Knuckle-walking
Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles.
[2][8] This form of hand-walking posture allows these tree-climbers to use their hands for terrestrial locomotion while retaining long fingers for gripping and climbing.
Their knuckle-walking involves flexing the tips of their fingers and carrying their body weight down on the dorsal surface of their middle phalanges.
Another difference is that the hand bones of gorillas lack key features that were once thought to limit the extension of the wrist during knuckle-walking in chimpanzees.
For example, the ridges and concavities features of the capitate and hamate bones have been interpreted to enhance stability of weight-bearing; on this basis, they have been used to identify knuckle-walking in fossils.
A gorilla's forearm and wrist bones lock together to be able to sustain the weight of the animal and create a strong supporting structure.
[14] Gorillas use this form of walking because their hips are attached differently from humans, so standing on two legs for a long period of time would eventually become painful.
High magnitudes of integration would indicate homoplasy of knuckle-walking in gorillas and chimpanzees, in which a trait is shared or similar between two species, but is not derived from a common ancestor.
[21] Similarities between gorillas and chimpanzees have been suggested to support a common origin for knuckle-walking, such as manual pressure distribution when practicing this form of locomotion.
[22] Another hypothesis proposes that African apes came from a bipedal ancestor, as no differences in hemoglobin are seen between Pan and Homo, suggesting that their divergence occurred relatively recently.
[24] It is important to keep this in mind when examining similarities and differences between African apes themselves, as well as knuckle-walkers and humans, when developing hypotheses on locomotive evolution.
... Pre-bipedal locomotion is probably best characterized as a repertoire consisting of terrestrial knuckle-walking, arboreal climbing and occasional suspensory activities, not unlike that observed in chimpanzees today".
Crucial to the knuckle-walking ancestor hypothesis is the role of the os centrale in the hominoid wrist, since the fusion of this bone with the scaphoid is among the clearest morphological synapomorphies of hominins and African apes.
[25] It has been shown that fused scaphoid-centrales display lower stress values during simulated knuckle-walking as compared to non-fused morphologies, hence supporting a biomechanical explanation for the fusion as a functional adaptation to this locomotor behavior.
[25] This suggests that this wrist morphology was probably retained from a recent common ancestor that showed knuckle-walking as part of its locomotor repertoire and that was probably later exapted for other functions (e.g. to withstand the shear stress during power-grip positions[26]).
[11] Kivell and Schmitt note "Features found in the hominin fossil record that have traditionally been associated with a broad definition of knuckle-walking are more likely reflecting the habitual Pan-like use of extended wrist postures that are particularly advantageous in an arboreal environment.
This, in turn, suggests that human bipedality evolved from a more arboreal ancestor occupying a generalized locomotor and ecological niche common to all living apes".
[30][31] It is also the method used by human infants when crawling on their knees or engaged in a "bear-crawl" (in which the legs are fully extended and weight is taken by the ankles).
[30][31] In olive baboons, rhesus macaques, and patas monkeys, such finger-walking turns to palm-walking when animals start to run.
