Editing Bipedalism (section)

==Evolution of human bipedalism==
{{Human timeline}}
{{Main|Human timeline|Human skeletal changes due to 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.<ref>{{Cite journal |author=Lovejoy, C.O. |title=Evolution of Human walking |journal=Scientific American |volume=259 |issue=5 |pages=82–89 |year=1988 |doi=10.1038/scientificamerican1188-118 |pmid=3212438 |bibcode=1988SciAm.259e.118L}}</ref> Bipedal specializations are found in ''[[Australopithecus]]'' fossils from 4.2 to 3.9 million years ago and recent studies have suggested that obligate bipedal hominid species were present as early as 7 million years ago.<ref name="Daver Guy Mackaye Likius 2022"/><ref>{{Cite book |title=Evolution: The First Four Billion Years |last=McHenry |first=H. M. |chapter=Human Evolution |editor=Michael Ruse |editor2=Joseph Travis |year=2009 |publisher=The Belknap Press of Harvard University Press |location=Cambridge, MA |isbn=978-0-674-03175-3 |page=[https://archive.org/details/evolutionfirstfo00mich/page/263 263] |chapter-url=https://archive.org/details/evolutionfirstfo00mich/page/263}}</ref> 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.<ref>{{Cite web |first=Erin |last=Wayman |date=August 6, 2012 |url=http://www.smithsonianmag.com/science-nature/becoming-human-the-evolution-of-walking-upright-13837658/?no-ist |title=Becoming Human: The Evolution of Walking Upright |website=Smithsonian.com |archive-url=http://archive.wikiwix.com/cache/20141003092005/http://www.smithsonianmag.com/science-nature/becoming-human-the-evolution-of-walking-upright-13837658/?no-ist |archive-date=October 3, 2014 |url-status=live}}</ref> 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.<ref>{{cite web |url=http://news.independent.co.uk/sci_tech/article3247561.ece |author=Steve Connor |date=13 December 2007 |title=A pregnant woman's spine is her flexible friend |work=The Independent |archive-date=2007-12-15 |archive-url=https://web.archive.org/web/20071215193431/http://news.independent.co.uk/sci_tech/article3247561.ece |url-status=dead}} Quoting {{Cite journal |doi=10.1038/nature06342 |volume=450| issue=7172 |title=Fetal load and the evolution of lumbar lordosis in bipedal hominins |journal=Nature |pages=1075–1078 |pmid=18075592 |date=December 2007 |vauthors=Whitcome KK, Shapiro LJ, Lieberman DE |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:3743553| bibcode=2007Natur.450.1075W |s2cid=10158}}</ref><ref>{{cite web |date=December 12, 2007 |author=Amitabh Avasthi |title=Why Pregnant Women Don't Tip Over |work=National Geographic News |url=http://news.nationalgeographic.com/news/2007/12/071212-pregnancy-tips_2.html |url-status=dead |archive-url=https://web.archive.org/web/20080911031547/http://news.nationalgeographic.com/news/2007/12/071212-pregnancy-tips_2.html |archive-date=2008-09-11}} This article has good pictures explaining the differences between bipedal and non-bipedal pregnancy loads.</ref> 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.<ref>{{Cite journal |last1=Sylvester |first1=Adam D. |year=2006 |title=Locomotor Coupling and the Origin of Hominin Bipedalism |journal=Journal of Theoretical Biology |volume=242 |issue=3 |pages=581–590 |doi=10.1016/j.jtbi.2006.04.016 |pmid=16782133 |bibcode=2006JThBi.242..581S}}</ref> 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.

