rocks and Mesozoic sandstone in the Yuanmou Basin are overlain by a thick series of later Tertiary and Quaternary fluviolacustrine sediments. The Late Miocene sediments bearing the Yuanmoupithecus fossils comprise the Xiaohe Formation. The stratigraphic sequences and sedimentary environments are similar at both Leilao and Xiaohe (Qi and Zhang, 2006). At Baozidongqing, about 1.5 km west of Xiaohe village, the Xiaohe Formation has a thickness of 70 m and consists of an alternating series of silts and gravels (Zheng and Zhang, 2006). Fossil mammals, including the hominoids (i.e., Yuanmoupithecus and Lufengpithecus), are principally recovered from three yellow gravel horizons, each ∼2–3 m in thickness (Zheng and Zhang, 2006). The section at Dashuqingliangzi near Leilao village consists of a series of silts with intercalated sands and gravels more than 70 m thick (Qi and Zhang, 2006). The fossils are derived from sands and gravels, with sandy silt lenses (Qi and Zhang, 2006). The mammalian fauna (Ni and Qiu, 2002; Dong et al., 2003, 2004; Qi, 2006; Qi and Ni, 2006a,b) indicates that the Xiaohe Formation is older than that from Lufeng, which is correlated by paleomagnetic stratigraphy to 6.2–6.9 Ma (Yue and Zhang, 2006). Biostratigraphic correlations based on the rodents, carnivores, and ungulates provide an estimated age for the Xiaohe fauna of 9–7 Ma (Dong and Qi, 2013). Paleomagnetic analyses (Yue et al., 2004; Zhu et al., 2005; Yue and Zhang, 2006) indicate that the hominoid-bearing stratigraphic units at Xiaohe and Leilao best correlate with an age of 8.2–7.1 Ma.