Bipedalism

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 /ˈbaɪpɛd/, meaning "two feet" (from the Latin bis for "double" and pes for "foot"). Types of bipedal movement include walking, running, or hopping.

Few modern species are habitual bipeds whose normal method of locomotion is two-legged. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, springhare,[4] hopping mice, pangolins and hominin apes (australopithecines and humans) as well as various other extinct groups evolving the trait independently. In the Triassic period some groups of archosaurs (a group that includes crocodiles and dinosaurs) developed bipedalism; among the dinosaurs, all the early forms and many later groups were habitual or exclusive bipeds; the birds are members of a clade of exclusively bipedal dinosaurs, the Theropods.

A larger number of modern species intermittently or briefly use a bipedal gait. Several lizard species move bipedally when running, usually to escape from threats. Many primate and bear species will adopt a bipedal gait in order to reach food or explore their environment, though there are a few cases where they walk on their hindlimbs only. Several arboreal primate species, such as gibbons and indriids, exclusively walk on two legs during the brief periods they spend on the ground. Many animals rear up on their hind legs whilst fighting or copulating. Some animals commonly stand on their hind legs, in order to reach food, to keep watch, to threaten a competitor or predator, or to pose in courtship, but do not move bipedally.

Struthio camelus in Serengeti crop
An ostrich, the fastest extant biped[1] at 70 km/h (43 mph)[2][a]
Muybridge runner
A Man Running - Eadweard Muybridge

Etymology

The word is derived from the Latin words bi(s) 'two' and ped- 'foot', as contrasted with quadruped 'four feet'.

Advantages

Limited and exclusive bipedalism can offer a species several advantages. Bipedalism raises the head; this allows a greater field of vision with improved detection of distant dangers or resources, access to deeper water for wading animals and allows the animals to reach higher food sources with their mouths. While upright, non-locomotory limbs become free for other uses, including manipulation (in primates and rodents), flight (in birds), digging (in giant pangolin), combat (in bears, great apes and the large monitor lizard) or camouflage (in certain species of octopus).

The maximum bipedal speed appears less fast than the maximum speed of quadrupedal movement with a flexible backbone – both the ostrich and the red kangaroo can reach speeds of 70 km/h (43 mph), while the cheetah can exceed 100 km/h (62 mph).[5][6] Even though bipedalism is slower at first, over long distances, it has allowed humans to outrun most other animals according to the endurance running hypothesis.[7] Bipedality in kangaroo rats has been hypothesized to improve locomotor performance, which could aid in escaping from predators.[8][9]

Facultative and obligate bipedalism

Zoologists often label behaviors, including bipedalism, as "facultative" (i.e. optional) or "obligate" (the animal has no reasonable alternative). Even this distinction is not completely clear-cut — for example, humans other than infants normally walk and run in biped fashion, but almost all can crawl on hands and knees when necessary. There are even reports of humans who normally walk on all fours with their feet but not their knees on the ground, but these cases are a result of conditions such as Uner Tan syndrome — very rare genetic neurological disorders rather than normal behavior.[10] Even if one ignores exceptions caused by some kind of injury or illness, there are many unclear cases, including the fact that "normal" humans can crawl on hands and knees. This article therefore avoids the terms "facultative" and "obligate", and focuses on the range of styles of locomotion normally used by various groups of animals. Normal humans may be considered "obligate" bipeds because the alternatives are very uncomfortable and usually only resorted to when walking is impossible.

Movement

There are a number of states of movement commonly associated with bipedalism.

  1. Standing. Staying still on both legs. In most bipeds this is an active process, requiring constant adjustment of balance.
  2. Walking. One foot in front of another, with at least one foot on the ground at any time.
  3. Running. One foot in front of another, with periods where both feet are off the ground.
  4. Jumping/hopping. Moving by a series of jumps with both feet moving together.

Bipedal animals

The great majority of living terrestrial vertebrates are quadrupeds, with bipedalism exhibited by only a handful of living groups. Humans, gibbons and large birds walk by raising one foot at a time. On the other hand, most macropods, smaller birds, lemurs and bipedal rodents move by hopping on both legs simultaneously. Tree kangaroos are able to walk or hop, most commonly alternating feet when moving arboreally and hopping on both feet simultaneously when on the ground.

Amphibians

There are no known living or fossil bipedal amphibians.

Extant reptiles

Many species of lizards become bipedal during high-speed, sprint locomotion, including the world's fastest lizard, the spiny-tailed iguana (genus Ctenosaura).

Early reptiles and lizards

The first known biped is the bolosaurid Eudibamus whose fossils date from 290 million years ago.[11][12] Its long hindlegs, short forelegs, and distinctive joints all suggest bipedalism. The species became extinct in the early Permian.

Archosaurs (includes birds, crocodiles, and dinosaurs)

Birds

All birds are bipeds when on the ground, a feature inherited from their dinosaur ancestors.

Other archosaurs

Bipedalism evolved more than once in archosaurs, the group that includes both dinosaurs and crocodilians.[13] All dinosaurs are thought to be descended from a fully bipedal ancestor, perhaps similar to Eoraptor. Bipedal movement also re-evolved in a number of other dinosaur lineages such as the iguanodons. Some extinct members of the crocodilian line, a sister group to the dinosaurs and birds, also evolved bipedal forms - a crocodile relative from the triassic, Effigia okeeffeae, is thought to be bipedal.[14] Pterosaurs were previously thought to have been bipedal, but recent trackways have all shown quadrupedal locomotion. Bipedalism also evolved independently among the dinosaurs. Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian-Triassic extinction event wiped out an estimated 95% of all life on Earth.[15][16] Radiometric dating of fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists suspect Eoraptor resembles the common ancestor of all dinosaurs;[17] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[18] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.

