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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 (Template:PronEng), meaning "two feet" (from the Latin bi for "two" and ped for "foot"). Types of bipedal movement include walking, running, or hopping, on two appendages (typically legs).

Relatively few modern species are habitual bipeds whose normal method of locomotion is two-legged. Within mammals, habitual bipedalism has evolved four times, with the macropods, kangaroo mice, springhare and homininan apes. In the Triassic period some groups of archosaurs (a group that includes the ancestors of crocodiles) developed bipedalism; among their descendants the dinosaurs all the early forms and many later groups were habitual or exclusive bipeds; the birds descended from one group of exclusively bipedal dinosaurs.

A larger number of modern species are capable of bipedal movement for a short time in exceptional circumstances. Several non-archosaurian lizard species move bipedally when running, usually to escape from threats. Many animals rear up on their hind legs while fighting. A few animals commonly stand on their hind legs, in order to reach food or to keep watch, but do not move bipedally.

There are two main types of bipedal locomotion: macropods, some smaller birds, and heteromyid rodents move by hopping on both legs simultaneously; other groups, including apes and larger birds, walk or run by moving one leg at a time.

Definition

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

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 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. 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.

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. Among mammals, bipedalism is a normal method of ground locomotion in various groups of primates (e.g. lemurs, gibbons and Hominina), in the macropods (kangaroos, wallabies, etc.) and in a few groups of rodents (kangaroo mice, spring hares). All birds are bipeds when on the ground, a feature inherited from their dinosaur ancestors. Bipedalism evolved more than once in archosaurs, the group that includes both dinosaurs and crocodilians. A few lizards can also move bipedally, but only in emergencies. There are no known living or fossil bipedal amphibians.

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, among primates that move bipedally, only the lemurs have tails.

Humans and large birds walk by raising one foot at a time. On the other hand most macropods, smaller birds and bipedal rodents move by hopping on both legs simultaneously. Tree kangaroos are able to utilize either form of locomotion, most commonly alternating feet when moving arboreally and hopping on both feet simultaneously when on the ground.

Dinosaurs and their descendants

All dinosaurs are believed to be descended from a fully bipedal ancestor, perhaps similar to Eoraptor. It is believed that maniraptors were able to reach speeds of up to 65 km/h moving bipedally, comparable to their large, flightless-bird descendants such as the ostrich. 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, was believed to be bipedal.

Mammals

Bipedal movement is less common among mammals, most of which are quadrupedal. All primates possess some bipedal ability, though non-human primates primarily use quadrupedal locomotion on land. Primates aside, the largest mammalian group using exclusive bipedal movement are the macropods (kangaroos, wallabies 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 primates that commonly move bipedally by an alternating gait rather than hopping is the ground pangolin.

Primates

Primates can be distinguished from other quadrupedal mammals as they exhibit a greater diversity in locomotor behaviors. These include arm swinging (brachiation), quadrumanous climbing, knuckle walking, and regular short bouts of bipedalism. In addition, quadrupedal locomotion in primates also exhibits significant differences from other mammals. These differences in gait characteristics are primarily adaptations to an arboreal environment. All primates can sit upright. Many primates can stand upright on their hind legs without any support. Chimpanzees, bonobos, gibbons and baboons exhibit relatively advanced forms of bipedalism. Injured chimpanzees and bonobos have been capable of sustained bipedalism.

Primates that live in tropical areas often wade through water in a bipedal stance. Bonobos, proboscis monkeys and baboons have been observed wading bipedally.Three captive primates, one macaque Natasha and two chimps, Oliver and Poko, were found to move bipedally. Natasha switched to exclusive bipedalism after an illness, while Poko was discovered in captivity in a tall, narrow cage. Oliver reverted to knuckle-walking after developing arthritis.

In addition, non-human primates often use bipedal locomotion when carrying food. One hypothesis for human bipedalism is thus that it evolved as a result of differentially successful survival from carrying food to share with group members, although there are other hypotheses, as below.

Limited bipedalism in mammals

Other mammals engage in limited, non-locomotory, bipedalism. 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 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 also use limited bipedalism when threatened, rearing up on its forelimbs 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 lizards such as agamids and monitor lizards. Many reptile species will also temporarily adopt bipedalism while fighting. 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 . 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 camouflage 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 camouflage (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 cheetah can exceed 100 km/h.

Evolution

Recent evidence regarding modern human sexual dimorphism (physical differences between men and women) 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 better bear lumbar load during pregnancy, an adaptation that non-bipedal primates would not need to make.

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 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 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. 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. Bipedalism evolved well before the large human brain or the development of stone tools. Bipedal specializations are found in australopithecus fossils from 4.2-3.9 million years ago 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.

