A Geometric Morphometric Study of Scapular Ontogeny in Modern Humans

This paper quantifies the size and shape change of the human scapula through ontogeny to better understand the human trajectory of growth. While previous work has touched on human scapular ontogeny, analysis using 3D geometric morphometrics focusing on humans alone has not been conducted. This work is important to improve our analyses of the immature hominin fossil record.

Deidentified CT scans of human nonadults (infancy to adolescence) and adults from The Cancer Imaging Archive were examined in this study. Twenty-one digital landmarks were placed on the scapula and analyzed using linear regression and geometric morphometrics.

The size of the scapula starts small compared to body size and grows faster than femur head diameter, used as a proxy for body size. Some features that distinguish humans from great ape scapulae also exhibit developmental change in children, notably the angle of the scapular spine. Nonadults have more laterally oriented scapular spines than adults. This suggests that the development of the adult human scapula starts from a shape that is different from other apes and converges slightly during growth, a finding supported by previous work.

These results expand upon our understanding of the development of the human shoulder and our interpretations of juvenile scapulae in the hominin fossil record. Human juveniles, who climb and engage in arboreal behavior more frequently than adults, have a scapula whose morphology is poorly suited to arboreal movement. Whether this is evolutionarily or functionally driven will be explored in further studies using comparative analyses.