Bipedalism

Bipedalism is standing, or moving for example by walking, running, or hopping, on two appendages (typically legs though it can also include hand walking). An animal or machine that usually moves in a bipedal manner is known as a biped (//), meaning "two feet" (Latin bi = two + ped = foot).



Types of bipedal 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) Hopping. Moving by a series of jumps with both feet moving together.

Bipedal animals
Bipedalism, involving the exclusive use of two legs for locomotion, has evolved multiple times in vertebrates. In reptiles, it evolved once with the thecodonts, anscestors to crocodiles, birds and dinosaurs, and once with the lizards such as the zebra-tailed lizard. Within mammals, exclusive bipedalism has evolved three times, with the kangaroos, kangaroo mice and humans. Within these groups there is still variations in locomotion - in large birds and humans locomotion occurs through walking by alternating legs. In comparison, kangaroos, smaller birds and kangaroo mice move through hopping on both legs simultaneously.

Mammals
Bipedal movement is less common among mammals, most of which are quadrupedal. The largest mammalian group using bipedal movement are the kangaroos and their relatives, which move via hopping. Other mammals also move via hopping, such as the kangaroo rat, springhare and certain primates such as the sifaka and sportive lemur. 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 bipedalism in mammals
Other mammals engage in limited, non-locomotory, bipedalism. The bonobo and proboscis monkey both live in forests that are often flooded and will wade through water in a bipedal stance; along with other primates, they will also also walk or stand bipedely on land, though generally for short periods. Two captive primates, the macaque Natasha and chimp Oliver both move bipedally. Natasha switched to exclusive bipedalism after an illness, while Oliver reverted to knuckle-walking after developing arthritis. A number of other animals, such as rats, racoons, and beavers will squat on their hindlegs in order to manipulate some objects but revert to four limbs when moving (the beaver may also move bipedally if transporting wood for their dams). Bears will fight in a bipedal stance in order to use their forelegs as weapons. Ground squirrels and meerkats will stand on hind legs to survey their surroundings, but will not walk bipedally. Dogs will stand on their hind legs if trained to do so, and some can even walk for a trick or in the case of birth defect destroying their forelimbs. 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 also use limited bipedalism when threatened, rearing up while facing the attacker so 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 certain lizards and monitor lizards will also temporarily adopt bipedalism while fighting. One genus of basilisk lizard can run bipedally across the surface of water for some distance. Bipedalism in the form of reared-up running can also be found in cockroaches, but is otherwise unknown in arthropods. Bipedalism is virtually solely found in 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 camoflage the octopus as a mat of algae or a floating coconut.

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 and the large monitor lizard) or camoflage (in certain species of octopus). Running speeds can be increased when an animal lacks a flexible backbone, though the maximum bipedal speed appears less fast than the maximum speed of quadrapedal movement with a flexible backbone - the ostrich reaches speeds of (65 km/h) and the red kangaroo (70 km/h), while the the cheetah can exceed 100 km/h.

Evolution
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.

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.

Humans
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.

Provisioning model
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.

Thermoregulatory model
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.

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, whilst others have suggested stone tools and weapons drove the change. The problem with this is that humans 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 most of their needs were to be had nearby. It is a common error to attribute to proto-humans the same capacities as modern humans.

Wading hypothesis
The Aquatic ape hypothesis 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, chimpanzee, lowland gorilla, orangutan, baboon and proboscis monkey. Bipedal wading provides the advantage of keeping the head above water for breathing.

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, apparently because it is less suited for swift movement. On the other hand, if humans developed their bipedalism at the water's edge, they wouldn't have been severely affected by receding forests.

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

Biomechanics
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.
 * Standing

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:
 * Walking
 * 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.
 * Running


 * Hopping

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 even the well-developed biceps of the arms.

Nervous system
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.

Respiration
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.

Bipedal robots
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 and QRIO.