Locomotor hand postures and the functional anatomy of the hominid wrist

Abstract

The African apes have stable wrists with limited mobility—probably for energetic efficiency while using the hand as a rigid, propulsive lever during knuckle‐walking (KW). In contrast, the Asian apes have mobile wrists to facilitate a variety of arboreal hand postures. Testing the KW ancestor hypothesis for hominins requires understanding mechanisms of wrist stability in African apes and subsequent identification of related anatomy in the fossil record.A CT‐based method to track 3D carpal kinematics was used to study midcarpal joint mechanics in Pan troglodytes and Pongo pygmaeus. Midcarpal extension in Pan is distinguished by scaphoid supination on the capitate head, which rapidly close‐packs the midcarpus. Coupled with scaphoid‐centrale fusion and a centrale‐capitate ligament, this limits extension and ensures that the scaphoid and capitate act as a rigid unit. Scaphoid‐centrale fusion and a Pan‐like scaphocapitate joint in early hominins suggest a similar midcarpal mechanism, which may indicate a KW ancestor; alternatively, the mechanism may be a more generalized quadrupedal adaptation. Studying carpal kinematics in quadrupedal monkeys will test this alternative hypothesis. Modifications to the scaphocapitate joint and loss of the centrale‐capitate ligament in later hominins probably resulted in the more mobile modern human wrist.Funding: NSF BCS‐622515, Wenner‐Gren Foundation, Sigma Xi, ASU