The form and function of modern-day humans' upper bodies appear to have evolved from living in a more forested setting. Living in this kind of environment would have made it so that being able to travel arboreally would have been advantageous at the time. Although different to human walking, bipedal locomotion in trees was thought to be advantageous.<ref>{{Cite journal |last=Kimura |first=Tasuku |date=2019 |title=How did humans acquire erect bipedal walking? |url=https://www.jstage.jst.go.jp/article/ase/127/1/127_190219/_article |journal=Anthropological Science |volume=127 |issue=1 |pages=1–12 |doi=10.1537/ase.190219|s2cid=132162687 |doi-access=free }}</ref> It has also been proposed that, like some modern-day apes, early hominins had undergone a [[knuckle-walking]] stage prior to adapting the back limbs for bipedality while retaining forearms capable of [[grasp]]ing.<ref>{{Cite journal |last1=Thorpe |first1=S. K. S. |last2=Holder |first2=R. L. |last3=Crompton |first3=R. H. |date=2007 |title=Origin of Human Bipedalism as an Adaptation for Locomotion on Flexible Branches |url=https://www.jstor.org/stable/20036393 |journal=Science |volume=316 |issue=5829 |pages=1328–1331 |doi=10.1126/science.1140799 |jstor=20036393 |pmid=17540902 |bibcode=2007Sci...316.1328T |s2cid=85992565 |issn=0036-8075}}</ref> Numerous causes for the evolution of human bipedalism involve freeing the hands for carrying and using tools, [[sexual dimorphism]] in provisioning, 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.<ref>{{Cite journal |last1=Niemitz |first1=Carsten |title=The evolution of the upright posture and gait—a review and a new synthesis |journal=Naturwissenschaften |date=2010 |doi=10.1007/s00114-009-0637-3 |volume=97 |issue=3 |pages=241–263 |pmid=20127307 |pmc=2819487 |bibcode=2010NW.....97..241N}}</ref> 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.<ref>{{Cite journal |last=Sigmon|first=Becky|year=1971|title=Bipedal behavior and the emergence of erect posture in man |journal=American Journal of Physical Anthropology |doi=10.1002/ajpa.1330340105 |pmid=4993117 |volume=34 |issue=1 |pages=55–60}}</ref> 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.<ref>{{Cite journal |last=Ko |first=Kwang Hyun |date=2015 |title=Origins of Bipedalism |journal=Brazilian Archives of Biology and Technology |doi=10.1590/S1516-89132015060399 |volume=58 |issue=6 |pages=929–934 |arxiv=1508.02739 |bibcode=2015arXiv150802739K |s2cid=761213}}</ref>

===Multiple factors===
Napier (1963) argued that it is 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 may equally have played their part as catalysts."''<ref>{{Cite book |title=The evolution of bipedal walking in the hominids |last=Napier |first=JR|publisher=Archives de Biologie (Liege) |year=1964}}</ref> Sigmon (1971) demonstrated that chimpanzees exhibit bipedalism in different contexts, and one single factor should be used to explain bipedalism: preadaptation for human bipedalism.<ref>{{Cite journal |last=Sigmon |first=Becky |year=1971 |title=Bipedal behavior and the emergence of erect posture in man |journal=American Journal of Physical Anthropology |doi=10.1590/S1516-89132015060399 |volume=58 |issue=6 |pages=929–934 |pmid=4993117 |arxiv=1508.02739 |bibcode=2015arXiv150802739K |s2cid=761213}}</ref> Day (1986) emphasized three major pressures that drove evolution of bipedalism: food acquisition, predator avoidance, and reproductive success.<ref>{{Cite book |title=Bipedalism: Pressures, origins and modes. Major topics in human evolution |last=Day |first=MH |publisher=Cambridge University Press |year=1986 |location=Cambridge }}</ref> Ko (2015) stated that there are two main questions regarding bipedalism 1. Why were the earliest hominins partially bipedal? and 2. Why did hominins become more bipedal over time? He argued that these questions can be answered with combination of prominent theories such as Savanna-based, Postural feeding, and Provisioning.<ref>{{Cite journal |last=Kwang Hyun |first=Ko |date=2015 |title=Origins of Bipedalism |journal=Brazilian Archives of Biology and Technology |doi=10.1590/S1516-89132015060399 |volume=58 |issue=6 |pages=929–934 |bibcode=2015arXiv150802739K |arxiv=1508.02739 |s2cid=761213}}</ref>

