Editing Pliocene (section)

==Environment and evolution of human ancestors==
The Pliocene is bookended by two significant events in the evolution of human ancestors. The first is the appearance of the [[Hominini|hominin]] ''[[Australopithecus anamensis]]'' in the early Pliocene, around 4.2 million years ago.<ref>{{Cite web|date=2019-08-28|title=Le plus vieux crâne fossile d'australopithèque a été découvert en Éthiopie|url=https://www.rfi.fr/fr/afrique/20190828-ethiopie-decouverte-plus-vieux-fossile-australopitheque|access-date=2023-02-18|website=RFI|language=fr
| first = Simon | last = Rozé}}</ref><ref name="Haile-Selassie2010c">{{cite journal |author=Haile-Selassie, Y |date=27 October 2010 |title=Phylogeny of early Australopithecus: new fossil evidence from the Woranso-Mille (central Afar, Ethiopia)|journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=365 |issue=1556 |pages=3323–3331 |pmid=20855306 |pmc=2981958 |doi=10.1098/rstb.2010.0064}}</ref><ref name=":12">{{Cite book|title=Understanding Humans: Introduction to Physical Anthropology and Archaeology|last=Lewis|first=Barry |display-authors=etal |year=2013|edition=11th|location=Belmont, CA: Wadsworth Publishing}}</ref> The second is the appearance of ''[[Homo]]'', the genus that includes [[Homo sapiens|modern humans]] and their closest extinct relatives, near the end of the Pliocene at 2.6 million years ago.<ref>{{cite book|title= The Cambridge Encyclopedia of Human Evolution | vauthors = Stringer CB | chapter=Evolution of early humans | veditors = Jones S, Martin R, Pilbeam D | year=1994 | publisher= Cambridge University Press | location= Cambridge  | page=242}}</ref> Key traits that evolved among hominins during the Pliocene include terrestrial bipedality and, by the end of the Pliocene, [[Encephalization quotient|encephalized brains]] (brains with a large [[neocortex]] relative to body mass<ref name=Potts2007>{{Citation |last=Potts |first=R. |title=Environmental hypotheses of Pliocene human evolution |date=2007 |work=Hominin Environments in the East African Pliocene: An Assessment of the Faunal Evidence |pages=25–49 |editor-last=Bobe |editor-first=René |series=Vertebrate Paleobiology and Paleoanthropology Series |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-1-4020-3098-7_2 |isbn=978-1-4020-3098-7 |editor2-last=Alemseged |editor2-first=Zeresenay |editor3-last=Behrensmeyer |editor3-first=Anna K.}}</ref>{{efn|Because of the 2009 reassignment of the Pliocene-Pleistocene boundary from 1.8 to 2.6 million years ago, older papers on Pliocene hominin evolution sometimes include events that would now be regarded as taking place in the early Pleistocene.}} and stone tool manufacture.<ref name="Potts1999"/>

Improvements in [[Chronological dating|dating methods]] and in the use of [[Proxy (climate)|climate proxies]] have provided scientists with the means to test hypotheses of the evolution of human ancestors.<ref name="Potts1999">{{cite journal |last1=Potts |first1=Richard |title=Environmental hypotheses of hominin evolution |journal=American Journal of Biological Anthropology |date=3 January 1999 |volume=107 |issue=S27 |pages=93–136 |doi=10.1002/(SICI)1096-8644(1998)107:27+<93::AID-AJPA5>3.0.CO;2-X|pmid=9881524 }}</ref><ref name=deMenocal2004>{{Cite journal |last=deMenocal |first=Peter B |date=3 March 2004 |title=African climate change and faunal evolution during the Pliocene–Pleistocene |journal=Earth and Planetary Science Letters |language=en |volume=220 |issue=1 |pages=3–24 |doi=10.1016/S0012-821X(04)00003-2 |bibcode=2004E&PSL.220....3D |issn=0012-821X|doi-access=free }}</ref> Early hypotheses of the evolution of human traits emphasized the selective pressures produced by particular habitats. For example, many scientists have long favored the [[savannah hypothesis]]. This proposes that the evolution of terrestrial bipedality and other traits was an adaptive response to Pliocene climate change that transformed forests into more open [[savannah]]. This was championed by [[Grafton Elliot Smith]] in his 1924 book, ''The Evolution of Man'', as "the unknown world beyond the trees", and was further elaborated by [[Raymond Dart]] as the [[killer ape theory]].<ref>{{cite journal |last1=Dart |first1=R |year=1953 |title=The predatory transition from ape to man |journal=Int. Anthrop. Ling. Rev |volume=1 |pages=201–218}}</ref> Other scientists, such as [[Sherwood L. Washburn]], emphasized an intrinsic model of hominin evolution. According to this model, early evolutionary developments triggered later developments. The model placed little emphasis on the surrounding environment.<ref>{{cite journal |last1=Washburn |first1=SL |year=1960 |title=Tools and human evolution |journal=Sci. Am. |volume=203 |number=3 |pages=63–75 |doi=10.1038/scientificamerican0960-62 |jstor=24940615|pmid=13843002 |bibcode=1960SciAm.203c..62W }}</ref> Anthropologists tended to focus on intrinsic models while geologists and vertebrate paleontologists tended to put greater emphasis on habitats.{{sfn|Potts|1999|pp=106–108}}

Alternatives to the savanna hypothesis include the woodland/forest hypothesis, which emphasizes the evolution of hominins in closed habitats, or hypotheses emphasizing the influence of colder habitats at higher latitudes or the influence of seasonal variation. More recent research has emphasized the variability selection hypothesis, which proposes that variability in climate fostered development of hominin traits.<ref name="Potts1999"/> Improved climate proxies show that the Pliocene climate of east Africa was highly variable, suggesting that adaptability to varying conditions was more important in driving hominin evolution than the steady pressure of a particular habitat.<ref name=Potts2007/>