Enthesis bilateral asymmetry in humans and African apes

Entheses (skeletal muscle and tendon attachment sites) have often been used to infer handedness and activity variability among human populations. However, the specific roles that intensity vs. frequency of muscle contractions play in modifying entheses are not well understood and the assumption that entheses reflect muscle activity levels has been challenged. This study explores the effect of habitual muscular activity on enthesis morphology in humans and African apes by investigating bilateral asymmetry in the forelimbs and hindlimbs of these taxa. Humans have generally more developed entheses in the lower limb while African apes have generally more developed entheses in the forelimbs. All species studied have more asymmetric forelimbs than hindlimbs except humans that show more asymmetrical expression of bony spurs in the lower limbs than in the upper limbs. When comparing species, humans are always more asymmetric in ethesis development than apes for both the forelimbs and hindlimbs, which reflects the relatively greater asymmetry in limb use in humans and the more symmetric use in apes. Enthesis development may reflect cross-symmetry patterns in humans and, more subtly, a moderate handedness in apes during manipulative activities. This study suggests that enthesis morphology provides information on muscle activity levels, with greater development of entheses associated with more habitual or powerful muscle use. The general similarity of ape and human responses to muscle activity suggests that muscle activity influenced enthesis development in Plio-Pleistocene hominins and that interpretation of muscle markings in these fossils can provide data for functional inferences in these extinct species.