===Savannah-based theory===
{{main|Savannah hypothesis}}
According to the Savanna-based theory, [[hominines]] came down from the tree's branches 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 (genus)|Pan]]'' and ''[[Gorilla (genus)|Gorilla]]'' instead of evolving it once as [[synapomorphy]] for ''Pan'' and ''Gorilla'' before losing it in Australopithecus.<ref>{{Cite journal |last1=Richmond |first1=B. G. |last2=Strait |first2=D. S. |year=2000 |title=Evidence that humans evolved from a knuckle-walking ancestor |journal=Nature |volume=404 |issue=6776 |pages=382–385 |doi=10.1038/35006045 |pmid=10746723 |bibcode=2000Natur.404..382R |s2cid=4303978}}</ref> 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.<ref name="Dean, F 2000">Dean, F. 2000. Primate diversity. W.W. Norton & Company, Inc: New York. Print.</ref> 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.<ref>{{Cite journal |last1=Wheeler |first1=P. E. |year=1984 |title=The Evolution of Bipedality and Loss of Functional Body Hair in Hominoids |journal=Journal of Human Evolution |volume=13 |issue=1 |pages=91–98 |doi=10.1016/s0047-2484(84)80079-2|bibcode=1984JHumE..13...91W }}</ref> In fact, [[Elizabeth Vrba]]'s [[turnover-pulse hypothesis|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.<ref name="Sunset on the savanna">{{cite web |last=Shreeve |first=James |date=July 1996 |url=http://www.cabrillo.edu/~crsmith/Sunset.Savanna.doc |title=Sunset on the savanna |work=Discover |archive-url=https://web.archive.org/web/20170928010036/http://www.cabrillo.edu/~crsmith/Sunset.Savanna.doc |archive-date=2017-09-28}}</ref>

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.<ref>{{Cite journal |last1=Green, Alemseged |first1=David, Zeresenay |title=Australopithecus afarensis Scapular Ontogeny, Function, and the Role of Climbing in Human Evolution |journal=Science |date=2012 |volume=338 |issue=6106 |pages=514–517 |doi=10.1126/science.1227123|pmid=23112331 |bibcode=2012Sci...338..514G |s2cid=206543814}}</ref> 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.<ref>{{Cite journal |last1=Thorpe |first1=S. K. |last2=Holder |first2=R. L. |last3=Crompton |first3=R. H. |year=2007 |title=Origin of human bipedalism as an adaptation for locomotion on flexible branches |doi=10.1126/science.1140799 |journal=Science |volume=316 |issue=5829 |pages=1328–31 |pmid=17540902| bibcode=2007Sci...316.1328T |s2cid=85992565}}</ref> 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.<ref name="Sunset on the savanna"/>

Some of the fossils found actually showed that there was still an adaptation to arboreal life. For example, [[Lucy (Australopithecus)|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.<ref name="Sunset on the savanna"/>

===Traveling efficiency hypothesis===
An alternative explanation is that 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.<ref>{{Cite journal |vauthors=Isbell LA, Young TP |title=The evolution of bipedalism in hominids and reduced group size in chimpanzees: alternative responses to decreasing resource availability |journal=Journal of Human Evolution |volume=30 |issue=5 |pages=389–397 |doi=10.1006/jhev.1996.0034 |year=1996|bibcode=1996JHumE..30..389I }}</ref><ref>{{Cite book |author1=Lewin, Roger |author2=Swisher, Carl Celso |author3=Curtis, Garniss H. |title=Java man: how two geologists' dramatic discoveries changed our understanding of the evolutionary path to modern humans |publisher=Scribner |location=New York |year=2000 |isbn=978-0-684-80000-4 |url=https://archive.org/details/javamanhowtwogeo00roge}}</ref> 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 not have been very difficult or energetically costing.<ref>{{Cite journal |last1=Pontzer |first1=H. |last2=Raichlen |first2=D. A. |last3=Rodman |first3=P. S. |year=2014 |title=Bipedal and quadrupedal locomotion in chimpanzees |journal=Journal of Human Evolution |volume=66 |pages=64–82| doi=10.1016/j.jhevol.2013.10.002 |pmid=24315239|bibcode=2014JHumE..66...64P }}</ref> This increased travel efficiency is likely to have been selected for as it assisted foraging across widely dispersed resources.

