Knuckle-walking

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A western lowland gorilla knuckle-walking. Western Lowland Gorilla.jpg
A western lowland gorilla 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. Gorillas and chimpanzees use this style of locomotion, as do anteaters and platypuses.

Contents

Knuckle-walking helps with actions other than locomotion on the ground. For 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. 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 common ancestor of chimpanzees and humans engaged in knuckle-walking, and humans evolved upright walking from knuckle-walking, a view thought to be supported by reanalysis of overlooked features on hominid fossils. [1] [2] 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. [3] This would imply that knuckle-walking evolved independently in the African great apes, which would mean a homoplasic evolution of this locomotor behaviour in gorillas and chimpanzees. [4] [5] 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. [6] [7]

Apes

Chimpanzees and gorillas engage in knuckle-walking. [2] 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 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. [2] [8] The wrist and elbow are extended throughout the last period in which the knuckle-walker's hand carried body weight. [9]

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. [10]

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. [10]

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. [10]

Knuckle-walking has been reported in some baboons. [11] 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] [12]

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. [13] 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. Humans would have to walk on all fours, as well, if they had the same type of hip placement. Gorillas sometimes do walk upright in instances where dangers are present.

Nonprimates

Giant anteaters [14] and platypuses [15] are also knuckle-walkers. Pangolins also sometimes walk on their knuckles. Another possible knuckle-walking taxon was the extinct chalicotheres, which looked something like a cross between a horse and a gorilla. [16] The ground sloths may have also walked on their knuckles.

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. [15]

Knuckle-walking of chimpanzees and gorillas, arguably, originally started from fist-walking as found in orangutans. [17] 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. [18]

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. [19] 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. [20]

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. [21] 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. [22]

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. [22] 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 hominin wrist and hand bones retain morphological evidence of early knuckle-walking. [2] [12] The argument is not that they were knuckle-walkers themselves, but that it is an example of "phylogenetic 'lag'". [2] "The retention of knuckle-walking morphology in the earliest hominids indicates that bipedalism evolved from an ancestor already adapted for terrestrial locomotion. ... 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". [12] See Vestigiality. 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. [23] 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. [23] This suggests that this wrist morphology was probably retained from an last 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 [24] ). Nevertheless, it is relevant to keep in mind that extant knuckle-walkers display diverse positional behaviors, and that knuckle-walking does not preclude climbing or exclude the possible importance of arboreality in the evolution of bipedalism in the hominin lineage.

Knuckle-walking, though has been suggested to have evolved independently and separately in Pan and Gorilla, so was not present in the human ancestors. [10] [25] 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. [10] 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". [10] Arguments for the independent evolution of knuckle-walking [10] [25] have not gone without criticism, however. [26] 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." [26]

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 proximal phalanges. [27]

Quadrupedal primate walking can be done on the palms. This occurs in many primates when walking on all fours on tree branches. [28] [29] 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. [30] 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 cerebellar ataxia that impairs the balance needed for bipedality. [31] Not only did they walk on their palms of their hands, but they also could do so holding objects in their fingers. [31]

Primates can also walk on their fingers. [28] [29] In olive baboons, rhesus macaques, and patas monkeys, such finger-walking turns to palm-walking when animals start to run. [29] This has been suggested to spread the forces better across the wrist bones to protect them. [29]

Related Research Articles

<i>Ardipithecus</i> Extinct genus of hominins

Ardipithecus is a genus of an extinct hominine that lived during the Late Miocene and Early Pliocene epochs in the Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the chimpanzees, the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate. Two fossil species are described in the literature: A. ramidus, which lived about 4.4 million years ago during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago. Behavioral analysis showed that Ardipithecus could be very similar to chimpanzees, indicating that the early human ancestors were very chimpanzee-like in behavior.

Bipedalism Terrestrial locomotion using two limbs

Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped, meaning "two feet". Types of bipedal movement include walking, running and hopping.

