Knuckle-Walking Signal in the Manual Phalanges and Metacarpals of the Great Apes (Pan and Gorilla)

To understand the role of knuckle walking in hominin evolution, we need to better understand behavioral variation in knuckle walking, and specifically whether behavioral similarities between chimpanzees and gorillas are likely homologous. Differences in how the hands of gorillas and chimpanzees contact the ground while knuckle walking have been noted but not quantified: it is widely believed that gorillas maintain a pronated arm and contact the ground with digits 2-5 consistently, while chimpanzees have variable arm position and digit contact. Distribution of pressure across the manus, peak digital pressures, and hand position were quantified to further test these generalizations. Chimpanzees and gorillas make initial ground contact with the ulnar aspect of the hand, and pressure then moves radially. They differ in which digit usually makes final contact and receives maximum pressure, and in hand position during contact. Gorillas regularly use a palm-back hand position and touch-off with digit 2. They show less variation in pressure application across the digits. Chimpanzees are more variable in hand position and pressure application. These differences may relate to aspects of locomotor behavior that are unrelated to knuckle walking; chimpanzees are more suspensory than gorillas. Nevertheless there are some commonalities that appear to be synapomorphic, and that would in any case require explanation if they do not signal knuckle walking in the common ancestor of chimpanzees and gorillas (and by implication, humans). In both, hand position plays a key role in determining which digit acts as the final touch-off element.