===Postural feeding hypothesis===
The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at [[Indiana University]].<ref name="Hunt 1996">{{Cite journal |last=Hunt |first=Kevin |date=February 1996 |title=The postural feeding hypothesis: an ecological model for the evolution of bipedalism |journal=South African Journal of Science |volume=92 |pages=77–90 |url=https://www.researchgate.net/publication/288349918 |url-status=live |archive-date=2017-03-05 |archive-url=https://web.archive.org/web/20170305035720/https://www.researchgate.net/publication/288349918_The_postural_feeding_hypothesis_An_ecological_model_for_the_evolution_of_bipedalism}}</ref> 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 [[human]]s. For this reason, Hunt argues that bipedalism evolved more as a terrestrial feeding posture than as a walking posture.<ref name="Hunt 1996"/>

A related study conducted by [[University of Birmingham]], Professor Susannah Thorpe examined the most arboreal great ape, the [[orangutan]], holding onto supporting branches in order to navigate branches that were too flexible or unstable otherwise. In more than 75 percent of observations, the orangutans used their forelimbs to stabilize themselves while navigating thinner branches. 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. Findings also could shed light on discrepancies observed in the anatomy of A. afarensis, such as the ankle joint, which allowed it to "wobble" and long, highly flexible forelimbs. If bipedalism started from upright navigation in trees, it could explain both increased flexibility in the ankle as well as long forelimbs which grab hold of branches.<ref name=SCI>{{cite news| title=Walk Like an Orangutan: Ape's stroll through the trees may shed light on evolution of human bipedalism| url=https://www.science.org/content/article/walk-orangutan| first=Ann| last=Gibbons| magazine=[[Science Magazine]]| date=31 May 2007}}</ref><ref name=SCAM>{{cite news| title=Orangutans Show First Walking May Have Been on Trees| url=https://www.scientificamerican.com/article/first-walking-may-have-been-on-trees/| first=JR| last=Minkel| magazine=[[Scientific American]]| date=31 May 2007}}</ref><ref name=NATR>{{cite news| title=Upright orangutans point way to walking| url=https://www.nature.com/news/2007/070528/full/news070528-8.html| first=Matt| last=Kaplan| magazine=[[Nature Magazine]]| date=31 May 2007}}</ref><ref name=NSMG>{{cite news| title=Our upright walking started in the trees| url=https://www.newscientist.com/article/dn11965-our-upright-walking-started-in-the-trees/| first=Rowan| last=Hooper| magazine=[[New Scientist Magazine]]| date=31 May 2007}}</ref><ref name=UBHM>{{cite journal| title=Walking the walk: evolution of human bipedalism| url=https://www.cs.bham.ac.uk/research/projects/cogaff/talks/wonac/sue.d/Physics-at-Brum-08-v2.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.cs.bham.ac.uk/research/projects/cogaff/talks/wonac/sue.d/Physics-at-Brum-08-v2.pdf |archive-date=2022-10-09 |url-status=live| first=Susannah| last=Thorpe| journal=[[University of Birmingham]]| date=2007}}</ref><ref>{{Cite journal |last=Stanford |first=Craig B. |date=February 2006 |title=Arboreal bipedalism in wild chimpanzees: Implications for the evolution of hominid posture and locomotion |url=https://onlinelibrary.wiley.com/doi/10.1002/ajpa.20284 |journal=American Journal of Physical Anthropology |language=en |volume=129 |issue=2 |pages=225–231 |doi=10.1002/ajpa.20284 |pmid=16288480 |issn=0002-9483}}</ref>

