The upper limb of Homo naledi

Abstract

The evolutionary transition from an ape-like to human-like upper extremity occurred in the context of a behavioral shift from an upper limb predominantly involved in locomotion to one adapted for manipulation. Selection for overarm throwing and endurance running is thought to have further shaped modern human shoulder girdle morphology and its position about the thorax. Homo naledi (Dinaledi Chamber, Rising Star Cave, Cradle of Humankind, South Africa) combines an australopith-like cranial capacity with dental characteristics akin to early Homo. Although the hand, foot, and lower limb display many derived morphologies, the upper limb retains many primitive traits. Here, we describe the H. naledi upper extremity (excluding the hand) in detail and in a comparative context to evaluate the diversity of clavicular, scapular, humeral, radial, and ulnar morphology among early hominins and later Homo. Homo naledi had a scapula with a markedly cranially-oriented glenoid, a humerus with extremely low torsion, and an australopith-like clavicle. These traits indicate that the H. naledi scapula was situated superiorly and laterally on the thorax. This shoulder girdle configuration is more similar to that of Australopithecus and distinct from that of modern humans, whose scapulae are positioned low and dorsally about the thorax. Although early Homo erectus maintains many primitive clavicular and humeral features, its derived scapular morphology suggests a loss of climbing adaptations. In contrast, the H. naledi upper limb is markedly primitive, retaining morphology conducive to climbing while lacking many of the derived features related to effective throwing or running purported to characterize other members of early Homo.

New fossil remains of Homo naledi from the Lesedi Chamber, South Africa

“…The complete U.W. 102a-021 clavicle confirms the morphological assessment of fragmentary clavicles from the Dinaledi Chamber (Feuerriegel et al, 2017). The U.W.…”

“…sediba and derived traits shared either with H. rudolfensis or with archaic and modern humans. Even though H. naledi crania are most similar in shape to the smallest H. erectus crania from Dmanisi (Schroeder et al, 2017), they are distinct from the Dmanisi sample in numerous aspects of cranial, dental, and postcranial morphology (Berger et al, 2015; Laird et al, 2017; Marchi et al, 2017; Feuerriegel et al, 2017; Rightmire et al, 2017), and Bayesian analysis of cranial and dental morphology provides strong evidence against the hypothesis of a sister taxon relationship for these samples (Dembo et al, 2016). In summary, no traits link H. naledi exclusively or specifically with H. erectus , and many traits distinguish the two.…”

“…sediba (Churchill et al, 2013) and H. habilis (Oxnard, 1969; Ohman, 1986; Voisin, 2001), suggesting a superiorly positioned shoulder (Feuerriegel et al, 2017). The U.W.…”

“…The most distinctive aspect of the humerus material of H. naledi in comparison with other hominin species is the very low humeral torsion angle in the adult U.W. 101–283 humerus, in which the head faces nearly directly posteriorly (Feuerriegel et al, 2017). This aspect cannot be assessed directly in the fragments from the Lesedi Chamber.…”

“…No derived features of the H. naledi skeleton require a close or exclusive relationship with modern humans. Overall, H. naledi resembles more primitive species of Homo such as H. erectus, H. habilis, or H. rudolfensis much more than it resembles archaic or modern humans (Berger et al, 2015; Laird et al, 2017; Marchi et al, 2017; Feuerriegel et al, 2017; Williams et al, 2016; Schroeder et al, 2017). H. naledi does, however, possess a number of derived features that are otherwise known only from modern humans and Neandertals (Supplementary file 1; Figures 34 and 35; Berger et al, 2015; Dembo et al, 2016; Kivell et al, 2015; Harcourt-Smith et al, 2015; Williams et al, 2016).…”