Mammals

A number of groups of extant mammals have independently evolved bipedalism as their main form of locomotion - for example humans, giant pangolins, the extinct giant ground sloths, numerous species of jumping rodents and macropods. Humans, as their bipedalism has been extensively studied, are documented in the next section. Macropods are believed to have evolved bipedal hopping only once in their evolution, at some time no later than 45 million years ago.[19] Bipedal movement is less common among mammals, most of which are quadrupedal. All primates possess some bipedal ability, though most species primarily use quadrupedal locomotion on land. Primates aside, the macropods (kangaroos, wallabies and their relatives), kangaroo rats and mice, hopping mice and springhare move bipedally by hopping. Very few mammals other than primates commonly move bipedally by an alternating gait rather than hopping. Exceptions are the ground pangolin and in some circumstances the tree kangaroo.[20] One black bear, Pedals, became famous locally and on the internet for having a frequent bipedal gait, although this is attributed to injuries on the bear's front paws.

Primates

Most bipedal animals move with their backs close to horizontal, using a long tail to balance the weight of their bodies. The primate version of bipedalism is unusual because the back is close to upright (completely upright in humans), and the tail may be absent entirely. Many primates can stand upright on their hind legs without any support. common chimpanzees, bonobos, gibbons[21] and baboons[22] exhibit forms of bipedalism. On the ground sifakas move like all indrids with bipedal sideways hopping movements of the hind legs, holding their forelimbs up for balance.[23] Geladas, although usually quadrupedal, will sometimes move between adjacent feeding patches with a squatting, shuffling bipedal form of locomotion.[24]

Humans are the only primates who are normally biped, due to an extra curve in the spine which stabilizes the upright position, as well as shorter arms relative to the legs than is the case for the nonhuman great apes. The evolution of human bipedalism began in primates about four million years ago,[25] or as early as seven million years ago with Sahelanthropus.[26] One hypothesis for human bipedalism is that it evolved as a result of differentially successful survival from carrying food to share with group members,[27] although there are alternative hypotheses.

Injured individuals

Injured chimpanzees and bonobos have been capable of sustained bipedalism.[28]

Three captive primates, one macaque Natasha[29] and two chimps, Oliver and Poko (chimpanzee), were found to move bipedally. Natasha switched to exclusive bipedalism after an illness, while Poko was discovered in captivity in a tall, narrow cage.[30][31] Oliver reverted to knuckle-walking after developing arthritis. Non-human primates often use bipedal locomotion when carrying food.

Limited bipedalism

Limited bipedalism in mammals

Other mammals engage in limited, non-locomotory, bipedalism. A number of other animals, such as rats, raccoons, and beavers will squat on their hindlegs to manipulate some objects but revert to four limbs when moving (the beaver will move bipedally if transporting wood for their dams, as will the raccoon when holding food). Bears will fight in a bipedal stance to use their forelegs as weapons. A number of mammals will adopt a bipedal stance in specific situations such as for feeding or fighting. Ground squirrels and meerkats will stand on hind legs to survey their surroundings, but will not walk bipedally. Dogs (e.g. Faith) can stand or move on two legs if trained, or if birth defect or injury precludes quadrupedalism. The gerenuk antelope stands on its hind legs while eating from trees, as did the extinct giant ground sloth and chalicotheres. The spotted skunk will walk on its front legs when threatened, rearing up on its front legs while facing the attacker so that its anal glands, capable of spraying an offensive oil, face its attacker.

Limited bipedalism in non-mammals

Bipedalism is unknown among the amphibians. Among the non-archosaur reptiles bipedalism is rare, but it is found in the 'reared-up' running of lizards such as agamids and monitor lizards. Many reptile species will also temporarily adopt bipedalism while fighting.[32] One genus of basilisk lizard can run bipedally across the surface of water for some distance. Among arthropods, cockroaches are known to move bipedally at high speeds.[33] Bipedalism is rarely found outside terrestrial animals, though at least two types of octopus walk bipedally on the sea floor using two of their arms, allowing the remaining arms to be used to camouflage the octopus as a mat of algae or a floating coconut.[34]

Evolution of human bipedalism

There are at least twelve distinct hypotheses as to how and why bipedalism evolved in humans, and also some debate as to when. Bipedalism evolved well before the large human brain or the development of stone tools.[35] Bipedal specializations are found in Australopithecus fossils from 4.2-3.9 million years ago,[36] although Sahelanthropus may have walked on two legs as early as seven million years ago.[26] Nonetheless, the evolution of bipedalism was accompanied by significant evolutions in the spine including the forward movement in position of the foramen magnum, where the spinal cord leaves the cranium.[37] Recent evidence regarding modern human sexual dimorphism (physical differences between male and female) in the lumbar spine has been seen in pre-modern primates such as Australopithecus africanus. This dimorphism has been seen as an evolutionary adaptation of females to bear lumbar load better during pregnancy, an adaptation that non-bipedal primates would not need to make.[38][39] Adapting bipedalism would have required less shoulder stability, which allowed the shoulder and other limbs to become more independent of each other and adapt for specific suspensory behaviors. In addition to the change in shoulder stability, changing locomotion would have increased the demand for shoulder mobility, which would have propelled the evolution of bipedalism forward.[40] The different hypotheses are not necessarily mutually exclusive and a number of selective forces may have acted together to lead to human bipedalism. It is important to distinguish between adaptations for bipedalism and adaptations for running, which came later still.