Human evolution Evolutionary process leading to anatomically modern humans

Human evolution is the evolutionary process within the history of primates that led to the emergence of Homo sapiens as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development of traits such as human bipedalism and language, as well as interbreeding with other hominins, which indicate that human evolution was not linear but a web.

Homininae Subfamily of mammals

Homininae, also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini ―and 2) the tribe Gorillini (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas, and to humans and chimpanzees via the tribe Hominini and subtribes Hominina and Panina. There are two living species of Panina and two living species of gorillas, but only one extant human species. Traces of extinct Homo species, including Homo floresiensis and Homo denisova, have been found with dates as recent as 40,000 years ago. Organisms in this subfamily are described as hominine or hominines.

Quadrupedalism

Quadrupedalism is a form of terrestrial locomotion where a tetrapod animal uses all four limbs (legs) to bear weight, walk, and run. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped. Most quadrupeds are terrestrial vertebrates, including mammals and reptiles, though some are largely aquatic such as turtles, amphibians, and pinnipeds.

<i>Australopithecus</i> Genus of hominin ancestral to modern humans

Australopithecus is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genera Homo, Paranthropus, and Kenyanthropus evolved from Australopithecus. Australopithecus is a member of the subtribe Australopithecina, which also includes Ardipithecus, though the term "australopithecine" is sometimes used to refer only to members of Australopithecus. Species include A. garhi, A. africanus, A. sediba, A. afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as to whether some Australopithecus species should be reclassified into new genera, or if Paranthropus and Kenyanthropus are synonymous with Australopithecus, in part because of the taxonomic inconsistency.

<i>Oreopithecus</i> Extinct genus of hominid from the Miocene

Oreopithecus is an extinct genus of hominoid primate from the Miocene epoch whose fossils have been found in today's Tuscany and Sardinia in Italy. It existed nine to seven million years ago in the Tusco-Sardinian area when this region was an isolated island in a chain of islands stretching from central Europe to northern Africa in what was becoming the Mediterranean Sea.

Brachiation Form of arboreal locomotion involving swinging by the arm

Brachiation, or arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. During brachiation, the body is alternately supported under each forelimb. This form of locomotion is the primary means of locomotion for the small gibbons and siamangs of southeast Asia. Gibbons in particular use brachiation for as much as 80% of their locomotor activities. Some New World monkeys, such as spider monkeys and muriquis, were initially classified as semibrachiators and move through the trees with a combination of leaping and brachiation. Some New World species also practice suspensory behaviors by using their prehensile tail, which acts as a fifth grasping hand. Evidence has shown that the extinct ape Proconsul from the Milocene of East Africa developed an early form of suspensory behaviour, and was therefore referred to as a probrachiator.

In terrestrial animals, plantigrade locomotion means walking with the toes and metatarsals flat on the ground. It is one of three forms of locomotion adopted by terrestrial mammals. The other options are digitigrade, walking on the toes with the heel and wrist permanently raised, and ungulates, walking on the nail or nails of the toes with the heel/wrist and the digits permanently raised. The leg of a plantigrade mammal includes the bones of the upper leg (femur/humerus) and lower leg. The leg of a digitigrade mammal also includes the metatarsals/metacarpals, the bones that in a human compose the arch of the foot and the palm of the hand. The leg of an unguligrade mammal also includes the phalanges, the finger and toe bones.

Forelimb

A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. In bipedal animals with an upright posture, the term upper limb is often used.

Orthograde is a term derived from Greek ὀρθός, orthos + Latin gradi that describes a manner of walking which is upright, with the independent motion of limbs. Both New and Old World monkeys are primarily arboreal, and they have a tendency to walk with their limbs swinging in parallel to one another. This differs from the manner of walking demonstrated by the apes.

Arches of the foot

The arches of the foot, formed by the tarsal and metatarsal bones, strengthened by ligaments and tendons, allow the foot to support the weight of the body in the erect posture with the least weight.

