Editing Australopithecus afarensis (section)

==Anatomy==
===Skull===
{{Multiple image|total_width=380|image1=BH-021-T-A-afarensis-Lucy-3qtrR-Lo.jpg|image2=Australopithecus afarensis skull - Naturmuseum Senckenberg - DSC02102.JPG|footer=Two ''A. afarensis'' skulls}}

''A. afarensis'' had a tall face, a delicate brow ridge, and [[prognathism]] (the jaw jutted outwards). One of the biggest skulls, AL 444–2, is about the size of a female gorilla skull.<ref>{{cite book|first1=W. H.|last1=Kimbel|first2=Y.|last2=Yak|first3=D. C.|last3=Johanson|author3-link=Donald Johanson|chapter=A. L. 444-2: the skull as a whole|title=The skull of Australopithecus afarensis|date=11 March 2004|publisher=Oxford University Press|isbn=978-0-19-803569-5}}</ref> The first relatively complete jawbone was discovered in 2002, AL 822–1. This specimen strongly resembles the deep and robust gorilla jawbone. However, unlike gorillas, the strength of the [[sagittal crest|sagittal]] and [[nuchal lines|nuchal]] crests (which support the [[temporalis muscle]] used in biting) do not vary between sexes. The crests are similar to those of chimpanzees and female gorillas.<ref name="Rak2007">{{cite journal|first1=Y.|last1=Rak|first2=A.|last2=Ginzburg|first3=E.|last3=Geffen|year=2007|title=Gorilla-like anatomy on ''Australopithecus afarensis'' mandibles suggests ''Au. afarensis'' link to robust australopiths|journal=Proceedings of the National Academy of Sciences|volume=104|issue=16|pages=6568–6572|doi=10.1073/pnas.0606454104|pmid=17426152|bibcode=2007PNAS..104.6568R|pmc=1871826|doi-access=free}}</ref> Compared to earlier hominins, the [[incisor]]s of ''A. afarensis'' are reduced in breadth, the [[canine tooth|canines]] reduced in size and lost the honing mechanism which continually sharpens them, the [[premolar]]s are [[molar (tooth)|molar]]-shaped, and the molars are taller.<ref>{{cite journal|first1=C. V.|last1=Ward|first2=J. M.|last2=Plavcan|first3=F. K.|last3=Manthi|year=2010|title=Anterior dental evolution in the ''Australopithecus anamensis''–''afarensis'' lineage|journal=Philosophical Transactions of the Royal Society B|volume=365|issue=1556|pages=3333–3344|doi=10.1098/rstb.2010.0039|pmc=2981954|pmid=20855307}}</ref> The molars of australopiths are generally large and flat with thick [[tooth enamel|enamel]], which is ideal for crushing hard and brittle foods.<ref>{{cite journal|first1=M. F.|last1=Teaford|first2=P. S.|last2=Ungar|year=2000|title=Diet and the evolution of the earliest human ancestors|journal=Proceedings of the National Academy of Sciences|volume=97|issue=25|pages=13506–13511|doi=10.1073/pnas.260368897|pmid=11095758|bibcode=2000PNAS...9713506T|pmc=17605|doi-access=free}}</ref>

The brain volume of Lucy was estimated to have been 365–417 cc, specimen AL 822-1 about 374–392 cc, AL 333-45 about 486–492 [[cubic centimetre|cc]], and AL 444-2 about 519–526 cc. This would make for an average of about 445 cc. The brain volumes of the infant (about 2.5 years of age) specimens DIK-1-1 and AL 333-105 are 273–277 and 310–315 cc, respectively. Using these measurements, the brain growth rate of ''A. afarensis'' was closer to the growth rate of modern humans than to the faster rate in chimpanzees. Though brain growth was prolonged, the duration was nonetheless much shorter than modern humans, which is why the adult ''A. afarensis'' brain was so much smaller. The ''A. afarensis'' brain was likely organised like non-human ape brains, with no evidence for humanlike brain configuration.<ref>{{cite journal|first1=P.|last1=Gunz|first2=S.|last2=Neubauer|first3=D.|last3=Falk|display-authors=et al.|year=2020|title=''Australopithecus afarensis'' endocasts suggest ape-like brain organization and prolonged brain growth|journal=Science Advances|volume=6|issue=14|page=eaaz4729|doi=10.1126/sciadv.aaz4729|pmid=32270044|pmc=7112758|bibcode=2020SciA....6.4729G|doi-access=free}}</ref>