Numerous causes for the evolution of human bipedalism involve freeing the hands for carrying and using tools, sexual dimorphism in provisoning, changes in climate and environment (from jungle to savanna) that favored a more elevated eye-position, and to reduce the amount of skin exposed to the tropical sun.[41] It is possible that bipedalism provided a variety of benefits to the hominin species, and scientists have suggested multiple reasons for evolution of human bipedalism.[42] There is also not only the question of why the earliest hominins were partially bipedal but also why hominins became more bipedal over time. For example, the postural feeding hypothesis describes how the earliest hominins became bipedal for the benefit of reaching food in trees while the savanna-based theory describes how the late hominins that started to settle on the ground became increasingly bipedal.[43]

Multiple factors

Napier (1963) argued that it was very unlikely that a single factor drove the evolution of bipedalism. He stated "It seems unlikely that any single factor was responsible for such a dramatic change in behaviour. In addition to the advantages of accruing from ability to carry objects - food or otherwise - the improvement of the visual range and the freeing of the hands for purposes of defence and offence must equally have played their part as catalysts.” [44] Sigmon argued that chimpanzees demonstrate bipedalism in different contexts, and one single factor should be used to explain bipedalism: preadaptation for human bipedalism.[45] Day (1986) emphasized three major pressures that drove evolution of bipedalism 1.food acquisition 2. predator avoidance 3. Reproductive success.[46] Ko (2015) states there are two questions regarding bipedalism 1. Why were the earliest hominins partially bipedal 2. why did hominins become more bipedal over time. He argues that these questions can be answered with combination of prominent theories such as Savanna-based, Postural feeding, and Provisioning.[47]

Savannah-based theory

According to the Savanna-based theory, hominines descended from the trees and adapted to life on the savanna by walking erect on two feet. The theory suggests that early hominids were forced to adapt to bipedal locomotion on the open savanna after they left the trees. One of the proposed mechanisms was the knuckle-walking hypothesis, which states that human ancestors used quadrupedal locomotion on the savanna, as evidenced by morphological characteristics found in Australopithecus anamensis and Australopithecus afarensis forelimbs, and that it is less parsimonious to assume that knuckle walking developed twice in genera Pan and Gorilla instead of evolving it once as synapomorphy for Pan and Gorilla before losing it in Australopithecus.[48] The evolution of an orthograde posture would have been very helpful on a savanna as it would allow the ability to look over tall grasses in order to watch out for predators, or terrestrially hunt and sneak up on prey.[49] It was also suggested in P.E. Wheeler's "The evolution of bipedality and loss of functional body hair in hominids", that a possible advantage of bipedalism in the savanna was reducing the amount of surface area of the body exposed to the sun, helping regulate body temperature.[50] In fact, Elizabeth Vrba’s turnover pulse hypothesis supports the savanna-based theory by explaining the shrinking of forested areas due to global warming and cooling, which forced animals out into the open grasslands and caused the need for hominids to acquire bipedality.[51]

Others state hominines had already achieved the bipedal adaptation that was used in the savanna. The fossil evidence reveals that early bipedal hominins were still adapted to climbing trees at the time they were also walking upright.[52] It is possible that bipedalism evolved in the trees, and was later applied to the savanna as a vestigial trait. Humans and orangutans are both unique to a bipedal reactive adaptation when climbing on thin branches, in which they have increased hip and knee extension in relation to the diameter of the branch, which can increase an arboreal feeding range and can be attributed to a convergent evolution of bipedalism evolving in arboreal environments.[53] Hominine fossils found in dry grassland environments led anthropologists to believe hominines lived, slept, walked upright, and died only in those environments because no hominine fossils were found in forested areas. However, fossilization is a rare occurrence—the conditions must be just right in order for an organism that dies to become fossilized for somebody to find later, which is also a rare occurrence. The fact that no hominine fossils were found in forests does not ultimately lead to the conclusion that no hominines ever died there. The convenience of the savanna-based theory caused this point to be overlooked for over a hundred years.[54]

Some of the fossils found actually showed that there was still an adaptation to arboreal life. For example, Lucy, the famous Australopithecus afarensis, found in Hadar in Ethiopia, which may have been forested at the time of Lucy’s death, had curved fingers that would still give her the ability to grasp tree branches, but she walked bipedally. “Little Foot,” a nearly-complete specimen of Australopithecus africanus, has a divergent big toe as well as the ankle strength to walk upright. “Little Foot” could grasp things using his feet like an ape, perhaps tree branches, and he was bipedal. Ancient pollen found in the soil in the locations in which these fossils were found suggest that the area used to be much more wet and covered in thick vegetation and has only recently become the arid desert it is now.[51]

Traveling efficiency hypothesis

An alternative explanation is the mixture of savanna and scattered forests increased terrestrial travel by proto-humans between clusters of trees, and bipedalism offered greater efficiency for long-distance travel between these clusters than quadrupedalism.[55][56] In an experiment monitoring chimpanzee metabolic rate via oxygen consumption, it was found that the quadrupedal and bipedal energy costs were very similar, implying that this transition in early ape-like ancestors would have not have been very difficult or energetically costing.[57] This increased travel efficiency is likely to have been selected for as it assisted the wide dispersal of early hominids across the savanna to create start populations.

