Editing Pygmy hippopotamus (section)

=== Evolution ===
{{main|Hippopotamus#Evolution}}
[[File:Anthracotherium magnum.jpg|thumb|[[Anthracothere]]s like ''[[Anthracotherium]]'' resembled pygmy hippos and are among their likely ancestors.]]
The evolution of the pygmy hippopotamus is most often studied in the context of its larger cousin. Both species were long believed to be most closely related to the family [[Suidae]] ([[pig]]s and hogs) or Tayassuidae ([[peccaries]]), but research within the last 10 years has determined that pygmy hippos and hippos are most closely related to [[cetacea]]ns ([[whale]]s and [[dolphin]]s). Hippos and whales shared a common semi-aquatic ancestor that branched off from other artiodactyls around {{mya|60|mya}}.<ref name="ScienceNews">{{Cite web|title=Scientists find missing link between the dolphin, whale and its closest relative, the hippo |date=2005-01-25 |access-date=2008-08-23 |url=http://www.sciencenewsdaily.org/story-2806.html |work=Science News Daily |url-status=dead |archive-url=https://web.archive.org/web/20070304214747/http://www.sciencenewsdaily.org/story-2806.html |archive-date=2007-03-04 }}</ref><ref name="DNA">{{Cite journal | title = More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen | author = Gatesy, J | journal = [[Molecular Biology and Evolution]] | volume = 14 | pages = 537–543 | pmid = 9159931 | issue = 5 | date=1 May 1997 | doi=10.1093/oxfordjournals.molbev.a025790| doi-access = free }}</ref>

This hypothesized ancestor likely split into two branches about six million years later.<ref name="Genomes">{{Cite journal | title = Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade | volume = 265 | issue = 1412 | year = 1998 | pages = 2251–5 | journal = [[Proceedings of the Royal Society]] | author = Ursing, B.M. |author2=U. Arnason  | doi = 10.1098/rspb.1998.0567 | pmid = 9881471 | pmc = 1689531}}</ref> One branch would [[Evolution of cetaceans|evolve into cetaceans]], the other branch became the [[anthracotheriidae|anthracotheres]], a large family of four-legged beasts, whose earliest member, from the Late [[Eocene]], would have resembled narrow hippopotami with comparatively small and thin heads.<ref name="Cetartiodactyla">{{cite journal |last=Boisserie |first=Jean-Renaud |author2= Fabrice Lihoreau |author3=Michel Brunet |date=February 2005|title= The position of Hippopotamidae within Cetartiodactyla|journal= [[Proceedings of the National Academy of Sciences]] |volume= 102 |issue= 5|pages= 1537–1541|doi= 10.1073/pnas.0409518102|pmid= 15677331 |pmc=547867|bibcode=2005PNAS..102.1537B |doi-access=free }}</ref>

Hippopotamids are deeply nested within the family [[Anthracotheriidae]]. The oldest known hippopotamid is the genus ''[[Kenyapotamus]]'', which lived in Africa from {{mya|16|8|mya}}. ''Kenyapotamus'' is known only through fragmentary fossils, but was similar in size to ''C.&nbsp;liberiensis''.<ref name="Origins">{{cite journal | last= Boisserie | first=Jean-Renaud | author2=Fabrice Lihoreau |author3=Michel Brunet |date=March 2005 |title=Origins of Hippopotamidae (Mammalia, Cetartiodactyla): towards resolution |journal=[[Zoologica Scripta]] |volume= 34|issue= 2|pages=119–143 | doi = 10.1111/j.1463-6409.2005.00183.x | s2cid=83768668 }}</ref> The Hippopotamidae are believed to have evolved in Africa, and while at one point the species spread across Asia and Europe, no hippopotami have ever been discovered in the Americas. Starting {{mya|7.5|1.8|mya}} the ''[[Archaeopotamus]]'', likely ancestors to the genus ''Hippopotamus'' and ''[[Hexaprotodon]]'', lived in Africa and the Middle East.<ref name="Linnean"/>

While the fossil record of hippos is still poorly understood, the lineages of the two modern genera, ''[[Hippopotamus (genus)|Hippopotamus]]'' and ''Choeropsis'', may have diverged as far back as {{mya|8|mya}}. The ancestral form of the pygmy hippopotamus may be the genus ''Saotherium''. ''Saotherium'' and ''Choeropsis'' are significantly more [[Basal (phylogenetics)|basal]] than ''Hippopotamus'' and ''Hexaprotodon'', and thus more closely resemble the ancestral species of hippos.<ref name="Linnean"/><ref name="Origins"/>