===Provisioning model===
One theory on the origin of bipedalism is the behavioral model presented by [[Owen Lovejoy (anthropologist)|C. Owen Lovejoy]], known as "male provisioning".<ref name="Price">{{Cite book |author1=T. Douglas Price |author2=Gary M. Feinman |year=2003 |title=Images of the Past, 5th edition |page=[https://archive.org/details/imagesofpast0005pric/page/68 68] |location=Boston, MA |publisher=McGraw Hill |isbn=978-0-07-340520-9 |url=https://archive.org/details/imagesofpast0005pric/page/68}}</ref> 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 [[hominid]]s 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]]''<ref>{{Cite journal |vauthors=Brunet M, Guy F, Pilbeam D, Mackaye HT, Likius A |title=A new hominid from the Upper Miocene of Chad, Central Africa |journal=Nature |date=11 July 2002 |volume=418 |pages=145–151 |doi=10.1038/nature00879 |pmid=12110880 |issue=6894 |display-authors=etal |bibcode=2002Natur.418..145B |s2cid=1316969|url=http://doc.rero.ch/record/13388/files/PAL_E190.pdf }}</ref> and ''[[Ardipithecus ramidus]]'',<ref>{{Cite journal |vauthors=Suwa G, Kono RT, Simpson SW, Asfaw B, Lovejoy CO, White TD |title=Paleobiological implications of the ''Ardipithecus ramidus'' dentition |journal=Science |date=2 October 2009 |volume=326 |issue=5949 |pages=94–99 |doi=10.1126/science.1175824 |pmid=19810195 |bibcode=2009Sci...326...94S |s2cid=3744438 |url=http://doc.rero.ch/record/211460/files/PAL_E4443.pdf |url-status=live |archive-date=2022-10-09 |archive-url=https://ghostarchive.org/archive/20221009/http://doc.rero.ch/record/211460/files/PAL_E4443.pdf}}</ref> which along with low body size dimorphism in ''Ardipithecus''<ref>{{cite journal |vauthors=White TD |title=Ardipithecus ramidus and the paleobiology of early hominids |display-authors=et al |journal=Science |date=2009 |volume=326 |issue=5949 |pages=75–86|doi=10.1126/science.1175802 |pmid=19810190 |bibcode=2009Sci...326...75W |s2cid=20189444 }}</ref> and ''Australopithecus'',<ref>{{cite journal |vauthors=Reno PL |title=An enlarged postcranial sample confirms Australopithecus afarensis dimorphism was similar to modern humans |display-authors=et al |journal=Philos Trans R Soc Lond B Biol Sci |date=2010 |volume=365 |issue=1556 |pages=3355–3363|doi=10.1098/rstb.2010.0086 |pmid=20855309 |pmc=2981962 }}</ref><ref>{{cite journal |vauthors=Harmon E |title=Size and shape variation in the proximal femur of Australopithecus africanus |journal=J Hum Evol |date=2009 |volume=56 |issue=6 |pages=551–559|doi=10.1016/j.jhevol.2009.01.002 |pmid=19446306 |bibcode=2009JHumE..56..551H }}</ref><ref>{{cite journal |vauthors=Reno PL, Lovejoy CO |title=From Lucy to Kadanuumuu: Balanced analyses of ''Australopithecus'' afarensisassemblages confirm only moderate skeletal dimorphism |journal=PeerJ |date=2015 |volume=3 |at=e925 |doi=10.7717/peerj.925|pmid=25945314 |pmc=4419524 |doi-access=free }}</ref> suggests a reduction in inter-male antagonistic behavior in early hominids.<ref name="ReferenceB">{{cite journal |vauthors=Lovejoy CO |title=Reexamining human origins in light of Ardipithecus ramidus |journal=Science |date=2009 |volume=326 |issue=5949 |pages=74e1–8 |doi=10.1126/science.1175834 |pmid=19810200 |bibcode=2009Sci...326...74L |s2cid=42790876 |url=http://doc.rero.ch/record/211449/files/PAL_E4439.pdf }}</ref> 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.<ref name="ReferenceB"/>

