Bipedalism is standing, or moving for example by walking, running, or hopping, on two appendages (typically legs). An animal or machine that usually moves in a bipedal manner is known as a biped (/'baɪ.pɛd/), meaning "two feet" (Latin bi = two + ped = foot).
Types of bipedal movement
There are a number of states of movement commonly associated with bipedalism.
- Standing. Staying still on both legs. In most bipeds this is an active process, requiring constant adjustment of balance.
- Walking. One foot in front of another, with at least one foot on the ground at any time.
- Running. One foot in front of another, with periods where both feet are off the ground.
- Hopping. Moving by a series of jumps with both feet moving together.
Bipedal movement has evolved a number of times other than in humans, mostly among the vertebrates. The most obvious example of bipedal movement is among the birds and their ancestors the theropod dinosaurs. All dinosaurs are believed to be descended from a fully bipedal ancestor, perhaps similar to Eoraptor. Indeed, among their descendants, the larger flightless birds, the ratites, such as the ostrich, perhaps epitomise the capacity to move bipedally, able to reach speeds of up to 65 km/h. Likewise many theropod dinosaurs, especially the maniraptors, are believed to have been able to move at similar speeds. 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, have also evolved bipedal forms - a crocodile relative from the triassic, Effigia okeeffeae, was believed to be bipedal . Larger birds tend to walk with alternating legs, whereas smaller birds will often hop. Penguins are interesting birds with regard to bipedality as they tend to hold their bodies upright, rather than horizontal as in other birds.
Bipedal movement is less common among mammals, most being quadrupedal. The largest mammalian group using bipedal movement are the kangaroos and their relatives. However these tend to move mostly by hopping, which is quite different from humans and many birds. There are also various groups of hopping rodents, such as the kangaroo rats and springhares. Some primates, such as sifakas and sportive lemurs, also moves by hopping when on the ground. Possibly the only mammals other than humans that commonly move bipedally by an alternating gait rather than hopping are gibbons when on the ground, and giant pangolins.
Limited examples of bipedalism are found in some other mammals. For example the bonobo ape and proboscis monkey, who both live in forests that are often flooded, will wade through water in a bipedal stance. On occasion bonobos and proboscis monkeys, and less frequently some other primates, will also walk or stand bipedely on land. A number of other animals, such as rats, will squat on their hindlegs in order to manipulate food objects. The raccoon often stands erect or squats in water to use its hands to manipulate food and rocks/sticks. Beavers will also move bipedally at times when carrying branches. Some animals, such as the bear, may raise up and move bipedally during physical confrontation, so as to better be able to use their forelegs as weapons. Also a number of mammals, such as ground squirrels or meerkats will stand on their hind legs, but not walk on them, in order to survey their surroundings. Finally, gerenuk antelope are known to stand on their hind legs in order to eat leaves from trees. The extinct giant ground sloth had hip joints whose form indicates that they also did this. Another extinct group, the bizarre rhinoceros/gorilla-like chalicotheres may also have behaved similarly. One unusual form of limited bipedalism is the spotted skunk which when threatened stands on its forelimbs. This allows the skunk, while still facing the attacker, to direct its anal glands towards the attacker. The anal glands can fire an offensively odorous oil.
Bipedalism is unknown among the amphibians. Among the non-archosaur reptiles bipedalism is rare, but it is found in the "reared-up" running of certain lizards. An interesting example is found in at least one genus of basilisk lizard that by this method can run across the surface of water for some distance. Bipedalism in the form of reared-up running can also be found in some insects such as the cockroach. Otherwise bipedal movement is unknown in arthropods. Bipeds are mostly terrestrial animals. However, at least two types of octopus are known to walk bipedally  on the sea floor, moving on the tips of two of their arms. According to the study, this form of locomotion may allow them to remain somewhat camouflaged while moving quickly, as the animals are still able to use the remaining six arms for disguise (taking a form like a coconut or seaweed), and as the necessary walking movements are hypothesized to be possible with minimal control from the brain.
Many animals that do not use bipedal locomotion in nature can be trained to walk on their hind legs. Animals missing limbs due to injury or congenital deformity may adapt to bipedal motion, either on two hind legs or on one front and one back leg. For videos of both kinds of bipedal motion in dogs see  and . Some unusual individual primates have also been known to be bipedal. There has been one recorded case of a captive macaque, Natasha, switching to bipedal walking completely after recovering from a serious illness , and at least one example of a captive chimp who only walked upright, Oliver. Some animals can also be trained to walk on their front limbs. Humans can learn to walk using solely their arms, see handstand and hand walking.