Postural feeding hypothesis

The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at Indiana University.[58] This hypothesis asserts that chimpanzees were only bipedal when they eat. While on the ground, they would reach up for fruit hanging from small trees and while in trees, bipedalism was used to reach up to grab for an overhead branch. These bipedal movements may have evolved into regular habits because they were so convenient in obtaining food. Also, Hunt's hypotheses states that these movements coevolved with chimpanzee arm-hanging, as this movement was very effective and efficient in harvesting food. When analyzing fossil anatomy, Australopithecus afarensis has very similar features of the hand and shoulder to the chimpanzee, which indicates hanging arms. Also, the Australopithecus hip and hind limb very clearly indicate bipedalism, but these fossils also indicate very inefficient locomotive movement when compared to humans. For this reason, Hunt argues that bipedalism evolved more as a terrestrial feeding posture than as a walking posture.(https://www.researchgate.net/publication/288349918_The_postural_feeding_hypothesis_An_ecological_model_for_the_evolution_of_bipedalism)

A similar study conducted by Thorpe et al. looked at how the most arboreal great ape, the orangutan, held onto supporting branches in order to navigate branches that were too flexible or unstable otherwise. They found that in more than 75% of locomotive instances the orangutans used their hands to stabilize themselves while they navigated thinner branches. They hypothesized that increased fragmentation of forests where A. afarensis as well as other ancestors of modern humans and other apes resided could have contributed to this increase of bipedalism in order to navigate the diminishing forests. Their findings also shed light on a couple of discrepancies observed in the anatomy of A. afarensis, such as the ankle joint, which allowed it to “wobble” and long, highly flexible forelimbs. The idea that bipedalism started from walking in trees explains both the increased flexibility in the ankle as well as the long limbs which would be used to grab hold of branches.

Provisioning model

One theory on the origin of bipedalism is the behavioral model presented by C. Owen Lovejoy, known as "male provisioning".[59] Lovejoy theorizes that the evolution of bipedalism was linked to monogamy. In the face of long inter-birth intervals and low reproductive rates typical of the apes, early hominids engaged in pair-bonding that enabled greater parental effort directed towards rearing offspring. Lovejoy proposes that male provisioning of food would improve the offspring survivorship and increase the pair's reproductive rate. Thus the male would leave his mate and offspring to search for food and return carrying the food in his arms walking on his legs. This model is supported by the reduction ("feminization") of the male canine teeth in early hominids such as Sahelanthropus tchadensis[60] and Ardipithecus ramidus,[61] which along with low body size dimorphism in Ardipithecus[62] and Australopithecus,[63] suggests a reduction in inter-male antagonistic behavior in early hominids.[64] In addition, this model is supported by a number of modern human traits associated with concealed ovulation (permanently enlarged breasts, lack of sexual swelling) and low sperm competition (moderate sized testes, low sperm mid-piece volume) that argues against recent adaptation to a polygynous reproductive system.[64]

However, this model has been debated, as others have argued that early bipedal hominids were instead polygynous. Among most monogamous primates, males and females are about the same size. That is sexual dimorphism is minimal, and other studies have suggested that Australopithecus afarensis males were nearly twice the weight of females. However, Lovejoy's model posits that the larger range a provisioning male would have to cover (to avoid competing with the female for resources she could attain herself) would select for increased male body size to limit predation risk.[65] Furthermore, as the species became more bipedal, specialized feet would prevent the infant from conveniently clinging to the mother - hampering the mother's freedom[66] and thus make her and her offspring more dependent on resources collected by others. Modern monogamous primates such as gibbons tend to be also territorial, but fossil evidence indicates that Australopithecus afarensis lived in large groups. However, while both gibbons and hominids have reduced canine sexual dimorphism, female gibbons enlarge ('masculinize') their canines so they can actively share in the defense of their home territory. Instead, the reduction of the male hominid canine is consistent with reduced inter-male aggression in a pair-bonded though group living primate.

Early bipedalism in homininae model

Recent studies of 4.4 million years old Ardipithecus ramidus suggest bipedalism. It is thus possible that bipedalism evolved very early in homininae and was reduced in chimpanzee and gorilla when they became more specialized. According to Richard Dawkins in his book "The Ancestor's Tale", chimps and bonobos are descended from Australopithecus gracile type species while gorillas are descended from Paranthropus. These apes may have once been bipedal, but then lost this ability when they were forced back into an arboreal habitat, presumably by those australopithecines from whom eventually evolved hominins. Early homininaes such as Ardipithecus ramidus may have possessed an arboreal type of bipedalism that later independently evolved towards knuckle-walking in chimpanzees and gorillas[67] and towards efficient walking and running in modern humans (see figure). It is also proposed that one cause of Neanderthal extinction was a less efficient running.

Warning display (aposematic) model

Joseph Jordania from the University of Melbourne recently (2011) suggested that bipedalism was one of the central elements of the general defense strategy of early hominids, based on aposematism, or warning display and intimidation of potential predators and competitors with exaggerated visual and audio signals. According to this model, hominids were trying to stay as visible and as loud as possible all the time. Several morphological and behavioral developments were employed to achieve this goal: upright bipedal posture, longer legs, long tightly coiled hair on the top of the head, body painting, threatening synchronous body movements, loud voice and extremely loud rhythmic singing/stomping/drumming on external subjects.[68] Slow locomotion and strong body odor (both characteristic for hominids and humans) are other features often employed by aposematic species to advertise their non-profitability for potential predators.

