Editing Human evolution (section)

=== Bipedalism ===
[[File:Akha cropped.png|thumb|upright|Bipedalism shown by a man and a woman]]

[[Bipedalism]] (walking on two legs) is the basic adaptation of the hominid and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominids. The earliest hominin, of presumably primitive bipedalism, is considered to be either ''[[Sahelanthropus]]''<ref name="Brunet2002">{{cite journal |last1=Brunet |first1=M. |last2=Guy |first2=F. |last3=Pilbeam |first3=D. |last4=Mackaye |first4=H. |last5=Likius |first5=A. |last6=Ahounta |first6=D. |last7=Beauvilain |first7=A. |last8=Blondel |first8=C. |last9=Bocherens |first9=H. |last10=Boisserie |first10=J. |last11=De Bonis |first11=L. |last12=Coppens |first12=Y. |last13=Dejax |first13=J. |last14=Denys |first14=C. |last15=Duringer |first15=P. |last16=Eisenmann |first16=V. |last17=Fanone |first17=G. |last18=Fronty |first18=P. |last19=Geraads |first19=D. |last20=Lehmann |first20=T. |last21=Lihoreau |first21=F. |last22=Louchart |first22=A. |last23=Mahamat |first23=A. |last24=Merceron |first24=G. |last25=Mouchelin |first25=G. |last26=Otero |first26=O. |last27=Pelaez Campomanes |first27=P. |last28=Ponce De Leon |first28=M. |last29=Rage |first29=J. |last30=Sapanet |first30=M. |last31=Schuster |first31=M. |last32=Sudre |first32=J. |last33=Tassy |first33=P. |last34=Valentin |first34=X. |last35=Vignaud |first35=P. |last36=Viriot |first36=L. |last37=Zazzo |first37=A. |last38=Zollikofer |first38=C. |display-authors=6 |author1-link=Michel Brunet (paleontologist) |author3-link=David Pilbeam |date=July 11, 2002 |title=A new hominid from the Upper Miocene of Chad, Central Africa |journal=[[Nature (journal)|Nature]] |volume=418 |issue=6894 |pages=145–151 |doi=10.1038/nature00879 |issn=0028-0836 |pmid=12110880 |bibcode=2002Natur.418..145B |s2cid=1316969 |url= http://doc.rero.ch/record/13388/files/PAL_E190.pdf |access-date=February 20, 2023 |archive-date=February 25, 2023 |archive-url= https://web.archive.org/web/20230225204437/https://doc.rero.ch/record/13388/files/PAL_E190.pdf |url-status=live}}}{{collapsible list |title=Full list of authors |bullets=true |Michel Brunet |Franck Guy |David Pilbeam |Hassane Taisso Mackaye |Andossa Likius |Djimdoumalbaye Ahounta |Alain Beauvilain |Cécile Blondel |Hervé Bocherens |Jean-Renaud Boisserie |Louis De Bonis |Yves Coppens |Jean Dejax |Christiane Denys |Philippe Duringer |Véra Eisenmann |Gongdibé Fanone |Pierre Fronty |Denis Geraads |Thomas Lehmann |Fabrice Lihoreau |Antoine Louchart |Adoum Mahamat |Gildas Merceron |Guy Mouchelin |Olga Otero |Pablo Pelaez Campomanes |Marcia Ponce De Leon |Jean-Claude Rage |Michel Sapanet |Mathieu Schuster |Jean Sudre |Pascal Tassy |Xavier Valentin |Patrick Vignaud |Laurent Viriot |Antoine Zazzo |Christoph Zollikofer}}</ref> or ''[[Orrorin]]'', both of which arose some 6 to 7&nbsp;million years ago. The non-bipedal knuckle-walkers, the [[gorilla]]s and chimpanzees, diverged from the hominin line over a period covering the same time, so either ''Sahelanthropus'' or ''Orrorin'' may be our last shared ancestor. ''[[Ardipithecus]]'', a full biped, arose approximately 5.6&nbsp;million years ago.<ref>{{cite journal |title=Ardipithecus ramidus and the Paleobiology of Early Hominids |journal=[[Science (journal)|Science]] |volume=326 |issue=5949 |pages=75–86 |date=2009 |last1=White |first1=T. D. |last2=Asfaw |first2=B. |last3=Beyene |first3=Y. |last4=Haile-Selassie |first4=Y. |last5=Lovejoy |first5=C. O. |last6=Suwa |first6=G. |last7=Woldegabriel |first7=G. |pmid=19810190 |bibcode=2009Sci...326...75W |s2cid=20189444 |doi=10.1126/science.1175802}}</ref>