===Size===
{{Multiple image|align=left|total_width=400px|image1=NHM - Australopithecus afarensis Modell 1.jpg|image2=NHM - Australopithecus afarensis Modell 2.jpg|footer=Reconstruction of a male (left) and female (right) ''A. afarensis'' at the [[Natural History Museum, Vienna]]}}
''A. afarensis'' specimens apparently exhibit a wide range of variation, which is generally explained as marked sexual dimorphism with males much bigger than females. In 1991, American anthropologist [[Henry McHenry (anthropologist)|Henry McHenry]] estimated body size by measuring the joint sizes of the leg bones and scaling down a human to meet that size. This yielded {{cvt|151|cm|ftin}} for a presumed male (AL 333–3), whereas Lucy was {{cvt|105|cm|ftin}}.<ref>{{cite journal|first=H. M.|last=McHenry|author-link=Henry McHenry (anthropologist)|year=1991|title=Femoral Lengths and Stature in Plio-Pleistocene Hominids|journal= American Journal of Physical Anthropology|volume=85|issue=2|pages=149–158|doi=10.1002/ajpa.1330850204|pmid=1882979}}</ref> In 1992, he estimated that males typically weighed about {{cvt|44.6|kg}} and females {{cvt|29.3|kg}} assuming body proportions were more humanlike than [[ape]]like. This gives a male to female body mass ratio of 1.52, compared to 1.22 in modern [[human]]s, 1.37 in [[chimpanzee]]s, and about 2 for [[gorilla]]s and [[orangutan]]s.<ref>{{cite journal|first=H. M.|last=McHenry|author-link=Henry McHenry (anthropologist)|year=1992|title=Body Size and Proportions in Early Hominids|journal= American Journal of Physical Anthropology|volume=87|issue=4|pages=407–431|doi=10.1002/ajpa.1330870404|pmid=1580350}}</ref> However, this commonly cited weight figure used only three presumed-female specimens, of which two were among the smallest specimens recorded for the species. It is also contested if australopiths even exhibited heightened sexual dimorphism at all, which if correct would mean the range of variation is normal body size disparity between different individuals regardless of sex. It has also been argued that the [[femoral head]] could be used for more accurate size modeling, and the femoral head size variation was the same for both sexes.<ref>{{cite journal|first1=P. L.|last1=Reno|first2=R. S.|last2=Meindl|first3=M. A.|last3=McCollum|first4=C. O.|last4=Lovejoy|author4-link=Owen Lovejoy|year=2003|title=Sexual dimorphism in ''Australopithecus afarensis'' was similar to that of modern humans|journal=Proceedings of the National Academy of Sciences|volume=100|issue=16|pages=4404–4409|doi=10.1073/pnas.1133180100|pmid=12878734|bibcode=2003PNAS..100.9404R|pmc=170931|doi-access=free}}</ref>

Lucy is one of the most complete Pliocene hominin skeletons, with over 40% preserved, but she was one of the smaller specimens of her species. Nonetheless, she has been the subject of several body mass estimates since her discovery, ranging from {{cvt|13–42|kg}} for absolute lower and upper bounds. Most studies report ranges within {{cvt|25–37|kg}}.<ref name=Brassey2017>{{cite journal |author1=Brassey, C. A. |author2=O'Mahoney, T. G. |author3=Chamberlain, A. T. |author4=Sellers, W. I. |title=A volumetric technique for fossil body mass estimation applied to ''Australopithecus afarensis'' |journal=[[Journal of Human Evolution]] |volume=115 |page=51|year=2018 |doi=10.1016/j.jhevol.2017.07.014 |pmid=28838563 |bibcode=2018JHumE.115...47B |url=https://e-space.mmu.ac.uk/618976/2/deleted_HUMEV-T-16-00432R3.pdf }}</ref>

For the five makers of the Laetoli fossil trackways (S1, S2, G1, G2 and G3), based on the relationship between footprint length and bodily dimensions in modern humans, S1 was estimated to have been considerably large at about {{cvt|165|cm|ftin}} tall and {{cvt|45|kg}} in weight, S2 {{cvt|145|cm|ftin}} and {{cvt|39.5|kg}}, G1 {{cvt|114|cm|ftin}} and {{cvt|30|kg}}, G2 {{cvt|142|cm|ftin}} and {{cvt|39|kg}}, and G3 {{cvt|132|cm|ftin}} and {{cvt|35|kg}}. Based on these, S1 is interpreted to have been a male, and the rest females (G1 and G3 possibly juveniles), with ''A. afarensis'' being a highly dimorphic species.<ref name=Masao2016/>

