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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 (/'bi.pɛd/), meaning "two feet" (Latin bi = two + ped = foot).
Diversity and evolution of bipedalism
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 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, 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. Bipealism has also evolved in the crocodilian line, a sister group to the dinosaurs and birds - a crocodile relative from the triassic, Effigia okeeffeae, was believed to be bipedal . 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 also from birds and theropods. There are also various groups of hopping rodents, such as the kangaroo rats. A primate, the sifaka, also moves by hopping when on the ground. Possibly the only mammal other than humans that commonly moves bipedally by an alternating gait rather than hopping is the giant pangolin.
Limited examples of bipedalism are found in some other mammals. One exception is found among some primates who live in forests that are often flooded, the bonobo ape and proboscis monkey. These primates 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 animals, such as rats, will squat on their hindlegs in order to manipulates 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 rhino/gorilla-like chalicotheres may also have behaved similarly.
Among the non-archosaur reptiles bipedalism is rare, and it is unknown among the amphibians, however it its 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. 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 almost exclusively terrestrial animals. However, at least two types of octopus are known to walk bipedally. This form of locomotion appears to allow them to remain somewhat camouflaged while moving quickly, taking a form like a coconut or seaweed and moving on the tips of two of its arms.
Many animals that do not use bipedal locomotion in nature can be trained to walk on hind legs. This includes dogs, elephants, horses and pretty much every mammal or reptile that has 4 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.
Some animals can also be trained to walk on front limbs, although this method lacks any practical benefits, save for gymnastic versatility for spectacle. Humans too, can learn to walk using their arms for aid, or solely their arms (handstand). This is unusual and requires mental and physical training, like many physical movements, otherwise it can result in injury due to lack of protection by atrophied or unbalanced muscles not developed for or genetically inclined for the movement.
Some unusual individual primates have been known to be bipedal. There has been one recorded case of a macaque 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.
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. Raccons and some primates may adopt a bipedal position in water, allowing them to stand or walk in deeper water while still breathing air.
- Faster speed. 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 ( 120 km/h).
- Greater reach. Gerunuk antelope adopt a bipedal position to browse the leaves from trees.
- Camoflage. Bipedalism in octopuses has been speculated to allow them to move while keeping the rest of their body still for camoflage.
There are many hypotheses as to how and why bipedelism evolved in humans, and also some debate as to when. Evidence point to it 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.”
One of the most elaborative 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. Because this theory is complex, many criticisms arise. First, all evidence indicates that early hominids, which are proven to be bipedal, were polygamous. Second, 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. This theory has too much evidence going against it for it to be considered a viable origin of bipedalism.
The thermoregulatory model explaining the origin of bipedalism is one of the simplest 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 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.
This theory proposes that humans evolved bipedalism as a result of bipedal wading. Bipedal wading is found among the semi-bipedal wading cousins of humans, the bonobo chimps, the lowland gorillas, and proboscis monkeys. Bipedal wading provides the advantage of keeping the head above water for breathing. This theory is part of a general theory of human evolution which often goes by the name of the aquatic ape hypothesis. Kuliakas 2001 argues that the skeletal morphology of the early hominan Australopithecus afarensis is consistent with adaptation for wading in water.
Turn-Over Pulse Hypothesis
The theory is part of a general theory of human evolution known as the savannah 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.
- Kuliukus, A., "Wading for Food: The Driving Force of the Evolution of Bipedalism." Nutrition and Health, 16(4), 267-290, (2002). html
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 intruption 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 nearly inevitable fall for the duration of another step.
Bipedalism requires strong leg muscules, particulary in the thighs. Contrast in domesticated poultry the meaty drumstick and thigh, against the small and bony wing. 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 any 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. Increasingly cheap and compact computing power, however, has made two-legged robots more feasible. In recent years, Honda and Sony have developed these machines.
- Study pushes bipedalism back 2 million years
- Video of Honda's humanoid robot Asimo running (Dec 16 2004) (see also Asimo)
- 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