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.<ref>{{Cite journal |vauthors=Lovejoy CO |title=The Origin of Man |journal=Science |year=1981 |volume=211 |issue=4480 |pages=341–350 |doi=10.1126/science.211.4480.341 |pmid=17748254 |bibcode=1981Sci...211..341L}}</ref> Furthermore, as the species became more bipedal, specialized feet would prevent the infant from conveniently clinging to the mother{{snd}} hampering the mother's freedom<ref>{{cite book |author1=Keith Oatley |author2=Dacher Keltner |author3=Jennifer M. Jenkins |title=Understanding Emotion |year=2006 |edition=2nd |page=235}}</ref> 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. Other recent studies of the foot structure of ''[[Ardipithecus ramidus]]'' suggest that the species was closely related to African-ape ancestors. This possibly provides a species close to the true connection between fully bipedal hominins and quadruped apes.<ref>{{Cite journal |last=Prang |first=Thomas Cody |date=2019-04-30 |title=The African ape-like foot of Ardipithecus ramidus and its implications for the origin of bipedalism |journal=eLife |language=en |volume=8 |pages=e44433 |doi=10.7554/eLife.44433 |issn=2050-084X |pmc=6491036 |pmid=31038121 |doi-access=free }}</ref> 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 [[homininae|hominine]]s such as ''[[Ardipithecus ramidus]]'' may have possessed an arboreal type of bipedalism that later independently evolved towards knuckle-walking in chimpanzees and gorillas<ref>{{Cite journal |pmid=19667206 |doi=10.1073/pnas.0901280106 |pmc=2732797 |volume=106 |issue=34 |title=Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor |date=August 2009 |journal=Proc. Natl. Acad. Sci. U.S.A. |pages=14241–6 |vauthors=Kivell TL, Schmitt D |bibcode=2009PNAS..10614241K |doi-access=free }}</ref> 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.<ref>[[Joseph Jordania]]. [[Why do People Sing? Music in Human Evolution]]. Logos, 2011</ref> 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.<ref name="Dean, F 2000"/>

===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.<ref>{{Cite journal |last=Wheeler |first=P. E. |year=1984 |title=The evolution of bipedality and loss of functional body hair in hominids |journal=J. Hum. Evol. |volume=13 |issue=1 |pages=91–98 |doi=10.1016/s0047-2484(84)80079-2 |bibcode=1984JHumE..13...91W }}</ref><ref>{{Cite journal |last=Wheeler |first=P. E. |year=1990 |title=The influence of thermoregulatory selection pressures on hominid evolution. |journal=Behav. Brain Sci. |volume=13 |issue=2 |page=366 |doi=10.1017/s0140525x00079218|s2cid=147314740 }}</ref><ref>{{Cite journal |last1=Wheeler |first1=P.E. |year=1991 |title=The influence of bipedalism on the energy and water budgets of early hominids |journal=J. Hum. Evol. |volume=21 |issue=2| pages=117–136 |doi=10.1016/0047-2484(91)90003-e|bibcode=1991JHumE..21..117W }}</ref> 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.<ref>{{Cite journal |last1=David-Barrett |first1=T. |last2=Dunbar |first2=R. |year=2016 |title=Bipedality and hair loss in human evolution revisited: The impact of altitude and activity scheduling |doi=10.1016/j.jhevol.2016.02.006 |journal=J. Hum. Evol. |volume=94 |pages=72–82 |pmid=27178459 |pmc=4874949|bibcode=2016JHumE..94...72D }}</ref>

