Did knuckle walking evolve twice?

Area of Science:

• Primate evolutionary biology
• Comparative anatomy
• Paleoanthropology

Background:

• African great apes (chimpanzees and gorillas) exhibit similar quadrupedal locomotion but have distinct hand morphologies.
• The evolutionary origin of knuckle walking, whether from a common ancestor or through parallel evolution, remains debated.

Purpose of the Study:

• To investigate the ontogenetic scaling and adult proportions of carpal bones in chimpanzees (Pan) and gorillas (Gorilla).
• To determine if observed morphological differences are attributable to developmental trajectories or behavioral adaptations in knuckle walking.

Main Methods:

• A morphometric analysis of 27 dimensions of the lunate, triquetral, hamate, and capitate bones in sub-adult and adult chimpanzees and gorillas.
• Comparative analysis of growth trajectories and adult proportions using reduced major axis (RMA) regressions.

Main Results:

• Only 56% of carpal dimensions showed ontogenetic scaling in sub-adults, with some dimensions exhibiting different adult proportions between species.
• Two main growth patterns were observed: shared trajectories (Pattern A) and transposed regressions (Pattern B), with gorillas showing earlier cessation of carpal growth.
• While some differences, particularly in the lunate, correlate with behavioral variations, most suggest heterochronic developmental modifications.

Conclusions:

• Carpal morphology differences between chimpanzees and gorillas are largely due to heterochronic modifications in developmental timing, not solely behavioral adaptations.
• The findings support the hypothesis that knuckle walking evolved in parallel in the chimpanzee and gorilla lineages.
• This study contributes to understanding the evolutionary divergence of locomotion and morphology in African great apes.