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

Area of Science:

• Paleoanthropology
• Primate Anatomy
• Evolutionary Biology

Background:

• Humans exhibit a strong species-wide right-hand preference, a trait not consistently observed in nonhuman primates.
• This suggests that pronounced handedness evolved after the divergence of hominins and other apes.
• Muscle attachment sites (entheses) on bones can reveal soft tissue anatomy and infer behaviors, but their link to hand preference needs clarification.

Purpose of the Study:

• To investigate if entheseal asymmetry, specifically of the opponens pollicis muscle flange, can reliably indicate hand preference.
• To compare entheseal asymmetry between humans and selected catarrhine primates to test if greater asymmetry correlates with more pronounced hand preference.

Main Methods:

• Assessed bilateral asymmetry of the opponens pollicis enthesis on first metacarpals from 3D models.
• Included samples of Homo sapiens, Macaca fascicularis, Gorilla spp., and Hylobates lar.
• Focused on the metacarpal shaft to isolate the enthesis and minimize variation from bone ends.

Main Results:

• Found significant right-directional asymmetry in the opponens pollicis enthesis for humans.
• Observed no significant bilateral asymmetry in Hylobates, Macaca, or Gorilla.

Conclusions:

• The opponens pollicis enthesis demonstrates a right/left hand bias in humans, supporting its use as an indicator of hand preference.
• The absence of significant asymmetry in nonhuman primates suggests their entheseal development does not reflect human-like hand preference.
• Nonhuman primates provide a valuable baseline for studying enthesis asymmetry, particularly concerning the opponens pollicis enthesis.