Bipedalism and associated traits can offer a species several advantages:
- Improved perception. Some evolutionary biologists have suggested that a crucial stage in the evolution of some or all bipeds was the ability to stand, which generally improves the ability to see (and perhaps otherwise detect) distant dangers or resources.
- Free forelimbs. In vertebrate species, for whom evolution of additional limbs would be an enormous genetic change, it can serve to free the front limbs for such other functions as manipulation (in primates), flight (in birds), digging (giant pangolin), or combat (bears).
- Wading. Raccoons and some primates may adopt a bipedal position in water, allowing them to stand or walk in deeper water while still breathing air.
- Faster movement. In animals without a flexible backbone, such as lizards or cockroaches, bipedalism may increase running speed. However the maximum bipedal speed appears less fast than the maximum speed of quadrapedal movement with a flexible backbone - compare the fastest bipeds the ostrich (65 km/h) or the red kangaroo (70 km/h) with the fastest quadruped, the cheetah (103 km/h).
- Greater reach. Gerunuk antelope adopt a bipedal position to browse the leaves from trees.
- Camouflage. It has been speculated that bipedalism in octopuses allows them to move while keeping the rest of their bodies still for camouflage.
- Face attacker while directing anal glands. The defense posture of the spotted skunk, which involves walking on its forelimbs, allows the skunk to face the attacker while simultaneously directing its anal glands at them. The anal glands can squirt an offensive smelling oil.
Bipedalism has a number of adaptive advantages, and has evolved independently in a number of lineages.
Early reptiles and lizards
The first known biped is the bolosaurid Eudibamus cursoris whose fossils date from 290 million years ago  . Its long hindlegs, short forelegs, and distinctive joints all suggest bipedalism. This may have given increased speed. The species was extinct before the dinosaurs appeared.
Independent of Eudibamus, some modern lizard species have developed the capacity to run on their hind legs for added speed. The praire dog effect where to stand and look around for such thing as predators or food is an evolution adaptation of early human.
Dinosaurs and birds
Bipedalism also developed 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. Radiometric dating of fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists believe Eoraptor resembles the common ancestor of all dinosaurs; if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. 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 (excluding humans)
A number of mammals will adopt a bipedal stance in specific situations such as for feeding or fighting. A number of groups of extant mammals have independently evolved bipedalism as their main form of locomotion - for example humans, giant pangolins, 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.
There are at least twelve distinct hypotheses as to how and why bipedalism evolved in humans, and also some debate as to when. Evidence points to bipedalism evolving before the expansion in human brain size. The different hypotheses are not necessarily mutually exclusive and a number of selective forces may have acted together to lead to human bipedalism.
Postural feeding hypothesis
The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at Indiana University. This theory asserts that chimpanzees were only bipedal when they ate. While on the ground, they would reach up for fruit hanging from small trees and while in trees, bipedalism was utilized by grabbing for an overhead branch. These bipedal movements may have evolved into regular habits because they were so convenient in obtaining food. Also, Hunt theorizes 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. As Hunt says, “A bipedal postural feeding adaptation may have been a preadaptation for the fully realized locomotor bipedalism apparent in Homo erectus.” A related hypothesis is that proto-humans learned upright posture not for picking fruit, as it is argued they would have stayed climbers if plucking fruit were all they were after, rather they learned to keep they head out of the water while searching for water plants, mollusca, and the like.
One of the most elaborate theories on the origin of bipedalism is the behavior model presented by C. Owen Lovejoy. Lovejoy theorizes that the evolution of bipedalism was a response to a monogamous society. As hominid males became monogamous, they would leave their families for the day in order to search for food. Once they found food for their family, the hominids would have to bring back the food, and the most effective way of doing this was through bipedalism. Some question whether early hominids really were monogamous though. Some evidence indicates that early hominids, which are proven to be bipedal, were polygamous. Among all monogamous primates, sexual dimorphism is mostly absent, but in Australopithecus afarensis males were found to be nearly twice the weight of females, an attribute scientists would expect in a polygamous species. Lastly, monogamous primates are highly territorial, but fossil evidence indicates that Australopithecus afarensis lived in large groups.