Various reasons have been proposed for the evolution of human bipedalism, including freeing the hands for tool use and carrying, sexual dimorphism in food gathering, changes in climate and habitat (from jungle to savanna) and to reduce the amount of skin exposed to the tropical sun. The first two explanations have been criticized for projecting modern social concerns and prejudices onto ancestral species. The latter two have been criticized for not making sense in the context of the forest and woodland biomes occupied by human ancestors. An alternative explanation is the mixture of savanna and scattered forests forced proto-humans to travel between clusters of trees and bipedalism offered greater efficiency for slow, long-distance travel between these clusters than knuckle-walking quadrupedism.

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.” {{citation}}: Empty citation (help)

Provisioning model

One theory on the origin of bipedalism is the behavioral model presented by C. Owen Lovejoy, known as "male provisioning". Lovejoy theorizes that the evolution of bipedalism was a product of monogamy. As hominid males became monogamous, Lovejoy contends, they would leave their mates and offspring for the day to search for food. Once they found food for their family, the male hominids would return carrying the food in their arms and walking on their hind legs.

There is no particular evidence, however, that early hominids were monogamous. And some evidence indicates that early bipedal hominids were in fact polygamous. Among all monogamous primates, males and females are about the same size. That is sexual dimorphism is minimal. In Australopithecus afarensis, males were thought to be nearly twice the weight of females (as well as a great deal taller), which suggests that they were polygamous. Modern monogamous primates are also highly territorial, but fossil evidence indicates that Australopithecus afarensis lived in large groups. There is likewise no evidence that female hominids did not forage themselves. Early hominids did not have the large brains that require that infants be born premature and helpless. Females in ape species similar to early hominids do not wait for food to be brought to them. In short, there is no direct evidence to support either monogamy or polygamy in early hominids and indirect evidence points to polygamy.

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.

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. 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.

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.

Aquatic ape hypothesis

The aquatic ape hypothesis proposes that humans evolved bipedalism as a result of bipedal wading but the theory does not have any scientific support and is primarily an exercise in comparative anatomy and speculation.

Savannah hypothesis

This proposes that the onset of drier conditions severely reduced the amount of wooded habitats. During this period, when the forests became thin, early hominids adapted to an environment which was now more like the liminal forest-savanna mosaic zones of equatorial Africa. The theory states that, in order to remain effective in gathering food, the hominids had to travel relatively long distances with food or tools, thus making quadrupedalism extremely inefficient. 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 drier conditions ecological niche, rather than the drier conditions being hypothetically bipedalism's initial stimulus. Bipedalism has been suggested as being further developed to give early hominids use of their hands for food cultivation and tool-use since they were no longer needed for locomotion.

Physiology

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.

Standing

Energy-efficient means of standing bipedally involve constant adjustment of balance, and of course these must avoid overcorrection.

Walking

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

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.

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.

This is not necessarily an advantage over quadrupeds, as not only can many quadrupeds breathe while running, but in mammals such as dogs, the act of running helps to expand and contract the lungs. The muscles of the trunk thus perform locomotive and respiratory tasks at the same time, making breathing while running more efficient in these animals than in bipeds.

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.

Bipedal molecule

In 2005, chemists at the University of California, Riverside developed the first bipedal molecule, 9,10-Dithioanthracene, which propels itself in a straight line when heated on a flat copper surface. Researchers believe the molecule has potential for use in molecular computers.