Other behavioural models

There are a variety of ideas which promote a specific change in behaviour as the key driver for the evolution of hominid bipedalism. For example, Wescott (1967) and later Jablonski & Chaplin (1993) suggest that bipedal threat displays could have been the transitional behaviour which led to some groups of apes beginning to adopt bipedal postures more often. Others (e.g. Dart 1925) have offered the idea that the need for more vigilance against predators could have provided the initial motivation. Dawkins (e.g. 2004) has argued that it could have begun as a kind of fashion that just caught on and then escalated through sexual selection. And it has even been suggested (e.g. Tanner 1981:165) that male phallic display could have been the initial incentive, as well as increased sexual signaling in upright female posture.[49]

Thermoregulatory model

The thermoregulatory model explaining the origin of bipedalism is one of the simplest theories so far advanced, but it is a viable explanation. Dr. Peter Wheeler, a professor of evolutionary biology, proposes that bipedalism raises the amount of body surface area higher above the ground which results in a reduction in heat gain and helps heat dissipation.[69][70][71] When a hominid is higher above the ground, the organism accesses more favorable wind speeds and temperatures. During heat seasons, greater wind flow results in a higher heat loss, which makes the organism more comfortable. Also, Wheeler explains that a vertical posture minimizes the direct exposure to the sun whereas quadrupedalism exposes more of the body to direct exposure. Analysis and interpretations of Ardipithecus reveal that this hypothesis needs modification to consider that the forest and woodland environmental preadaptation of early-stage hominid bipedalism preceded further refinement of bipedalism by the pressure of natural selection. This then allowed for the more efficient exploitation of the hotter conditions ecological niche, rather than the hotter conditions being hypothetically bipedalism's initial stimulus. A feedback mechanism from the advantages of bipedality in hot and open habitats would then in turn make a forest preadaptation solidify as a permanent state.[72]

Carrying models

Charles Darwin wrote that "Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to the act of obedience of his will". Darwin (1871:52) and many models on bipedal origins are based on this line of thought. Gordon Hewes (1961) suggested that the carrying of meat "over considerable distances" (Hewes 1961:689) was the key factor. Isaac (1978) and Sinclair et al. (1986) offered modifications of this idea, as indeed did Lovejoy (1981) with his "provisioning model" described above. Others, such as Nancy Tanner (1981), have suggested that infant carrying was key, while others again have suggested stone tools and weapons drove the change.[73] This stone-tools theory is very unlikely, as though ancient humans were known to hunt, the discovery of tools was not discovered for thousands of years after the origin of bipedalism, chronologically precluding it from being a driving force of evolution. (Wooden tools and spears fossilize poorly and therefore it is difficult to make a judgment about their potential usage.)

Wading models

The observation that large primates, including especially the great apes, that predominantly move quadrupedally on dry land, tend to switch to bipedal locomotion in waist deep water, has led to the idea that the origin of human bipedalism may have been influenced by waterside environments. This idea, labelled "the wading hypothesis",[74] was originally suggested by the Oxford marine biologist Alister Hardy who said: "It seems to me likely that Man learnt to stand erect first in water and then, as his balance improved, he found he became better equipped for standing up on the shore when he came out, and indeed also for running."[75] It was then promoted by Elaine Morgan, as part of the aquatic ape hypothesis, who cited bipedalism among a cluster of other human traits unique among primates, including voluntary control of breathing, hairlessness and subcutaneous fat.[76] The "aquatic ape hypothesis", as originally formulated, has not been accepted or considered a serious theory within the anthropological scholarly community.[77] Others, however, have sought to promote wading as a factor in the origin of human bipedalism without referring to further ("aquatic ape" related) factors. Since 2000 Carsten Niemitz has published a series of papers and a book[78] on a variant of the wading hypothesis, which he calls the "amphibian generalist theory" (German: Amphibische Generalistentheorie).

Other theories have been proposed that suggest wading and the exploitation of aquatic food sources (providing essential nutrients for human brain evolution[79] or critical fallback foods[80]) may have exerted evolutionary pressures on human ancestors promoting adaptations which later assisted full-time bipedalism. It has also been thought that consistent water-based food sources had developed early hominid dependency and facilitated dispersal along seas and rivers.[81]

Consequences

During the hominin’s early evolution, brains became larger, due to increased intelligence, and bipedalism became the norm. The consequences of these two changes in particular resulted in painful and difficult labor due to the increased favor of a narrow pelvis for bipedalism being countered by larger heads passing through the constricted birth canal. This phenomenon is commonly known as the obstetrical dilemma.

Physiology

Bipedal movement occurs in a number of ways, and requires many mechanical and neurological adaptations. Some of these are described below.

Biomechanics

Standing

Energy-efficient means of standing bipedally involve constant adjustment of balance, and of course these must avoid overcorrection. The difficulties associated with simple standing in upright humans are highlighted by the greatly increased risk of falling present in the elderly, even with minimal reductions in control system effectiveness.

Shoulder stability

Shoulder stability would decrease with the evolution of bipedalism. Shoulder mobility would increase because the need for a stable shoulder is only present in arboreal habitats. Shoulder mobility would support suspensory locomotion behaviors which are present in human bipedalism. The forelimbs are freed from weight-bearing requirements, which makes the shoulder a place of evidence for the evolution of bipedalism.[82]

Walking

Walking is characterized by an "inverted pendulum" movement in which the center of gravity vaults over a stiff leg with each step.[83] Force plates can be used to quantify the whole-body kinetic & potential energy, with walking displaying an out-of-phase relationship indicating exchange between the two.[83] This model applies to all walking organisms regardless of the number of legs, and thus bipedal locomotion does not differ in terms of whole-body kinetics.[84]

In humans, walking is composed of several separate processes:[83]

  • Vaulting over a stiff stance leg
  • Passive ballistic movement of the swing leg
  • A short 'push' from the ankle prior to toe-off, propelling the swing leg
  • Rotation of the hips about the axis of the spine, to increase stride length
  • Rotation of the hips about the horizontal axis to improve balance during stance

Running

Running is characterized by a spring-mass movement.[83] Kinetic and potential energy are in phase, and the energy is stored & released from a spring-like limb during foot contact.[83] Again, the whole-body kinetics are similar to animals with more limbs.[84]

Musculature

Bipedalism requires strong leg muscles, particularly in the thighs. Contrast in domesticated poultry the well muscled legs, against the small and bony wings. Likewise in humans, the quadriceps and hamstring muscles of the thigh are both so crucial to bipedal activities that each alone is much larger than the well-developed biceps of the arms.

