Knuckle-walking
Knuckle-walking is a form of
Knuckle-walking helps with actions other than locomotion on the ground. Gorillas use fingers for the manipulation of food, whereas chimpanzees use fingers for the manipulation of food and climbing. In anteaters and pangolins, the fingers have large claws for opening the mounds of social insects. Platypus fingers have webbing that extend past the fingers to aid in swimming, thus knuckle-walking is used to prevent stumbling. Gorillas move around by knuckle-walking, although they sometimes walk bipedally for short distances while carrying food or in defensive situations. Mountain gorillas use knuckle-walking plus other parts of their hand—fist-walking does not use the knuckles, using the backs of their hand, and using their palms.
Anthropologists once thought that the
Apes
Chimpanzees and gorillas engage in knuckle-walking.[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. It may also allow small objects to be carried in the fingers while walking on all fours. This is the most common type of movement for gorillas, although they also practice bipedalism.
Their knuckle-walking involves flexing the tips of their fingers and carrying their body weight down on the dorsal surface of their middle
Differences exist between knuckle-walking in chimpanzees and gorillas; juvenile chimpanzees engage in less knuckle-walking than juvenile gorillas. 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
Chimpanzee knuckle-walking and gorilla knuckle-walking have been suggested to be biomechanically and posturally distinct. Gorillas use a form of knuckle-walking that is "columnar". In this forelimb posture, the hand and wrist joints are aligned in a relatively straight, neutral posture. In contrast, chimpanzees use an extended wrist posture. These differences underlie the different characteristics of their hand bones.[11]
The difference has been attributed to the greater locomotion of chimpanzees in trees, compared to gorillas. The former frequently engage in both knuckle-walking and palm-walking branches. As a result, to preserve their balance in trees, chimpanzees, like other primates in trees, often extended their wrists. This need has produced different wrist bone anatomy, and through this, a different form of knuckle-walking.[11]
Knuckle-walking has been reported in some baboons.[12] Fossils attributed to Australopithecus anamensis and Au. afarensis also may have had specialized wrist morphology that was retained from an earlier knuckle-walking ancestor.[2][13]
Gorillas
Gorillas use the form of walking on all fours with the fingers on the hands of the front two limbs folded inward. 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. Gorillas sometimes do walk upright in instances where dangers are present.
Other mammals
Advantages
Knuckle-walking tends to evolve when the fingers of the forelimb are specialized for tasks other than locomotion on the ground. In the gorilla, the fingers are used for the manipulation of food, and in chimpanzees, for the manipulation of food and climbing. In anteaters and pangolins, the fingers have large claws for opening the mounds of social insects. Platypus fingers have webbing that extend past the fingers to aid in swimming, thus knuckle-walking is used to prevent stumbling.[16]
Knuckle-walking of chimpanzees and gorillas, arguably, originally started from fist-walking as found in orangutans.[18] African apes most likely diverged from ancestral arboreal apes (similar to orangutans) that were adapted to distribute their weight among tree branches and forest canopies. Adjustments made for terrestrial locomotion early on may have involved fist-walking, later evolving into knuckle-walking.[19]
Evolution of knuckle-walking
Competing hypotheses are given as to how knuckle-walking evolved as a form of locomotion, stemming from comparisons between African apes. 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. However, results show that they are not characterized by such high magnitudes, which does not support independent evolution of knuckle-walking.[20] 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. On the other hand, their behavioral differences have been hypothesized to suggest convergent evolution, or homoplasy.[21]
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. Examining protein sequence changes suggests that Gorilla diverged before the clade Homo-Pan, meaning that ancestral bipedalism would require parallel evolution of knuckle-walking in separate chimpanzee and gorilla radiations.[22] The fact that chimpanzees practice both arboreal and knuckle-walking locomotion implies that knuckle-walking evolved from an arboreal ancestor as a solution for terrestrial travel, while still maintaining competent climbing skills.[23]
Not all features associated with knuckle-walking are identical to the beings that practice it, as it suggests possible developmental differences. For example, brachiation and suspension are almost certainly homologous between siamangs and gibbons, yet they differ substantially in the relative growth of their locomotor skeletons. Differences in carpal growth are not necessarily a consequence of their function, as they could be related to differences in body mass, growth, etc.[23] 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.
Human evolution
One theory of the origins of human bipedality is that it evolved from a terrestrial knuckle-walking ancestor. This theory is opposed to the theory that such bipedalism arose from a more generalized arboreal ape ancestor. The terrestrial knuckle-walking theory argues that early
Knuckle-walking, though has been suggested to have evolved independently and separately in Pan and Gorilla, so was not present in the human ancestors.[11][26] This is supported by the evidence that gorillas and chimpanzees differ in their knuckle-walking-related wrist anatomy and in the biomechanics of their knuckle-walking.[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".[11] Arguments for the independent evolution of knuckle-walking[11][26] have not gone without criticism, however.[27] A more recent study of morphological integration in human and great ape wrists suggests that knuckle-walking did not evolve independently in gorillas and chimpanzees, which "places the emergence of hominins and the evolution of bipedalism in the context of a knuckle-walking background."[27]
Related forms of hand-walking
Primates can walk on their hands in other ways than on their knuckles. They can walk on fists such as orangutans. In this form, body weight is borne on the back of the
Quadrupedal primate walking can be done on the palms. This occurs in many primates when walking on all fours on tree branches.
Primates can also walk on their fingers.
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- ^ a b Humphrey, N., Keynes, R. & Skoyles, J. R. (2005). "Hand-walkers : five siblings who never stood up". Discussion Paper. Centre for Philosophy of Natural and Social Science, London, UK. "Archived copy" (PDF). Archived (PDF) from the original on 11 October 2006. Retrieved 13 November 2006.
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