The upper limb of Paranthropus boisei from Ileret, Kenya

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Abstract

Paranthropus boisei was first described in 1959 based on fossils from the Olduvai Gorge and now includes many fossils from Ethiopia to Malawi. Knowledge about its postcranial anatomy has remained elusive because, until recently, no postcranial remains could be reliably attributed to this taxon. Here, we report the first associated hand and upper limb skeleton (KNM-ER 47000) of P. boisei from 1.51 to 1.53 Ma sediments at Ileret, Kenya. While the fossils show a combination of primitive and derived traits, the overall anatomy is characterized by primitive traits that resemble those found in Australopithecus, including an oblique scapular spine, relatively long and curved ulna, lack of third metacarpal styloid process, gracile thumb metacarpal, and curved manual phalanges. Very thick cortical bone throughout the upper limb shows that P. boisei had great upper limb strength, supporting hypotheses that this species spent time climbing trees, although probably to a lesser extent than earlier australopiths. Hand anatomy shows that P. boisei, like earlier australopiths, was capable of the manual dexterity needed to create and use stone tools, but lacked the robust thumb of Homo erectus, which arguably reflects adaptations to the intensification of precision grips and tool use. KNM-ER 47000 provides conclusive evidence that early Pleistocene hominins diverged in postcranial and craniodental anatomy, supporting hypotheses of competitive displacement among these contemporaneous hominins.

Introduction

Over six decades of discoveries and analyses have made Paranthropus boisei one of the best documented early hominin taxa in terms of its craniodental anatomy, diet, and biogeography. It is well known for a number of remarkably derived characteristics, including a structurally strong facial skeleton, large and anteriorly positioned attachments for chewing muscles, distinctive zygomatic shape, huge postcanine and small front teeth, and thicker enamel than found in any hominin or modern primate (Tobias, 1967, Rak, 1983, Smith et al., 2015). Its postcranial anatomy, however, has remained largely unknown, hampering our understanding of many important aspects of the paleobiology of this member of the human family tree.

Domínguez-Rodrigo et al. (2013) reported their breakthrough discovery of postcranial fossils associated with premolars reliably attributable to P. boisei. The associated remains (OH 80) include shaft fragments of a tibia and femur, a proximal radius, and a distal humerus fragment. OH 80 provides compelling evidence that the postcranial skeleton of P. boisei is robust compared with that of other hominins but leaves open many questions about the postcranial anatomy and paleobiology of this species.

Here, we report the discovery of the first upper limb skeleton, KNM-ER 47000 (Fig. 1), attributable to P. boisei. It provides evidence of scapular morphology, hand and upper limb proportions, upper limb robusticity, and locomotor and manual dexterity capabilities. We provide basic morphometric and comparative analyses with living hominoids and fossils attributed to Australopithecus, Paranthropus, and early Homo from eastern and South Africa.

Section snippets

Discovery and stratigraphic and paleoecological context

KNM-ER 47000 was found along the slope of an exposure in area 1A of the Koobi Fora Formation (Fig. 2), at the site FwJj14E where hominin footprints were later discovered as a result of our work to document the discovery of KNM-ER 47000 (Bennett et al., 2009, Dingwall et al., 2013, Hatala et al., 2016, Hatala et al., 2017). Hillary Sale found the first element, the right third metacarpal (MC), on July 12, 2004, near the end of the field season. Intensive survey led to the discovery of several

Preservation and morphology

Most of KNM-ER 47000's elements are well preserved, with undamaged and undistorted articular and shaft anatomy. The close spatial association of recovered fragments along with refitting pieces that display similar sediment abrasion and trampling damage indicates recent erosion from nearby exposures. In addition, dry bone fractures suggest this specimen was exposed for 1–3 years before burial (Behrensmeyer, 1978). The humerus has a tooth mark on the anterior distal shaft produced by a large

Scapular shape

Two scapular shape variables were considered to assess glenohumeral joint (ventral bar/glenoid angle) and scapular spine orientation (axillary border/spine angle). These two variables enable comparison of KNM-ER 47000 with representatives of Australopithecus afarensis (A.L. 288-1 [first order cast from the Cleveland Museum of Natural Sciences] and KSD-VP-1/1 [data from the study by Melillo, 2016]), Australopithecus africanus (Sts 7 and StW 162 [original fossils]); Australopithecus sediba (MH2

Scapula

Both KNM-ER 47000A and KNM-WT 15000 have ventral bar/glenoid angles that approach the modern human mean and fall in the upper ranges observed for nonhuman hominids. This morphology is indicative of a more derived, laterally oriented shoulder joint than typically occurs in apes and contrasts with the more cranial (ape-like) glenoid orientation seen in Australopithecus (Fig. 8). At the same time, the KNM-ER 47000A scapula possesses an obliquely oriented spine that suggests a narrow, long

Discussion

The Okote Member (1.4–1.6 Ma) near Ileret has yielded taxonomically identifiable craniodental remains of three hominin taxa: H. erectus, H. habilis, and P. boisei (Constantino and Wood, 2007, Spoor et al., 2007). The northern areas of the Koobi Fora Formation include some of the most intensively prospected and studied paleoanthropological regions in Africa, with a half-century of active research and sampling, making it unlikely that any other hominin taxon lived there at the time and has

Acknowledgments

This research would not have been possible without the support and assistance of the Ileret Community. Research was facilitated by a research permit issued to the collaborative project that included the National Museums of Kenya and colleagues at The George Washington University issued by the National Council of Science and Technology. We are also grateful for the continued enthusiasm and support of the students of the Koobi Fora Field School. We thank Hannah Carter-Menn and Kathryn C. Braun,

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      Furthermore, according to Kivell et al. (2011), individuals belonging to a more recent Australopithecus species: A. sebiba (Malapa site, South Africa, 1.977 Ma) possessed forceful precision gripping and hand proportions that show enhanced dexterity, confirmed by other recent studies using trabecular bone analyses and musculoskeletal modeling (Bardo et al., 2018; Dunmore et al., 2020). Concerning Paranthropus genus, the anatomy of the hand of specimens belonging to P. boisei (e.g. specimens from East Turkana, Kenya), highlights that this species exhibit manual dexterity required to use or make stone tools, but did not possess the robust thumb observed in H. erectus, which are linked to the intensification of forceful precision grips and tool use (Richmond et al., 2020). In southern Africa, the remains (phalanges and metacarpals) discovered mainly from Member 1 and allocated to P. robustus (Susman, 1988, 1994, 1998), made it possible to highlight that these hominins possessed, in particular, the opposing muscle of the thumb and the fifth finger.

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