===Torso===

DIK-1-1 preserves an oval [[hyoid bone]] (which supports the [[tongue]]) more similar to those of chimpanzees and gorillas than the bar-shaped hyoid of humans and orangutans. This would suggest the presence of [[Larynx|laryngeal]] [[air sac]]s characteristic of non-human African apes (and large [[gibbon]]s).<ref name=Alamseged2006/> Air sacs may lower the risk of hyperventilating when producing faster extended call sequences by rebreathing exhaled air from the air sacs. The loss of these in humans could have been a result of speech and resulting low risk of hyperventilating from normal vocalisation patterns.<ref>{{cite journal|first1=G.|last1=Hewitt|first2=A.|last2=MacLarnon|first3=K. E.|last3=Jones|year=2002|title=The Functions of Laryngeal Air Sacs in Primates: A New Hypothesis|journal=Folia Primatologica|volume=73|issue=2–3|pages=70–94|doi=10.1159/000064786|pmid=12207055|s2cid=17329870}}</ref>

It was previously thought that the australopithecines' spine was more like that of non-human apes than humans, with weak [[neck vertebra]]e. However, the thickness of the neck vertebrae of KSD-VP-1/1 is similar to that of modern humans. Like humans, the series has a bulge and achieves maximum girth at C5 and 6, which in humans is associated with the [[brachial plexus]], responsible for nerves and muscle innervation in the arms and hands. This could perhaps speak to advanced motor functions in the hands of ''A. afarensis'' and competency at precision tasks compared to non-human apes, possibly implicated in stone tool use or production.<ref>{{cite journal|first=M. R.|last=Meyer|year=2015|title=The Spinal Cord in Hominin Evolution|journal=eLS|doi=10.1002/9780470015902.a0027058|url=https://www.researchgate.net/publication/306232081|pages=1–6|isbn=9780470015902}}</ref><ref name=Haile2015/>{{rp|63–111}} However, this could have been involved in head stability or posture rather than dexterity. A.L. 333-101 and A.L. 333-106 lack evidence of this feature. The neck vertebrae of KDS-VP-1/1 indicate that the [[nuchal ligament]], which stabilises the head while distance running in humans and other cursorial creatures, was either not well developed or absent.<ref name=Haile2015/>{{rp|92–95}} KSD-VP-1/1, preserving (among other skeletal elements) six rib fragments, indicates that ''A. afarensis'' had a bell-shaped [[ribcage]] instead of the barrel shaped ribcage exhibited in modern humans. Nonetheless, the constriction at the upper ribcage was not so marked as exhibited in non-human great apes and was quite similar to humans.<ref name=Haile2015/>{{rp|143–153}} Originally, the [[vertebral centra]] preserved in Lucy were interpreted as being the [[thoracic vertebrae|T]]6, T8, T10, T11 and [[lumbar vertebrae|L]]3, but a 2015 study instead interpreted them as being T6, T7, T9, T10 and L3.<ref>{{cite journal|first1=M. R.|last1=Meyer|first2=S. A.|last2=Williams|first3=M. P.|last3=Smith|first4=G. J.|last4=Sawyer|year=2015|title=Lucy's back: Reassessment of fossils associated with the A.L. 288-1 vertebral column|journal=Journal of Human Evolution|volume=84|pages=174–180|doi=10.1016/j.jhevol.2015.05.007|pmid=26058822|bibcode=2015JHumE..85..174M |s2cid=10410978 }}</ref> DIK-1-1 shows that australopithecines had twelve thoracic vertebrae like modern humans instead of thirteen like non-human apes.<ref>{{cite journal|first1=C. V.|last1=Ward|first2=T. K.|last2=Nalley|first3=F.|last3=Spoor|first4=P.|last4=Tafforeau|first5=Z.|last5=Alemseged|year=2017|title=Thoracic Vertebral Count and Thoracolumbar Transition in ''Australopithecus afarensis''|journal=Proceedings of the National Academy of Sciences|volume=114|issue=23|pages=6000–6004|doi=10.1073/pnas.1702229114|pmc=5468642|pmid=28533391|bibcode=2017PNAS..114.6000W |doi-access=free}}</ref> Like humans, australopiths likely had five lumbar vertebrae, and this series was likely long and flexible in contrast to the short and inflexible non-human great ape lumbar series.<ref name=Haile2015>{{cite book|first1=Y.|last1=Haile-Selassie|author1-link=Yohannes Haile-Selassie|first2=D. F.|last2=Su|year=2015|title=The Postcranial Anatomy of Australopithecus afarensis: New Insights from KSD-VP-1/1|series=Vertebrate Paleobiology and Paleoanthropology|publisher=Springer|doi=10.1007/978-94-017-7429-1|isbn=978-94-017-7429-1|s2cid=133164058}}</ref>{{rp|143–153}}