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. All these theories lack any observational data, and are in the category of "armchair" theories.
The thermoregulatory model explaining the origin of bipedalism is one of the simplest and most fanciful theories on the table, 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. 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.
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, whilst others have suggested stone tools and weapons drove the change. The problem with this is that people were bipedal long before they had tools and weapons to carry. Furthermore, it doesn't appear that early proto-humans would have had to carry anything very far, as must of their needs were to be had nearby. It is a common error to attribute to proto-humans the same capacities as modern humans.
This theory proposes that humans evolved bipedalism as a result of bipedal wading. Mammals that switch from quadrupedalism on land to bipedal wading appear mainly to be found among large primates, especially apes, with relatively few exceptions such as the grizzly bear. Bipedal wading has been observed in the bonobo, chimpanzees, the lowland gorillas, orang utans, baboons and proboscis monkey. Bipedal wading provides the advantage of keeping the head above water for breathing. This theory is a refinement of a general theory of human evolution which often goes by the name of the aquatic ape hypothesis. Kuliukas 2001 argues that the skeletal morphology of the early hominan Australopithecus afarensis is consistent with adaptation for wading in water and that their habitats were probably sufficiently prone to flooding for this behaviour to have been selected for. Furfthermore, all hominid remains have been found deposited in wet conditions, though that's also true of most fossils. The fossil record supports the wading theory. Likewise, all humans still live within walking distance of potable water, having been unable to escape our physiological needs; this being so, Occams Razor suggests that we have never left the water which shaped us.
Turn-over pulse hypothesis
The theory is part of a general theory of human evolution known as the savanna hypothesis. This theory asserts that a major climate change occurred which induced an onset of drier conditions. These dry conditions severely reduced the amount of wooded habitats in the Pliocene era, about 2.5 million years ago. During this period where the forests became thin, the Australopithecus organisms had to evolve and change their habitats from the forest to grasslands. In order to remain effective in gathering food, the hominids had to travel long distances with food or tools, thus making quadrupedalism extremely inefficient. These hominids evolved into bipeds which made their treks along the grasslands much more efficient. All other theories of bipedalism, aside from the "wading hypothesis" are derivatives of this one. Some of the problems related to this theory have to do with dates. Bipedalism is evident in Australopithecus afarensis a million years before the thinning of the forests in question. It would be a million-and-a-half years or more before these proto-humans would have weapons sufficient to defend themselves or bring down large game. Midden piles from the time suggest a diet rich in turtles. Also, the savannas into which the humans were supposed to have gone, had limited edible vegetation, and an omnivore, like a human, would most likely head for a more diverse food supply, such as the eco-edge between the land and water. The few large apes which have adapted to savanna living have all become dietary specialists surviving on a single plant, the very opposite of an omnivore. None of those apes has developed bipedalism, because as the ones who tried it were eaten, leaving the swift four-legged creatures to survive. On the other hand, if humans developed their bipedalism at the water's edge, they wouldn't have been severely affected by receding forests.
- 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).
- Hardy, Alistair Hardy, "Was Man More Aquatic in the PAst?," The New Scientis, 17 March 1960, pp. 642-645, U. of Victoria (1960)
- 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. "Origin of Habitual Terrestrial Bipedalism in the Ancestor of the Hominidae", Journal of Human Evolution, 24, 259-280, (1993).
- Kuliukas, A., "Wading for Food: The Driving Force of the Evolution of Bipedalism." Nutrition and Health, 16(4), 267-290, (2002). html
- Lovejoy, C. O., "The Origin of Man." Science, 211, 341-350, (1981).
- Morgan, E., "The Aquatic Ape: A theory of Human Evolution," Souvenir Press, London (1982).
- Tanner, N.M., "On Becoming Human", Cambridge University Press (Cambridge), (1981).
- Wescott, R.W., "Hominid Uprightness and Primate Display", American Anthropologist, 69, 738,(1967).
- Wheeler, P. E., "The Evolution of Bipedality and Loss of Functional Body Hair in Hominoids." Journal of Human Evolution, 13, 91-98, (1984).
- Vrba, E., "The Pulse that Produced Us." Natural History, 102(5), 47-51, (1993).
