Abstract
The glenohumeral joint, the most mobile joint in the body of hominoids, is involved in the locomotion of all extant primates apart from humans. Over the last few decades, our knowledge of how variation in its morphological characteristics relates to different locomotor behaviors within extant primates has greatly improved, including features of the proximal humerus and the glenoid cavity of the scapula, as well as the muscles that function to move the joint (the rotator cuff muscles). The glenohumeral joint is a region with a strong morphofunctional signal, and hence, its study can shed light on the locomotor behaviors of crucial ancestral nodes in the evolutionary history of hominoids (e.g., the last common ancestor between humans and chimpanzees). Hominoids, in particular, are distinct in showing round and relatively big proximal humeri with lowered tubercles and flattened and oval glenoid cavities, morphology suited to engage in a wide range of motions, which enables the use of locomotor behaviors such as suspension. The comparison with extant taxa has enabled more informed functional interpretations of morphology in extinct primates, including hominoids, from the Early Miocene through to the emergence of hominins. Here, I review our current understanding of glenohumeral joint functional morphology and its evolution throughout the Miocene and Pleistocene, as well as highlighting the areas where a deeper study of this joint is still needed.
Highlights
The evolution of primate locomotion and, especially, ape locomotion is fundamental to the understanding of human origins
The forelimb is critical to a diversity of locomotor behaviors, ranging from terrestrial knuckle-walking to suspension to ricochetal brachiation, but is largely removed from ocomotion in humans Within the fore imb the g enohumera joint the articulation between the scapula’s glenoid fossa and the proximal humerus is the primary joint invo ved in arm movement and the most mobile joint in the body in hominoids
This review considers the shoulder girdle and locomotor behavior of Miocene apes in general and, when possible, their glenohumeral joint
Summary
The evolution of primate locomotion and, especially, ape locomotion is fundamental to the understanding of human origins. Gorilla (gorillas) and Pan (chimpanzees) present a glenohumeral joint seemingly preserving all the traits necessary to engage in arboreal and suspensory locomotion (Arias-Martorell et al, 2015a; Hunt, 1991b; Larson & Stern, 1987), despite their main locomotor behavior being a modified form of quadrupedalism known as knuckle-walking (where they use of the back of the middle phalanges to make contact with the ground) Their humeral articular surface is still relatively big, globular and rounded, showing adaptations to a great range of motion. A higher degree of cranial/ superior orientation of the muscle’s facet seems to be related to the function of the infraspinatus muscle as the main stabilizer of the glenohumeral joint against forces pushing the humerus head away or along the glenoid cavity, mainly during pendant suspension and the support phase of arm-swinging/brachiation (Larson, 1995; Larson & Stern, 1986; Roberts, 1974). Questions such as what was the locomotor behavior of the LCA between humans and chimpanzees, or what was the extent of the role suspension played in the early days of the hominins only the discovery of new fossils (especially of the critical period of 8–6 mya) and the proper analysis of old ones will tell
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