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Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the in a partially flexed posture that allows body weight to press down on the ground through the . Gorillas and chimpanzees use this style of locomotion, as do and .
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 , 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 Bipedalism for short distances while carrying food or in defensive situations. 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 common ancestor of chimpanzees and humans engaged in knuckle-walking, and Human evolved upright walking from knuckle-walking, a view thought to be supported by reanalysis of overlooked features on hominid fossils. Since then, scientists discovered Ardipithecus ramidus, a human-like hominid descended from the common ancestor of chimpanzees and humans. Ar. ramidus engaged in upright walking, but not knuckle-walking. This leads to the conclusion that chimpanzees evolved knuckle-walking after they split from humans six million years ago, and humans evolved upright walking without knuckle-walking. This would imply that knuckle-walking evolved independently in the African , which would mean a Homoplasy (independent) evolution of this locomotor behaviour in gorillas and chimpanzees. However, other studies have argued the opposite by pointing out that the differences in knuckle-walking between gorillas and chimpanzees can be explained by differences in positional behaviour, kinematics, and the biomechanics of weight-bearing.
Their knuckle-walking involves flexing the tips of their fingers and carrying their body weight down on the dorsal surface of their middle phalanges. The outer fingers are held clear off the ground. The wrist is held in a stable, locked position during the support phase of knuckle-walking by means of strongly flexed interphalangeal joints, and extended metacarpophalangeal joints. The palm, as a result, is positioned perpendicular to the ground and in line with the forearm. The wrist and elbow are extended throughout the last period in which the knuckle-walker's hand carried body weight.
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 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. These are found in all chimpanzees, but in only two out of five gorillas. They are also less prominent when found in gorillas. They are, however, found in primates that do not knuckle-walk.
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.
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.
Knuckle-walking has been reported in some . Fossils attributed to Australopithecus anamensis and Au. afarensis also may have had specialized wrist morphology that was retained from an earlier knuckle-walking ancestor.
Knuckle-walking of and , arguably, originally started from fist-walking as found in . African 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.
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. 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.
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 and , 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. 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.
Knuckle-walking, though has been suggested to have evolved independently and separately in Pan and Gorilla, so was not present in the human ancestors. 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. 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". Arguments for the independent evolution of knuckle-walking have not gone without criticism, however. Another 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."
Quadrupedal primate walking can be done on the palms. This occurs in many primates when walking on all fours on tree branches. 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). A few older children and some adults retain the ability to walk quadrupedally, even after acquiring bipedalism. A BBC2 and NOVA episode, "The Family That Walks on All Fours", reported on the Ulas family in which five individuals grew up walking normally upon the palms of their hands and fully extended legs due to a recessive genetic mutation that causes a nonprogressive congenital ataxia that impairs the balance needed for bipedality.Humphrey, N., Keynes family & Skoyles, J. R. (2005). "Hand-walkers : five siblings who never stood up". Discussion Paper. Centre for Philosophy of Natural and Social Science, London, UK. Not only did they walk on their palms of their hands, but they also could do so holding objects in their fingers.
Primates can also walk on their fingers. In , , and , such finger-walking turns to palm-walking when animals start to run. This has been suggested to spread the forces better across the wrist bones to protect them.
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