Paranthropus boisei is arguably the most striking fossil hominin species with its highly derived craniodental morphology. It is also among the more common hominin species represented in the Plio-Pleistocene deposits of the Omo-Turkana basin of Ethiopia and Kenya. However, nearly all fossils recovered to date that are attributed to this species are craniodental remains. As a consequence, the postcranial anatomy of P. boisei remains poorly understood (recent reviews in Grine et al., 2022; Lague and Ward, in press).

At present, the only postcranial fossils clearly associated with definitive P. boisei teeth are those belonging to OH 80, a 1.34 million-year-old skeleton recovered in situ from excavations carried out at Olduvai Gorge, Tanzania (Domínguez-Rodrigo et al., 2013). OH 80 preserves two large maxillary premolars and a partial molar with cusp proportions and enamel thickness consistent with attribution to P. boisei. Associated with these teeth are a distal humeral diaphysis, somewhat less than half of a proximal radius, and a proximal femoral shaft fragment. These represent the first postcranial elements securely attributable to P. boisei. All these postcranial elements are large in size. Given the impressive diversity in size of craniodental fossils attributable to P. boisei, this species appears to have been highly sexually dimorphic (Kimbel and White, 1988; Wood and Lieberman, 2001; Wood and Constantino, 2007). On the basis of its notably large size, OH 80 appears to represent a male P. boisei individual (Domínguez-Rodrigo et al., 2013).

As noted by Lague and Ward (in press), other postcrania have been suggested to represent P. boisei based only on the fact that they were recovered from localities where the hominin dental remains are exclusively or overwhelmingly those of P. boisei, and these derive largely from Area 6A at Koobi Fora (Grausz et al., 1988; Walker et al., 1989; McHenry, 1991, 1992, 1994; Wood and Constantino, 2007) and OMO 323-10003 of the Shungura Formation, Ethiopia (Daver et al., 2018). Other attributions are based on equally loose evidence, such as similarities to fossils from South African sites (e.g., Swartkrans) that may represent Paranthropus robustus (reviewed in Grine et al., 2022; Lague and Ward, in press).

Precisely because of the rarity of associated craniodental and postcranial fossils of P. boisei, any associated specimens stand to significantly improve our understanding of the postcranial anatomy of this enigmatic species. For this reason, a second partial skeleton, KNM-ER 1500, that has been argued to be attributable to P. boisei1, assumes special importance. KNM-ER 1500 (Leakey, 1973; Grausz et al., 1988) consists of fragmentary and badly weathered bones discovered by John Kimingech as part of a team from the Koobi Fora Research Project (Fig. 1; Table 1). It was recovered from the upper Burgi Member of Area 130 at Koobi Fora, roughly 2–5 m below the KBS Tuff (Leakey, 1973; Day et al., 1976; Leakey and Leakey, 1978) and is dated to 1.9 Ma (Brown et al., 2006). This date places KNM-ER 1500 within the time period during which P. boisei, Homo habilis, Homo rudolfensis, and Homo erectus are known from the Koobi Fora deposits (reviews in Wood, 1991; Hammond et al., 2021; Grine et al., 2022), which are the only hominins currently recognized from eastern Africa in this time period (McGechie and Ward, 2018). The KNM-ER 1500 skeleton preserves 15 fragmentary and badly weathered postcranial elements representing both limbs and the axial skeleton along with a small fragment of mandibular corpus (Fig. 1; Table 1).2 Given the small size of the bones in this partial skeleton, if it is indeed attributable to P. boisei, KNM-ER 1500 would very likely represent a female individual.

Determining the taxonomic attribution of KNM-ER 1500 has remained problematic, given the fragmentary and weathered state of these fossils (Wood and Constantino, 2007). KNM-ER 1500 was originally attributed to Australopithecus by Leakey (1973) based on the apparent primitiveness of its morphology, although the morphological attributes that led to this assignation was (were) not articulated. It should be noted that Leakey (1973) regarded all australopiths as belonging to the genus Australopithecus. After its description by Day et al. (1976: 402–404), a small piece of bone (KNM-ER 1500q) that had been regarded as an ‘unidentified fragment’ was identified by Grausz et al. (1988) as a fragment of the mandibular corpus (Fig. 2). KNM-ER 1500q consists of a portion of the right corpus that preserves the base, extending upwards to the anteroinferior margin of the mental foramen and a point about 25 mm anterior to the foramen. A large section of cortical bone is missing along the external surface of the fragment. Although DeGusta (1994) suggested that this fragment derives from the left side of the mandible, in basal view, it is clear that the corpus swings medially anterior to the mental foramen, confirming its identification by Grausz et al. (1988) as being from the right side. This mandibular fragment is significant for evaluating the taxonomic attribution of KNM-ER 1500. Grausz et al. (1988) noted a blunt marginal crest and dramatic widening of the corpus from its base in cross-sectional view, which they held to be features consistent with its attribution to Australopithecus boisei. Importantly, they observed that KNM-ER 1500q resembles the P. boisei mandible KNM-ER 15390. Wood (1991), on the other hand, argued that the morphology of this fragment can also be matched in some specimens assigned to early Homo (e.g., KNM-ER 1802, which he attributed to H. rudolfensis). As such, he suggested that the morphology of KNM-ER 1500q cannot be considered particularly diagnostic of P. boisei. He noted that “any more detailed taxonomic allocation of this specimen will have to depend on assessment of the postcranial evidence” (Wood, 1991: 182).

Refining our understanding of the taxonomic attribution of KNM-ER 1500 has implications for understanding the biology and behavior of P. boisei, as well as for interpreting the morphological and adaptive diversity of Plio-Pleistocene hominins in eastern Africa. Here, we review previous suggestions for the taxonomic attribution of KNM-ER 1500 and the basis on which they were made and offer new evidence supporting the hypothesis that this partial skeleton can be attributed with some confidence to P. boisei.