===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.<ref>{{cite book |last=Tanner |first=Nancy Makepeace |year=1981 |url=http://catalogue.nla.gov.au/Record/1425592 |title=On Becoming Human |url-status=live |archive-url=https://web.archive.org/web/20130522084709/http://catalogue.nla.gov.au/Record/1425592 |archive-date=2013-05-22 |place=Cambridge |publisher=Cambridge University Press}}</ref> 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",<ref>{{Cite journal |last=Kuliukas |first=A. |title=Wading Hypotheses of the Origin of Human Bipedalism |journal=Human Evolution |volume=28 |issue=3–4 |pages=213–236 |year=2013}}</ref> was originally suggested by the [[University of Oxford|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."<ref>{{Cite journal |last=Hardy |first=Alister C. |year=1960 |title=Was man more aquatic in the past? |journal=[[New Scientist]] |volume=7 |issue=174 |author-link=Alister Hardy |pages=642–645 |url=http://www.riverapes.com/AAH/Hardy/Hardy1960.pdf |archive-url=https://web.archive.org/web/20090326175059/http://www.riverapes.com/AAH/Hardy/Hardy1960.pdf |archive-date=26 March 2009 }}</ref> 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.<ref name=Morgan1997>{{Cite book |last=Morgan |first=Elaine |author-link=Elaine Morgan |title=The Aquatic Ape Hypothesis |year=1997 |publisher=Souvenir Press |isbn=978-0-285-63518-0}}</ref> The "[[aquatic ape hypothesis]]", as originally formulated, has not been accepted or considered a serious theory within the anthropological scholarly community.<ref name=Meier>{{Cite book |last=Meier |first=R. |year=2003 |title=The complete idiot's guide to human prehistory |pages=[https://archive.org/details/completeidiotsgu00meie/page/57 57–59] |publisher=Alpha Books |isbn=978-0-02-864421-9 |url=https://archive.org/details/completeidiotsgu00meie}}</ref> 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<ref name=Niemitz2004>{{Cite book |last=Niemitz |first=Carsten |author-link=Carsten Niemitz |title=Das Geheimnis des Aufrechten Gangs ~ Unsere Evolution Verlief Anders |year=2004 |publisher=Beck |isbn=978-3-406-51606-1}}</ref> on a variant of the wading hypothesis, which he calls the "amphibian generalist theory" ({{langx|de|Amphibische Generalistentheorie}}).

Other theories have been proposed that suggest wading and the exploitation of aquatic food sources (providing essential nutrients for human brain evolution<ref>{{Cite book |last=Cunnane |first=Stephen C |title=Survival of the fattest: the key to human brain evolution |publisher=World Scientific Publishing Company |year=2005 |isbn=978-981-256-191-6 |pages=[https://books.google.com/books?id=kKNpHN3dDaUC&pg=PA259 259]}}</ref> or critical fallback foods<ref>{{Cite journal |vauthors=Wrangham R, Cheney D, Seyfarth R, Sarmiento E |title=Shallow-water habitats as sources of fallback foods for hominins |journal=Am. J. Phys. Anthropol. |volume=140 |issue=4 |pages=630–42 |date=December 2009 |pmid=19890871 |doi=10.1002/ajpa.21122 |s2cid=36325131 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:8947970}}</ref>) 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.<ref>{{cite journal |vauthors=Verhaegena M, Puechb PF, Munro S |date=2002 |title=Aquaboreal ancestors? |journal= Trends in Ecology & Evolution|volume=17 |issue=5 |pages=212–217|doi=10.1016/S0169-5347(02)02490-4 }}</ref>

===Consequences===
Prehistoric fossil records show that early hominins first developed bipedalism before being followed by an increase in brain size.<ref>{{Cite book |last=DeSilva |first=Jeremy |title=First Steps: How Upright Walking Made Us Human |publisher=HarperCollins |year=2021 |isbn=978-0062938497 |page=17}}</ref> 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]].

Non-human primates habitually deliver their young on their own, but the same cannot be said for modern-day humans. Isolated [[birth]] appears to be rare and actively avoided cross-culturally, even if birthing methods may differ between said cultures. This is due to the fact that the narrowing of the hips and the change in the pelvic angle caused a discrepancy in the ratio of the size of the head to the [[Vagina|birth canal]]. The result of this is that there is greater difficulty in birthing for hominins in general, let alone to be doing it by oneself.<ref>{{Cite journal |last=Trevathan |first=Wenda R. |date=1996 |title=The Evolution of Bipedalism and Assisted Birth |url=https://www.jstor.org/stable/649332 |journal=Medical Anthropology Quarterly |volume=10 |issue=2 |pages=287–290 |doi=10.1525/maq.1996.10.2.02a00100 |jstor=649332 |pmid=8744088 |issn=0745-5194}}</ref>