Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the finger

Finger

A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....

s in a partially flexed posture that allows body weight to press down on the ground through the knuckle

Knuckle

The knuckles are the joints of the fingers and toes, which are brought into prominence when the hand is clenched and a fist is made. The word is cognate to similar words in other Germanic languages, such as the Dutch "Knokkel" or German "Knöchel" , i.e., Knöchlein, the diminutive of the German...

s.

Gorilla

Gorilla

Gorillas are the largest extant species of primates. They are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Africa. Gorillas are divided into two species and either four or five subspecies...

s and chimpanzee

Chimpanzee

Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...

s use this style of locomotion

Animal locomotion

Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...

 as do anteater

Anteater

Anteaters, also known as antbear, are the four mammal species of the suborder Vermilingua commonly known for eating ants and termites. Together with the sloths, they compose the order Pilosa...

s and platypus

Platypus

The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

es.

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 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 scientists to conclude that chimpanzees evolved knuckle-walking after they split from humans 6 million years ago, and humans evolved upright walking without knuckle-walking.

Apes

Chimpanzees and gorillas engage in knuckle-walking. This form of hand-walking posture allows these tree climbers to use their hands for terrestrial locomotion while retaining long fingers for climbing. It may also allow small objects to be carried in the fingers while walking on all fours. Although this is the most common type of movement for gorillas, they also will stand upright on two legs, called bipedalism. They do this by counterbalancing their weight by swinging their arms parallel to the opposite leg like a human.

Their knuckle walking involves flexing the tips of their fingers and carrying their body weight down on the dorsal surface of their middle phalanges (middle segments of their fingers). The outer fingers are held clear on 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 joint

Metacarpophalangeal joint

The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

s. The palm as a result is positioned perpendicular to the ground and in-line with the forearm. The wrist and elbow

Elbow

The human elbow is the region surrounding the elbow-joint—the ginglymus or hinge joint in the middle of the arm. Three bones form the elbow joint: the humerus of the upper arm, and the paired radius and ulna of the forearm....

 are extended throughout the last period in which the knuckle-walker's hand carried body weight.

There are differences between 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.

It has been suggested that chimpanzee knuckle walking and gorilla knuckle walking are biomechanically and posturally distinct. Gorillas use a form of knuckle-walking which 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 baboon

Baboon

Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger...

s. It has also been suggested that fossils attributed to Australopithecus anamensis

Australopithecus anamensis

Australopithecus anamensis is a stem-human species that lived approximately four million years ago. Nearly one hundred fossil specimens are known from Kenya and Ethiopia, representing over 20 individuals.- Discovery :...

and A. afarensis

Australopithecus anamensis

Australopithecus anamensis is a stem-human species that lived approximately four million years ago. Nearly one hundred fossil specimens are known from Kenya and Ethiopia, representing over 20 individuals.- Discovery :...

had specialized wrist morphology that was retained from an earlier knuckle-walking ancestor.

Nonprimates

Giant anteaters

Giant Anteater

The Giant Anteater, Myrmecophaga tridactyla, is the largest species of anteater. It is the only species in the genus Myrmecophaga. It is found in Central and South America from Honduras to northern Argentina...

 and platypus

Platypus

The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

es are also knuckle-walkers. Pangolin

Pangolin

A pangolin , also scaly anteater or Trenggiling, is a mammal of the order Pholidota. There is only one extant family and one genus of pangolins, comprising eight species. There are also a number of extinct taxa. Pangolins have large keratin scales covering their skin and are the only mammals with...

s also sometimes walk on their knuckles. Another possible knuckle-walking taxon

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 was the extinct chalicothere

Chalicothere

Chalicotheres were a group of herbivorous, odd-toed ungulate mammals spread throughout North America, Europe, Asia, and Africa during the Early Eocene to Early Pleistocene subepochs living from 55.8 mya—781,000 years ago, existing for approximately .They evolved around 40 million years ago from...

s, which looked something like a cross between a horse

Equidae

Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...

 and a gorilla. The ground sloth

Ground sloth

Ground sloths are a diverse group of extinct sloths, in the mammalian superorder Xenarthra. Their most recent survivors lived in the Antilles, where it has been proposed they may have survived until 1550 CE; however, the youngest AMS radiocarbon date reported is 4190 BP, calibrated to c. 4700 BP...

s 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 for 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.

It has been argued that knuckle-walking of chimpanzees and gorillas originally started from fist-walking as found in orangutans.

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. The argument is not that they were knuckle-walkers themselves but that it is an example of "phylogenetic “lag”". "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".

It has however been suggested that knuckle-walking evolved independently and separately in Pan and gorillas and so was not present in the human ancestors. This is supported by the evidence that gorilla and chimpanzee differ in their knuckle-walking related wrist anatomy and in the biomechanics of their knuckle-walking.

Indeed as Tracy L. Kivell and Daniel 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. 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."

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 orangutan

Orangutan

Orangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping...

s. In this body weight is borne on the back of the proximal phalanges

Proximal phalanges

Proximal phalanges are bones found in the limbs of most vertebrates. In humans, they are the bones at the base of a toe or finger, the prominent, knobby ends of which are often called the knuckles....

.

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 in taken by the ankles). A few older children and some adults retain the ability to walk quadrupedally, even after acquiring bipedally. A BBC2 and NOVA

NOVA (TV series)

Nova is a popular science television series from the U.S. produced by WGBH Boston. It can be seen on the Public Broadcasting Service in the United States, and in more than 100 other countries...

in The Family That Walks On All Fours

The Family That Walks On All Fours

The Family That Walks On All Fours is a BBC2 documentary that explored the science and the story of five individuals in the Ulas family in Turkey that walk with a previously unreported quadruped gait . The documentary was created by Passionate Productions and was broadcast on Friday 17 March 2006....

 reported on the Ulas family

Ulas family

The Ulas family is a large Kurdish family of 19 from rural southern Turkey, five of whom walk on all fours with their feet and the palms of their hands in what is called a "bear crawl". Their quadrupedal gait has never been reported in anatomically intact adult humans. The gait is different from...

 in which five individuals grew up walking normally upon the palms of their hands and fully extended legs due to a recessive genetic mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 that causes a non-progressive congenital cerebellar ataxia

Ataxia

Ataxia is a neurological sign and symptom that consists of gross lack of coordination of muscle movements. Ataxia is a non-specific clinical manifestation implying dysfunction of the parts of the nervous system that coordinate movement, such as the cerebellum...

 that impairs the balance need for bipedality. Not only did they walk on their palms of their hands but could do so holding objects in their fingers.

Primates can also walk on their fingers. In Olive Baboon

Olive Baboon

The olive baboon , also called the Anubis baboon, is a member of the family Cercopithecidae . The species is the most widely spread of all baboons: it is found in 25 countries throughout Africa, extending south from Mali to Ethiopia and to Tanzania. Isolated populations are also found in some...

s, Rhesus Macaque

Rhesus Macaque

The Rhesus macaque , also called the Rhesus monkey, is one of the best-known species of Old World monkeys. It is listed as Least Concern in the IUCN Red List of Threatened Species in view of its wide distribution, presumed large population, and its tolerance of a broad range of habitats...

s, and Patas Monkey

Patas Monkey

The patas monkey , also known as the Wadi monkey or Hussar monkey, is a ground-dwelling monkey distributed over semi-arid areas of West Africa, and into East Africa. It is the only species classified in the genus Erythrocebus...

s 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.