A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional circumstances (facultative), while normally walking or running on four limbs or more. In contrast, obligate bipedalism is where walking or running on two legs is the primary method of locomotion. Facultative bipedalism has been observed in several families of lizards and multiple species of primates, including sifakas, capuchin monkeys, baboons, gibbons, gorillas, bonobos and chimpanzeess. Different facultatively bipedal species employ different types of bipedalism corresponding to the varying reasons they have for engaging in facultative bipedalism. In primates, bipedalism is often associated with food gathering and transport. In lizards, it has been debated whether bipedal locomotion is an advantage for speed and energy conservation or whether it is governed solely by the mechanics of the acceleration and lizard's center of mass. Facultative bipedalism is often divided into high-speed (lizards) and low-speed (gibbons), but some species cannot be easily categorized into one of these two. Facultative bipedalism has also been observed in cockroaches and some desert rodents.

Lucy (<i>Australopithecus</i>) Skeleton of a female of the hominin species Australopithecus afarensis

Lucy is the common name of AL 288-1, several hundred pieces of fossilized bone representing 40 percent of the skeleton of a female of the hominin species Australopithecus afarensis. In Ethiopia, the assembly is also known as Dinkinesh, which means "you are marvelous" in the Amharic language. Lucy was discovered in 1974 in Africa, at Hadar, a site in the Awash Valley of the Afar Triangle in Ethiopia, by paleoanthropologist Donald Johanson of the Cleveland Museum of Natural History.

<i>Ardipithecus ramidus</i> Extinct hominin from Early Pliocene Ethiopia

Ardipithecus ramidus is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). A. ramidus, unlike modern hominids, has adaptations for both walking on two legs (bipedality) and life in the trees (arboreality). However, it would not have been as efficient at bipedality as humans, nor at arboreality as non-human great apes. Its discovery, along with Miocene apes, has reworked academic understanding of the chimpanzee-human last common ancestor from appearing much like modern day chimpanzees, orangutans and gorillas to being a creature without a modern anatomical cognate.

The evolution of human bipedalism, which began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus, or about 12 million years ago with Danuvius guggenmosi, has led to morphological alterations to the human skeleton including changes to the arrangement and size of the bones of the foot, hip size and shape, knee size, leg length, and the shape and orientation of the vertebral column. The evolutionary factors that produced these changes have been the subject of several theories.

Hand Extremity at the end of an arm or forelimb

A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala are often described as having "hands" instead of paws on their front limbs. The raccoon is usually described as having "hands" though opposable thumbs are lacking.

Chimpanzee–human last common ancestor Chimpanzee–human last common ancestor

The chimpanzee–human last common ancestor (CHLCA) is the last common ancestor shared by the extant Homo (human) and Pan genera of Hominini. Due to complex hybrid speciation, it is not possible to give a precise estimate on the age of this ancestral population. While "original divergence" between populations may have occurred as early as 13 million years ago (Miocene), hybridization may have been ongoing until as recently as 4 million years ago (Pliocene).

Vertical clinging and leaping (VCL) is a type of arboreal locomotion seen most commonly among the strepsirrhine primates and haplorrhine tarsiers. The animal begins at rest with its torso upright and elbows fixed, with both hands clinging to a vertical support, such as the side of a tree or bamboo stalk. To move from one support to another, it pushes off from one vertical support with its hindlimbs, landing on another vertical support after an extended period of free flight. Vertical clinging and leaping primates have evolved a specialized anatomy to compensate for the physical implications of this form of locomotion. These key morphological specializations have been identified in prosimian fossils from as early as the Eocene.

Danuvius guggenmosi is an extinct species of great ape that lived 11.6 million years ago during the Middle–Late Miocene in southern Germany. It is the sole member of the genus Danuvius. The area at this time was probably a woodland with a seasonal climate. A male specimen was estimated to have weighed about 31 kg (68 lb), and two females 17 and 19 kg. Both genus and species were described in November 2019.

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