Figure 3. Stratigraphic section (left) shown with the image (right) of the outcrop highlighting the positions of the three major tuffs. All hominin fossils were found on the surface or in secondarily deposited sediment below the Ileret Tuff (1.52 ± 0.01 Ma). Some of the remains were found above the Lower Ileret Tuff (1.53 ± 0.01 Ma) indicating that the bones must have been buried above it. The large excavation area is visible on the right side of the outcrop and gully; the lower footprint level (Bennett et al., 2009, Dingwall et al., 2013, Hatala et al., 2017) is exposed on the left side of the image. All fragments of KNM-ER 47000 were located on the surface of the lower portion of the outcrop or secondarily buried in sediment eroded from the drainage that extends up the slope from the excavation site toward the right margin of the picture.
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 characterised 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.
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Figure 1. Right upper limbs of a modern human (left), chimpanzee (center), and KNM-ER 47000 (right), which preserves lateral portions of the scapula, the distal portion of the humerus, most of the ulna, and most of metacarpals (MCs) 1-3 and proximal phalanges 2-4. KNM-ER 47000 has primitive traits including a gracile thumb MC, lack of MC 3 styloid process, curved phalanges with prominent flexor sheaths, a long and curved ulna, a humerus with thick cortical bone and a prominent brachioradialis flange, and obliquely oriented scapular spine. Derived traits include a relatively long thumb, short manual phalanges, and a lateral scapular glenoid orientation. Scale bar at right is 10 cm.