Respiration

A biped has the ability to breathe while running, without strong coupling to stride cycle. Humans usually take a breath every other stride when their aerobic system is functioning. During a sprint the anaerobic system kicks in and breathing slows until the anaerobic system can no longer sustain a sprint.

Bipedal robots

Asimo
ASIMO - a bipedal robot

For nearly the whole of the 20th century, bipedal robots were very difficult to construct and robot locomotion involved only wheels, treads, or multiple legs. Recent cheap and compact computing power has made two-legged robots more feasible. Some notable biped robots are ASIMO, HUBO, MABEL and QRIO. Recently, spurred by the success of creating a fully passive, un-powered bipedal walking robot,[85] those working on such machines have begun using principles gleaned from the study of human and animal locomotion, which often relies on passive mechanisms to minimize power consumption.

See also

Notes

  1. ^ The red kangaroo can attain a similar speed for short distances.[3]

References

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Further reading

  • Darwin, C., "The Descent of Man and Selection in Relation to Sex", Murray (London), (1871).
  • Dart, R.A., "Australopithecus africanus: The Ape Man of South Africa" Nature, 145, 195-199, (1925).
  • Dawkins, R., "The Ancestor's Tale", Weidenfeld and Nicolson (London), (2004).
  • Hewes, G.W., "Food Transport and the Origin of Hominid Bipedalism" American Anthropologist, 63, 687-710, (1961).
  • Hunt, K.D., "The Evolution of Human Bipedality" Journal of Human Evolution, 26, 183-202, (1994).
  • Isaac, G.I., "The Archeological Evidence for the Activities of Early African Hominids" In:Early Hominids of Africa (Jolly, C.J. (Ed.)), Duckworth (London), 219-254, (1978).
  • Jablonski, N.G.; Chaplin, G. (1993). "Origin of Habitual Terrestrial Bipedalism in the Ancestor of the Hominidae". Journal of Human Evolution. 24 (4): 259–280. doi:10.1006/jhev.1993.1021.
  • Lovejoy, C. O. (1981). "The Origin of Man". Science. 211 (4480): 341–350. Bibcode:1981Sci...211..341L. doi:10.1126/science.211.4480.341. PMID 17748254.
  • Tanner, N.M., "On Becoming Human", Cambridge University Press (Cambridge), (1981)
  • Wescott, R.W. (1967). "Hominid Uprightness and Primate Display". American Anthropologist. 69 (6): 738. doi:10.1525/aa.1967.69.6.02a00110.
  • Wheeler, P. E. (1984) "The Evolution of Bipedality and Loss of Functional Body Hair in Hominoids." Journal of Human Evolution, 13, 91-98,
  • Vrba, E. (1993). "The Pulse that Produced Us". Natural History. 102 (5): 47–51.

External links

Aquatic ape hypothesis

The aquatic ape hypothesis (AAH), also referred to as aquatic ape theory (AAT) and more recently the waterside model, is the idea that certain ancestors of modern humans were more aquatic than other great apes and even many modern humans, and, as such, were habitual waders, swimmers and divers. The hypothesis in its present form was proposed by the marine biologist Alister Hardy in 1960, who argued that a branch of apes was forced by competition over terrestrial habitats to hunt for food such as shellfish on the sea shore and sea bed leading to adaptations that explained distinctive characteristics of modern humans such as functional hairlessness and bipedalism. This proposal was built upon by Elaine Morgan in her 1972 book The Descent of Woman, which drew attention to what she saw as the sexism inherent in the then prevalent savannah-based “man the hunter” theories of human evolution as presented in popular anthropological works by Robert Ardrey, Lionel Tiger and others.

Morgan removed the feminist content in several later books and her ideas were discussed at a 1987 conference devoted to the idea. Her 1990 book Scars of Evolution received some favorable reviews but the thesis was subject to criticism from the anthropologist John Langdon in 1997, who characterized it as an "umbrella hypothesis" with inconsistencies that were unresolved and a claim to parsimony that was false.The hypothesis remains highly controversial and is generally more popular with the lay public than with scientists. Though much of the mainstream academic community ignored or derided the initial proposal, a small group of academics in the last 15 years have undertaken research programmes linked to the AAH.

Australopithecus

Australopithecus ( OS-trə-lo-PITH-i-kəs; from Latin australis, meaning 'southern', and Greek πίθηκος (pithekos), meaning 'ape', informal australopithecine or australopith (although the term australopithecine has a broader meaning as a member of the subtribe Australopithecina,  which includes this genus as well as the Paranthropus, Kenyanthropus, Ardipithecus, and Praeanthropus genera)  is a genus of hominins. From paleontological and archaeological evidence, the genus Australopithecus apparently evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct two million years ago. Australopithecus is not literally extinct (in the sense of having no living descendants) as the Kenyanthropus, Paranthropus and Homo genera probably emerged as sister of a late Australopithecus species such as Australopithecus africanus and/or A. sediba. During that time, a number of australopithecine species emerged, including Australopithecus afarensis, A. africanus, A. anamensis, A. bahrelghazali, A. deyiremeda (proposed), A. garhi, and A. sediba.

For some other hominid species of this time – A. robustus, A. boisei and A. aethiopicus – some debate exists whether they truly constitute members of the genus Australopithecus. If so, they would be considered 'robust australopiths', while the others would be 'gracile australopiths'. However, if these more robust species do constitute their own genus, they would be under the genus name Paranthropus, a genus described by Robert Broom when the first discovery was made in 1938, which makes these species P. robustus, P. boisei and P. aethiopicus.

