Enthesis Size and Hand Preference: Asymmetry in Humans Contrasts With Symmetry in Nonhuman Primates

Humans display species-wide right-hand preference across tasks, but this pattern has not been observed at comparable levels in nonhuman primates, suggesting the behavior arose after the panin-hominin split. Muscle attachment sites (entheses) are used to infer soft tissue anatomy and reconstruct behaviors within skeletal populations, but whether entheseal size asymmetry can reflect hand preference remains unclear. If entheseal asymmetry is linked to hand preference, we expect to see greater asymmetry in human hands, where hand preference is more pronounced, compared to nonhuman primates. We tested for bilateral asymmetry in the size of the opponens pollicis muscle flange using a sample of humans and catarrhine primates to determine if enthesis development can be a reliable indicator of hand preference.

We assess the asymmetry of the opponens pollicis enthesis between paired (left/right) first metacarpals using distance-based heat maps generated from three-dimensional models of Homo sapiens (n = 85 individuals), Macaca fascicularis (n = 58 individuals), Gorilla spp. (n = 8 individuals), and Hylobates lar (n = 44 individuals). Metacarpals were cropped to isolate the metacarpal shaft and capture the majority of the enthesis while eliminating variation from the metacarpal ends.

We found right-directional asymmetry for humans; no significant differences are observed for Hylobates, Macaca, and Gorilla.

The opponens pollicis enthesis shows right/left hand bias in humans. The lack of significant asymmetry in nonhuman primates suggests entheseal development in these species does not reflect the same level of hand preference observed in humans. Nonhuman primates can serve as a baseline for studying enthesis asymmetry based on the size of the opponens pollicis enthesis.