See also

• Quadrupedalism
• Orthograde posture

Notes

1. NC Heglund, GA Cavagna and CR Taylor 1982 Energetics and mechanics of terrestrial locomotion. III. Energy changes of the centre of mass as a function of speed and body size in birds and mammals Journal of Experimental Biology 97:1
2. Dhingra, Philip (2004-05-25). "Comparative bipedalism: How the rest of the animal kingdom walks on two legs". Retrieved 2007-10-29.
3. "Human Hand-Walkers: Five Siblings Who Never Stood Up" (PDF). Centre for Philosophy of Natural and Social Science, London School of Economics. 2005. {{cite web}}: Unknown parameter |authors= ignored (help)
4. Hutchinson, J.R. (2006). "The evolution of locomotion in archosaurs". Comptes Rendus Palevol. 5 (3–4): 519–530. doi:10.1016/j.crpv.2005.09.002.
5. Handwerk, Brian (2006-01-26). "Dino-Era Fossil Reveals Two-Footed Croc Relative". National Geographic. Retrieved 2007-10-29.
6. Fleagle, J.G (1994). "Primate locomotion and posture". The Cambridge Encyclopedia of Human Evolution. Cambridge: Cambridge University Press. pp. 75–79. ISBN 0-521-3270-3. {{cite book}}: Check |isbn= value: length (help); Unknown parameter |editors= ignored (|editor= suggested) (help) Also ISBN-0-521-46786-1 (paperback)
7. Schmitt, Daniel (2003). "Insights into the evolution of human bipedalism from experimental studies of humans and other primates". Journal of Experimental Biology. 206: 1437. doi:10.1242/jeb.00279.
9. Bauer, Harold (1976). "Chimpanzee bipedal locomotion in the Gombe National Park, East Africa". Primates. 18: 913. doi:10.1007/BF02382940.
10. Waldman, Dan (2004-07-21). "Monkey apes humans by walking on two legs". MSNBC. Retrieved 2007-10-29.
11. University of Liverpool - Research Intelligence Issue 22 - Walking tall after all
12. Tetrapod Zoology : Bipedal orangs, gait of a dinosaur, and new-look Ichthyostega: exciting times in functional anatomy part I
13. Bipedality in chimpanzee (Pan troglodytes) and bonobo (Pan paniscus): Testing hypotheses on the evolution of bipedalism
14. Sharma, Jayanth (2007-03-08). "The Story behind the Picture - Monitor Lizards Combat" (php). Wildlife Times. Retrieved 2007-10-29.
15. Huffard CL, Boneka F, Full RJ (2005). "Underwater bipedal locomotion by octopuses in disguise". Science. 307 (5717): 1927. doi:10.1126/science.1109616. PMID 15790846.{{cite journal}}: CS1 maint: multiple names: authors list (link)
16. The Independent's article A pregnant woman's spine is her flexible friend, by Steve Connor from The Independent (Published: 13 December 2007) quoting Shapiro, Liza, University of Texas at Austin Dept. of Anthropology about her article, Whitcome, et al., Nature advance online publication, doi:10.1038/nature06342 (2007).
17. Why Pregnant Women Don't Tip Over. Amitabh Avasthi for National Geographic News, December 12, 2007. This article has good pictures explaining the differences between bipedal and non-bipedal pregnancy loads.
18. "Upright lizard leaves dinosaur standing". cnn.com. 2000-11-03. Retrieved 2007-10-17.
19. Berman, David S.; et al. (2000). "Early Permian Bipedal Reptile". Science. 290 (5493): 969–972. doi:10.1126/science.290.5493.969. PMID 11062126. {{cite journal}}: Explicit use of et al. in: |author= (help)
20. Citation for Permian/Triassic extinction event, percentage of animal species that went extinct. See commentary
21. Another citation for P/T event data. See commentary
22. Hayward, T. (1997). The First Dinosaurs. Dinosaur Cards. Orbis Publishing Ltd. D36040612.
23. Sereno, Paul C. (1993). "Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria". Nature. 361: 64–66. doi:10.1038/361064a0. Retrieved 2008-06-28. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
24. Burk, Angela (1988). "The Phylogenetic Position of the Musky Rat-Kangaroo and the Evolution of Bipedal Hopping in Kangaroos (Macropodidae: Diprotodontia)". Systematic Biology. 47 (3): 457–474. doi:10.1080/106351598260824. PMID 12066687. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
25. Lovejoy, C.O. (1988). "Evolution of Human walking". Scientific American. 259 (5): 82–89.
26. McHenry, H.M (2009). "Human Evolution". Evolution: The First Four Billion Years. Cambridge, Massachusetts: The Belknap Press of Harvard University Press. p. 263. ISBN 978-0-674-03175-3. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
27. Lewin, Roger; Swisher, Carl Celso; Curtis, Garniss H. (2000). Java man: how two geologists' dramatic discoveries changed our understanding of the evolutionary path to modern humans. New York: Scribner. ISBN 0-684-80000-4.{{cite book}}: CS1 maint: multiple names: authors list (link)
28. T. Douglas Price, Gary M. Feinman (2003). Images of the Past, 5th edition. Boston: McGraw Hill. p. 68. ISBN 978-0-07-340520-9. {{cite book}}: Cite has empty unknown parameter: |month= (help)
29. Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Souvenir Press. ISBN 0-285-63518-2.
30. Bridgeman, B (2003). Psychology & evolution: the origins of mind. SAGE Publications. pp. 64. ISBN 0761924795.
31. Meier, R (2003). The complete idiot's guide to human prehistory. Alpha Books. pp. 57-59. ISBN 0028644212.
32. Kwon KY, Wong KL, Pawin G, Bartels L, Stolbov S, Rahman TS. (2005). Unidirectional adsorbate motion on a high-symmetry surface: "walking" molecules can stay the course. Phys Rev Lett. 95(16):166101. PMID 16241817

References

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• Locomotion