The early bipeds eventually evolved into the australopithecines and still later into the genus ''[[Homo]]''. There are several theories of the adaptation value of bipedalism. It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion,<ref name="Kwang Hyun 2015 929–934">{{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 |arxiv=1508.02739 |bibcode=2015arXiv150802739K |s2cid=761213}}</ref> enabled long-distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun; features all advantageous for thriving in the new savanna and woodland environment created as a result of the East African Rift Valley uplift versus the previous closed forest habitat.<ref name="Kwang Hyun 2015 929–934" />{{sfn|DeSalle|Tattersall|2008|p=[https://books.google.com/books?id=Bf4Sitw7YaIC&pg=PA146 146]}}{{sfn|Curry|2008|pp=106–109}} A 2007 study provides support for the hypothesis that bipedalism evolved because it used less energy than quadrupedal knuckle-walking.<ref>{{cite news |date=July 17, 2007 |title=Study Identifies Energy Efficiency as Reason for Evolution of Upright Walking |work=[[ScienceDaily]] |url= https://www.sciencedaily.com/releases/2007/07/070716191140.htm |access-date=April 9, 2015 |archive-date=May 4, 2015 |archive-url= https://web.archive.org/web/20150504174649/http://www.sciencedaily.com/releases/2007/07/070716191140.htm |url-status=live}}<br />{{cite web |url= http://uanews.org/story/study-identifies-energy-efficiency-reason-evolution-upright-walking |title=Study identifies energy efficiency as reason for evolution of upright walking |date=July 16, 2007 |website=UANews |publisher=[[University of Arizona]] |location=Tucson |access-date=April 23, 2015 |archive-date=July 3, 2022 |archive-url= https://web.archive.org/web/20220703195556/http://uanews.org/story/study-identifies-energy-efficiency-reason-evolution-upright-walking |url-status=usurped}}</ref><ref>{{cite journal |last1=Sockol |first1=Michael D. |last2=Raichlen |first2=David A. |last3=Pontzer |first3=Herman |date=July 24, 2007 |title=Chimpanzee locomotor energetics and the origin of human bipedalism |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=104 |issue=30 |pages=12265–12269 |doi=10.1073/pnas.0703267104 |issn=0027-8424 |pmc=1941460 |pmid=17636134 |bibcode=2007PNAS..10412265S |doi-access=free}}</ref> However, recent studies suggest that bipedality without the [[Control of fire by early humans|ability to use fire]] would not have allowed global dispersal.<ref>{{cite journal |last1=David-Barrett |first1=T. |last2=Dunbar |first2=R. I. M. |date=2016 |title=Bipedality and Hair-loss Revisited: The Impact of Altitude and Activity Scheduling |journal=Journal of Human Evolution |volume=94 |pages=72–82 |pmid=27178459 |pmc=4874949 |doi=10.1016/j.jhevol.2016.02.006}}</ref> This change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for [[brachiation]]. Another change is the shape of the big toe. Recent studies suggest that australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe. This was progressively lost in habilines.

Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the [[Human vertebral column|vertebral column]], feet and ankles, and skull.{{sfn|Aiello|Dean|1990}} The [[femur]] evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became increasingly robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the [[lumbar vertebrae]] became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion. The arms and forearms shortened relative to the legs making it easier to run. The [[foramen magnum]] migrated under the skull and more anterior.{{sfn|Kondo|1985}}

The most significant changes occurred in the pelvic region, where the long downward facing [[Ilium (bone)|iliac blade]] was shortened and widened as a requirement for keeping the center of gravity stable while walking;{{sfn|Srivastava|2009|p=[https://books.google.com/books?id=kCerOsM8XMwC&pg=PA87 87]}} bipedal hominids have a shorter but broader, bowl-like pelvis due to this. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, thus permitting the passage of newborns due to the increase in cranial size. This is limited to the upper portion, since further increase can hinder normal bipedal movement.{{sfn|Strickberger|2000|pp=[https://books.google.com/books?id=feMItLo5gwgC&pg=PA476 475–476]}}

The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth, which is much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal and rotate about 90 degrees upon exit.{{sfn|Trevathan|2011|p=[https://books.google.com/books?id=Potlqpl-jxgC&pg=PA20 20]}} The smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human offspring, who are unable to walk much before 12 months and have greater [[neoteny]], compared to other primates, who are mobile at a much earlier age.{{sfn|Curry|2008|pp=106–109}} The increased brain growth after birth and the increased dependency of children on mothers had a major effect upon the female reproductive cycle,<ref>{{cite book |last=Zuk |first=Marlene |date=2014 |title=Paleofantasy: What Evolution Really Tells Us About Sex, Diet, and How We Live |publisher=W.W. Norton & Company |isbn=978-0-393-34792-0 |oclc=846889455}}{{page needed|date=December 2021}}</ref> and the more frequent appearance of [[alloparenting]] in humans when compared with other hominids.<ref>{{cite book |last=Hrdy |first=Sarah Blaffer |date=2011 |title=Mothers and Others: The Evolutionary Origins of Mutual Understanding |publisher=Harvard University Press |isbn=978-0-674-06032-6 |oclc=940575388}}{{page needed|date=December 2021}}</ref> Delayed human sexual maturity also led to the evolution of [[menopause]] with one explanation, the [[grandmother hypothesis]], providing that elderly women could better pass on their genes by taking care of their daughter's offspring, as compared to having more children of their own.<ref>{{cite journal |last=Wayman |first=Erin |date=August 19, 2013 |title=Killer whales, grandmas and what men want: Evolutionary biologists consider menopause |url= https://www.sciencenews.org/article/killer-whales-grandmas-and-what-men-want-evolutionary-biologists-consider-menopause |journal=[[Science News]] |issn=0036-8423 |access-date=April 24, 2015 |archive-date=November 6, 2014 |archive-url= https://web.archive.org/web/20141106194849/https://www.sciencenews.org/article/killer-whales-grandmas-and-what-men-want-evolutionary-biologists-consider-menopause |url-status=live}}</ref><ref>{{Cite journal |last=Blell |first=Mwenza |date=September 29, 2017 |title=Grandmother Hypothesis, Grandmother Effect, and Residence Patterns |journal=The International Encyclopedia of Anthropology |pages=1–5 |doi=10.1002/9781118924396.wbiea2162 |isbn=978-1-118-92439-6 |doi-access=free}}</ref>