Australopithecus species played a significant part in human evolution, the genus Homo being derived from Australopithecus at some time after three million years ago.

In addition, they were the first hominids to possess certain genes, known as the duplicated SRGAP2, which increased the length and ability of neurons in the brain. One of the australopith species evolved into the genus Homo in Africa around two million years ago (e.g. Homo habilis), and eventually modern humans, H. sapiens sapiens.In January 2019, scientists reported that A. sediba is distinct from, but shares anatomical similarities to, both the older A. africanus, and the younger H. habilis.

Australopithecus afarensis

Australopithecus afarensis (Latin: "Southern ape from Afar") is an extinct hominin that lived between 3.9 and 2.9 million years ago in Africa. A. afarensis was slenderly built, like the younger Australopithecus africanus. A. afarensis is thought to be more closely related to the genus Homo (which includes the modern human species Homo sapiens), whether as a direct ancestor or a close relative of an unknown ancestor, than any other known primate from the same time. Some researchers include A. afarensis in the genus Praeanthropus.The most famous fossil is the partial skeleton named Lucy (3.2 million years old) found by Donald Johanson, Yves Coppens and Maurice Taïeb, who, in celebration of their find, repeatedly played the Beatles song "Lucy in the Sky with Diamonds".

Axilla

The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the joint where the arm connects to the shoulder. It also provides the under-arm sweat gland.

In humans, the formation of body odor happens mostly in the axillary region. These odorant substances serve as pheromones which play a role related to mating. The underarm regions seem more important than the genital region for body odor which may be related to human bipedalism.

Brachiation

Brachiation (from "brachium", Latin for "arm"), 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.Upon further observations and more in depth understandings of the anatomy and behaviour of primates, the terms semibrachiator and probrachiator have largely fallen out of favour within the scientific community. Currently, researchers classify gibbons and siamangs as the only true brachiators and classify the great apes as modified brachiators. All other brachiation behaviours that do not meet either of these classifications are referred to as forearm suspensory postures and locomotion.Some traits that allow primates to brachiate include a short spine (particularity the lumbar spine), short fingernails (instead of claws), long curved fingers, reduced thumbs, long forelimbs and freely rotating wrists. Modern humans retain many physical characteristics that suggest a brachiator ancestor, including flexible shoulder joints and fingers well-suited for grasping. In lesser apes, these characteristics were adaptations for brachiation. Although great apes do not normally brachiate (with the exception of orangutans), our human anatomy suggests that brachiation may be an exaptation to bipedalism, and healthy modern humans are still capable of brachiating. Some children's parks include monkey bars which children play on by brachiating.

As well as shaping the evolution of gibbon body structure, brachiation has influenced the style and order of their behaviour. For example, unlike other primates who carry infants on their back, gibbons will carry young ventrally. It also affects their play activities, copulation, and fighting. It is thought that gibbons gain evolutionary advantages through brachiation and being suspended by both hands (bimanual suspension) when feeding. While smaller primates cannot hold themselves by both hands for long periods, and larger primates are too heavy to exploit food resources on the ends of branches, gibbons can remain suspended for a significant period and use their long arms to reach food in terminal branches more easily. Another theory postulates that brachiation is a quieter and less obvious mode of locomotion than quadrupedal jumping and climbing thereby more successfully avoiding predators.

Concealed ovulation

Concealed ovulation or hidden estrus in a species is the lack of any perceptible change in an adult female (for instance, a change in appearance or scent) when she is fertile and near ovulation. Some examples of perceptible changes are swelling and redness of the genitalia in baboons and bonobos, and pheromone release in the feline family. In contrast, the females of humans and a few other species that undergo hidden estrus have few external signs of fecundity, making it difficult for a mate to consciously deduce, by means of external signs only, whether or not a female is near ovulation.

Euparkeria

Euparkeria (; meaning "Parker's good animal", named in honor of W.K. Parker) is an extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, and was close to the ancestry of Archosauria, the group that includes dinosaurs, pterosaurs, and modern birds and crocodilians.

Euparkeria had hind limbs that were slightly longer than its forelimbs, which has been taken as evidence that it may have been able to rear up on its hind legs as a facultative biped. Although Euparkeria is close to the ancestry of fully bipedal archosaurs such as early dinosaurs, it probably developed bipedalism independently. Euparkeria was not as well adapted to bipedal locomotion as dinosaurs and its normal movement was probably more analogous to a crocodilian high walk.

Evolutionary musicology

Evolutionary musicology is a subfield of biomusicology that grounds the psychological mechanisms of music perception and production in evolutionary theory. It covers vocal communication in non-human animal species, theories of the evolution of human music, and cross-cultural human universals in musical ability and processing.

Facultative bipedalism

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, and chimpanzees. 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.

Homininae

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 Hominini tribe (with the genus Homo including modern humans and numerous extinct species; the subtribe Australopithecina, comprising at least two extinct genera; and the subtribe Panina, represented only by the genus Pan, which includes common chimpanzees and bonobos)―and 2) the Gorillini tribe (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans (subfamily Ponginae) split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas (through the tribe Gorillini), and to humans and chimpanzees via the tribe Hominini and subtribes Hominina and Panina (see the evolutionary tree below). There are two living species of Panina (chimpanzees and bonobos) and two living species of gorillas, but only one extant human species. Traces of hypothetical 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 (not to be confused with the terms hominins or hominini).

Hominini

The Hominini, or hominins, form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the genus Homo (humans), but excludes the genus Gorilla (gorillas). As of 2019, there is no consensus on whether it should include the genus Pan (chimpanzees and bonobos), the question being closely tied to the complex speciation process connecting humans and chimpanzees and the development of bipedalism in proto-humans.