===Upper limbs===
[[File:Lucy Skeleton.jpg|left|thumb|"[[Lucy (Australopithecus)|Lucy]]" skeleton]]
Like other australopiths, the ''A. afarensis'' skeleton exhibits a mosaic anatomy with some aspects similar to modern humans and others to non-human great apes. The pelvis and leg bones clearly indicate weight-bearing ability, equating to habitual bipedalism, but the upper limbs are reminiscent of orangutans, which would indicate [[arboreal]] locomotion. However, this is much debated, as tree-climbing adaptations could simply be basal traits inherited from the great ape [[last common ancestor]] in the absence of major selective pressures at this stage to adopt a more humanlike arm anatomy.<ref>{{cite journal|first1=J.|last1=Arias-Martorell|first2=J. M.|last2=Potau|first3=G.|last3=Bello-Hellegouarch|first4=A.|last4=Pérez-Pérez|year=2015|title=Like Father, Like Son: Assessment of the Morphological Affinities of A.L. 288–1 (''A. afarensis''), Sts 7 (''A. africanus'') and Omo 119–73–2718 (''Australopithecus'' sp.) through a Three-Dimensional Shape Analysis of the Shoulder Joint|journal=PLOS ONE|volume=10|issue=2|page=e0117408|doi=10.1371/journal.pone.0117408|pmc=4317181|pmid=25651542|bibcode=2015PLoSO..1017408A|doi-access=free}}</ref>

The shoulder joint is somewhat in a shrugging position, closer to the head, like in non-human apes.<ref name=Green2012/> Juvenile modern humans have a somewhat similar configuration, but this changes to the normal human condition with age; such a change does not appear to have occurred in ''A. afarensis'' development. It was once argued that this was simply a byproduct of being a small-bodied species, but the discovery of the similarly sized ''[[H. floresiensis]]'' with a more or less human shoulder configuration and larger ''A. afarensis'' specimens retaining the shrugging shoulders show this to not have been the case. The [[spine of scapula|scapular spine]] (reflecting the strength of the back muscles) is closer to the range of gorillas.<ref name=Green2012>{{Cite journal | last1 = Green | first1 = D. J. | last2 = Alemseged | first2 = Z. | doi = 10.1126/science.1227123 | title = ''Australopithecus afarensis'' Scapular Ontogeny, Function, and the Role of Climbing in Human Evolution | journal = Science | volume = 338 | issue = 6106 | pages = 514–517 | year = 2012 | pmid =  23112331|bibcode = 2012Sci...338..514G | s2cid = 206543814 }}</ref>

The forearm of ''A. afarensis'' is incompletely known, yielding various brachial indexes ([[radius (bone)|radial]] length divided by [[humeral]] length) comparable to non-human great apes at the upper estimate and to modern humans at the lower estimate. The most complete [[ulna]] specimen, AL 438–1, is within the range of modern humans and other African apes. However, the L40-19 ulna is much longer, though well below that exhibited in orangutans and gibbons. The AL 438-1 [[metacarpals]] are proportionally similar to those of modern humans and orangutans.<ref>{{cite journal|first1=M. S. M.|last1=Drapeau|first2=C. V.|last2=Ward|year=2007|title=Forelimb Segment Length Proportions in Extant Hominoids and ''Australopithecus afarensis''|journal=American Journal of Physical Anthropology|volume=132|issue=3|pages=327–343|doi=10.1002/ajpa.20533|pmid=17154362}}</ref> The ''A. afarensis'' hand is quite humanlike, though there are some aspects similar to orangutan hands which would have allowed stronger flexion of the fingers, and it probably could not handle large spherical or cylindrical objects very efficiently. Nonetheless, the hand seems to have been able to have produced a [[precision grip]] necessary in using [[stone tool]]s.<ref>{{cite journal|first=M. W.|last=Marzke|year=1983|title=Joint functions and grips of the ''Australopithecus afarensis'' hand, with special reference to the region of the capitate|journal=Journal of Human Evolution|volume=12|issue=2|pages=197–211|doi=10.1016/S0047-2484(83)80025-6|bibcode=1983JHumE..12..197M }}</ref> However, it is unclear if the hand was capable of producing stone tools.<ref>{{cite journal|first1=M.|last1=Domalain|first2=A.|last2=Bertin|first3=G.|last3=Daver|year=2017|title=Was ''Australopithecus afarensis'' able to make the Lomekwian stone tools? Towards a realistic biomechanical simulation of hand force capability in fossil hominins and new insights on the role of the fifth digit|journal=Comptes Rendus Palevol|volume=16|issue=5–6|pages=572–584|doi=10.1016/j.crpv.2016.09.003|bibcode=2017CRPal..16..572D |doi-access=free}}</ref>