Physiology of bipedalism
Bipedal movement occurs in a number of ways, and requires many mechanical and neurological adaptations. Some of these are described below.
Engineers who study bipedal walking or running describe it as a repeatedly interrupted fall. The phenomenon of "tripping" is informative with regards to the "controlled falling" concept of walking and running. The common way to think of tripping is as pulling a leg out from under a walker or runner. In fact, however, merely stopping the movement of one leg of a walker, and merely slowing one leg of a runner, is sufficient to amount to tripping them. They were already "falling", and preventing the tripped leg from aborting that fall is sufficient to cause bipeds to collapse to the ground.
Energy-efficient means of standing bipedally involve constant adjustment of balance, and of course these must avoid overcorrection.
Efficient walking is more complicated than standing. It entails tipping slightly off-balance forward and to the side, and correcting balance with the right timing. In humans, walking is composed of several separate processes:
- rocking back and forth between feet
- pushing with the toe to maintain speed
- combined interruption in rocking and ankle twist to turn
- shortening and extending the knees to prolong the "forward fall"
Running is an inherently continuous process, in contrast to walking; a bipedal creature or device, when efficiently running, is in a constant state of falling forward. This is maintained as relatively smooth motion only by repeatedly "catching oneself" with the right timing, but in the case of running only delaying the otherwise inevitable fall for the duration of another step.
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 even the well-developed biceps of the arms.
The famous knee jerk (or patellar reflex) emphasizes the necessary bipedal control system: the only function served by the nerves involved being connected as they are is to ensure quick response to imminent disturbance of erect posture; it not only occurs without conscious mental activity, but also involves none of the nerves which lead from the leg to the brain.
A less well-known aspect of bipedal neuroanatomy can be demonstrated in human infants who have not yet developed toward the ability to stand up. They can nevertheless run with great dexterity, provided they are supported in a vertical position and offered the stimulus of a moving treadmill beneath their feet.
A biped also has the ability to breathe whilst it runs. Humans usually take a breath every other stride when their aerobic system is functioning. During a sprint, at which point the anaerobic system kicks in, breathing slows until the anaerobic system can no longer sustain a sprint.
For nearly the whole of the 20th century, bipedal robots were very difficult to construct. Robots which could move usually did so using wheels, treads, or multiple legs (see robot locomotion). Increasingly cheap and compact computing power, however, has made two-legged robots more feasible. Some notable biped robots are ASIMO, developed by Honda, HUBO and Albert Einstein HUBO developed by KAIST and QRIO, developed by Sony.
- Berman, David S.; et al. (2000). "Early Permian Bipedal Reptile". Science. 290 (5493): 969–972.
- Citation for Permian/Triassic extinction event, percentage of animal species that went extinct. See commentary
- Another citation for P/T event data. See commentary
- Hayward, T. (1997). The First Dinosaurs. Dinosaur Cards. Orbis Publishing Ltd. D36040612.
- Sereno, P.C., C.A. Forster, R.R. Rogers, and A.M. Monetta. 1993. Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria. Nature 361:64-66.
- Angela Burk, Michael Westerman, Mark Springer. (1998) The Phylogenetic Position of the Musky Rat-Kangaroo and the Evolution of Bipedal Hopping in Kangaroos (Macropodidae: Diprotodontia) Systematic Biology, Vol. 47, No. 3 , pp. 457-474
- Study pushes bipedalism back 2 million years
- True stories of human children raised by animals, mostly wolves, who cannot walk on two legs, mimicking almost exactly the culture and lifestyle, instincts and habits of the particular animal that raises them
- Information about bipedal octopuses, with link to original paper and videos
- Why australopithecines became bipedal
- Time-warp family who walk on all fours
- The dawn of man, article on the evolution of bipedalism
- Comparative bipedalism - how the rest of the animal kingdom walks on two legs
- Video of Faith, a dog born without her front legs and trained to walk upright
- Video of Dominic, a greyhound adapted to non-upright bipedal motion after losing both right legs
- Video of Honda's humanoid robot Asimo running (Dec 16 2004)
- Albert Einstein Hubo: by Hanson Robotics and KAIST
- Two fighting Common Indian Monitor Lizard males "standing" make the cover of "Sanctuary Asia Magazine"
- The Story behind the Picture - "Monitor Lizards Combat" from the website of Jayanth Sharma's "WILDLIFE TIMES"