Functional morphology of the scaphoid in extant African apes, humans and fossil hominins

Objectives

The morphology of the hominoid scaphoid has played a key role in functional and evolutionary hypotheses related to the emergence of hominin bipedalism and tool use. However, the scaphoid's complex morphology is challenging to comparatively analyze via traditional 2D linear measurements. This study quantifies scaphoid morphology utilizing 3D geometric morphometrics (3D GM) in African apes, humans and extinct hominins to provide a more holistic functional understanding of scaphoid morphology.

Materials and Methods

We use 3D GM, including anatomical and sliding semi-landmarks, to quantify scaphoid morphology in a comparative sample of African apes (Gorilla, Gorilla beringei, Pan paniscus and Pan troglodytes) (N = 54), extant and fossil Homo sapiens (N = 20) and nine fossil hominin scaphoids from Australopithecus sp., Australopithecus sediba, Homo naledi and Neandertals.

Results

Principal component analysis indicates that extant species can be distinguished by differences in scaphoid shape that are consistent with variation in hand use during locomotion and manipulation. The australopith scaphoids plot between the African ape and modern human distributions, whereas H. naledi falls between Gorilla and human distributions.

Discussion

Results confirm previous studies describing differences between extant African apes and modern human scaphoids that were interpreted as advantageous for knuckle-walking and forceful manipulation, respectively. However, we highlight greater variation between Pan and Gorilla than previously recognized. The fossil hominin scaphoids present differing mosaics of joint orientation and shape, creating a distinct overall morphology in each hominin species. This may reflect differing functional pressures acting upon hominin wrists resulting from disparate combinations of locomotor and manipulative behaviors.