===Lower limbs===
The australopith pelvis is [[pelvis#Caldwell–Moloy classification|platypelloid]] and maintains a relatively wider distance between the [[hip socket]]s and a more oval shape. Despite being much smaller, Lucy's [[pelvic inlet]] is {{cvt|132|mm}} wide, about the same breadth as that of a modern human woman. These were likely adaptations to minimise how far the [[centre of mass]] drops while walking upright in order to compensate for the short legs (rotating the hips may have been more important for ''A. afarensis''). Likewise, later ''Homo'' could reduce relative pelvic inlet size probably due to the elongation of the legs. Pelvic inlet size may not have been due to fetal head size (which would have increased [[birth canal]] and thus pelvic inlet width) as an ''A. afarensis'' newborn would have had a similar or smaller head size compared to that of a newborn chimpanzee.<ref name=Gruss2015/><ref>{{cite journal|first=Y.|last=Rak|year=1991|title=Lucy's pelvic anatomy: its role in bipedal gait|journal=Journal of Human Evolution|volume=20|issue=4|pages=283–290|doi=10.1016/0047-2484(91)90011-J|bibcode=1991JHumE..20..283R }}</ref> It is debated if the platypelloid pelvis provided poorer leverage for the [[hamstring]]s or not.<ref name=Gruss2015>{{cite journal|first1=L. T.|last1=Gruss|first2=D.|last2=Schmitt|year=2015|title=The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation|journal=Philosophical Transactions of the Royal Society B|volume=370|issue=1663|page=20140063|pmc=4305164|pmid=25602067|doi=10.1098/rstb.2014.0063}}</ref>

[[File:Resti di australopithecus afarensis detto selam, di tre anni circa, da dikika (afar), 3,3 milioni di anni fa.jpg|thumb|upright=1.5|[[DIK-1-1]] skeleton; notice the diverging left big toe bone]]
The [[heel bone]] of ''A. afarensis'' adults and modern humans have the same adaptations for bipedality, indicating a developed grade of walking. The big toe is not dextrous as is in non-human apes (it is adducted), which would make walking more energy efficient at the expense of arboreal locomotion, no longer able to grasp onto tree branches with the feet.<ref name=Latimer&Lovejoy1989>{{cite journal | last1=Latimer | first1=B. | last2=Lovejoy | first2=C. O. | title=The calcaneus of ''Australopithecus afarensis'' and its implications for the evolution of bipedality | journal=American Journal of Physical Anthropology |volume=78 |issue=3 |pages=369–386 |year=1989 |doi=10.1002/ajpa.1330780306 |pmid=2929741 }}</ref> However, the foot of the infantile specimen DIK-1-1 indicates some mobility of the big toe, though not to the degree in non-human primates. This would have reduced walking efficiency, but a partially dextrous foot in the juvenile stage may have been important in climbing activities for food or safety, or made it easier for the infant to cling onto and be carried by an adult.<ref>{{cite journal|first1=J. M.|last1=DeSilva|first2=C. M.|last2=Gill|first3=T. C.|last3=Prang|display-authors=et al.|year=2018|title=A nearly complete foot from Dikika, Ethiopia and its implications for the ontogeny and function of ''Australopithecus afarensis''|journal=Science Advances|volume=4|issue=7|page=eaar7723|doi=10.1126/sciadv.aar7723|pmid=29978043|pmc=6031372|bibcode=2018SciA....4.7723D }}</ref>