The tribe was originally introduced by John Edward Gray (1824), long before any details on the speciation of Pan and Homo were known. Gray's tribe Hominini by definition includes both Pan and Homo. This definition is still adhered to in the proposal by Mann and Weiss (1996), which divides Hominini into three subtribes, Panina (containing Pan), Hominina ("homininans", containing Homo "humans"), and Australopithecina (containing several extinct "australopithecine" genera).Alternatively, Hominini is taken to exclude Pan. In this case, Panini ("panins", Delson 1977) may be used to refer to the tribe containing Pan as its only genus.Minority dissenting nomenclatures include Gorilla in Hominini and Pan in Homo (Goodman et al. 1998), or both Pan and Gorilla in Homo (Watson et al. 2001).

Human evolution

Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates—in particular genus Homo—and leading to the emergence of Homo sapiens as a distinct species of the hominid family, 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.The study of human evolution involves several scientific disciplines, including physical anthropology, primatology, archaeology, paleontology, neurobiology, ethology, linguistics, evolutionary psychology, embryology and genetics. Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, and the earliest fossils appear in the Paleocene, around 55 million years ago.Within the Hominoidea (apes) superfamily, the Hominidae family diverged from the Hylobatidae (gibbon) family some 15–20 million years ago; African great apes (subfamily Homininae) diverged from orangutans (Ponginae) about 14 million years ago; the Hominini tribe (humans, Australopithecines and other extinct biped genera, and chimpanzee) parted from the Gorillini tribe (gorillas) between 8–9 million years ago; and, in turn, the subtribes Hominina (humans and biped ancestors) and Panina (chimps) separated 4–7 million years ago.

Human skeletal changes due to bipedalism

The evolution of human bipedalism, which began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus, 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.

Humanoid

A humanoid (; from English human and -oid "resembling") is something that has an appearance resembling a human without actually being one. The earliest recorded use of the term, in 1870, referred to indigenous peoples in areas colonized by Europeans. By the 20th century, the term came to describe fossils which were morphologically similar, but not identical, to those of the human skeleton.Although this usage was common in the sciences for much of the 20th century, it is now considered rare. More generally, the term can refer to anything with distinctly human characteristics or adaptations, such as possessing opposable anterior forelimb-appendages (i.e. thumbs), visible spectrum-binocular vision (i.e. having two eyes), or biomechanic plantigrade-bipedalism (i.e. the ability to walk on heels and metatarsals in an upright position). Science fiction media frequently present sentient extraterrestrial lifeforms as humanoid as a byproduct of convergent evolution theory.

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. In technical terms, knuckle-walking is locomotion with the manus (Latin for hand) distally flexed on contact with the substratum.

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 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. 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 doesn’t not use the knuckles, using the backs of their hand, and using their palms.

Gorillas and chimpanzees use this style of locomotion as do anteaters and platypuses.

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.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.

Laetoli

Laetoli is a site in Tanzania, dated to the Plio-Pleistocene and famous for its hominin footprints, preserved in volcanic ash. The site of the Laetoli footprints (Site G) is located 45 km south of Olduvai gorge. The location and tracks were discovered by archaeologist Mary Leakey and her team in 1976, and were excavated by 1978. Based on analysis of the footfall impressions "The Laetoli Footprints" provided convincing evidence for the theory of bipedalism in Pliocene hominins and received significant recognition by scientists and the public. Since 1998, paleontological expeditions have continued under the leadership of Amandus Kwekason of the National Museum of Tanzania and Terry Harrison of New York University, leading to the recovery of more than a dozen new hominin finds, as well as a comprehensive reconstruction of the paleoecology.Dated to 3.7 million years ago, they were the oldest known evidence of hominin bipedalism at that time. Subsequently, older Ardipithecus ramidus fossils were found with features that suggest bipedalism. With the footprints there were other discoveries excavated at Laetoli including hominin and animal skeletal remains. Analysis of the footprints and skeletal structure showed clear evidence that bipedalism preceded enlarged brains in hominins. At a species level, the identity of the hominins who made the trace is difficult to construe precisely; Australopithecus afarensis is the species most commonly proposed.

List of humanoid aliens

This is a list of humanoid alien characters who have traits similar to that of human beings including bipedalism, opposable thumbs, facial features, etc.

Obligate

As an adjective, obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:

Obligate aerobe, an organism that cannot survive without oxygen

Obligate anaerobe, an organism that cannot survive in the presence of oxygen

Obligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphere

Obligate biped, Bipedalism designed to walk on two legs

Obligate carnivore, an organism dependent for survival on a diet of animal flesh.

Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.

Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell

Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host

Obligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions

Obligate symbionts, organisms that can only live together in a symbiosis

Tripedalism

Tripedalism (from the Latin tri = three + ped = foot) is locomotion by the use of three legs. It has been conjectured that parrots (Psittaciformes) display tripedalism during climbing gaits, though this has not yet been documented thoroughly in scientific literature. Tripedal gaits were also observed by K. Hunt in primates. This is usually observed when the animal is using one limb to grasp a carried object and is thus a non-standard gait. Apart from the parrot conjecture, there are no known species where three legs are standard, although the movement of some macropods such as kangaroos, which can alternate between resting their weight on their muscular tails and their two hind legs, may be an example of tripedal locomotion in animals. There are also the tripod fish. Several species of these fish rest on the ocean bottom on two rays from its two pelvic fins and one ray from its caudal fin.

Tripedalism contrasts with the common bipedalism of two-legged animals and quadrupedalism of four-legged animals.

Gait class
Anatomy
Specific
Taxonomy
(Hominins)
Ancestors
Models
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