page_title: Even-toed ungulate
section_title: Research history
hierarchical_section_title: Even-toed ungulate / Taxonomy and phylogeny / Research history
caption_attribution_description: English: Hippopotamus amphibius in Kruger National Park Deutsch: Flusspferd im Krüger-Nationalpark
context_page_description: The even-toed ungulates are ungulates – hoofed animals – which bear weight equally on two of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, odd-toed ungulates bear weight on one of the five toes: the third toe. Another difference between the two is that even-toed ungulates digest plant cellulose in one or more stomach chambers rather than in their intestine as the odd-toed ungulates do.
  The aquatic cetaceans evolved from even-toed ungulates, so modern taxonomic classification combines the two under the name Cetartiodactyla.
  The roughly 270 land-based even-toed ungulate species include pigs, peccaries, hippopotamuses, antelopes, mouse deer, deer, giraffes, camels, llamas, alpacas, sheep, goats, and cattle. Many of these are of great dietary, economic, and cultural importance to humans.
context_section_description: In the 1990s, biological systematics used not only morphology and fossils to classify organisms, but also molecular biology. Molecular biology involves sequencing an organism's DNA and RNA and comparing the sequence with that of other living beings – the more similar they are, the more closely they are related. Comparison of even-toed ungulate and cetaceans genetic material has shown that the closest living relatives of whales and hippopotamuses is the paraphyletic group Artiodactyla.
  Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994. However, they did not recognize hippopotamuses and classified the ruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on casein genes, SINEs, fibrinogen sequences, cytochrome and rRNA sequences, IRBP (and vWF) gene sequences, adrenergic receptors, and apolipoproteins.
  
  In 2001, the fossil limbs of a Pakicetus (amphibioid cetacean the size of a wolf) and Ichthyolestes (an early whale the size of a fox) were found in Pakistan. They were both archaeocetes ("ancient whales") from about 48 million years ago (in the Eocene). These findings showed that archaeocetes were more terrestrial than previously thought, and that the special construction of the talus (ankle bone) with a double-rolled joint surface, previously thought to be unique to even-toed ungulates, were also in early cetaceans. The mesonychids, another type of ungulate, did not show this special construction of the talus, and thus was concluded to not have the same ancestors as cetaceans.
  The oldest cetaceans date back to the early Eocene (53 million years ago), whereas the oldest known hippopotamus dates back only to the Miocene (15 million years ago). Some doubts have arisen regarding the relationship between the two, as there is a 40 million year gap between their first appearances in the fossil record. It seems unlikely that there were ancestral hippos that left no remains, given the high number of even-toed ungulate fossils. Some studies proposed the late emergence of hippos is because they are relatives of peccaries and split recently, but molecular findings contradict this. Research is therefore focused on anthracortheres (family Anthracotheriidae); one dating from the Eocene to Miocene was declared to be "hippo-like" upon discovery in the 19th century. A study from 2005 showed that the anthracotheres and hippopotamuses have very similar skulls, but differed in the adaptations of their teeth. It was nevertheless believed that cetaceans and anthracothereres descended from a common ancestor, and that hippopotamuses developed from anthracotheres. A study published in 2015 was able to confirm this, but also revealed that hippopotamuses were derived from older anthracotheriens. The newly introduced genus Epirigenys from eastern Africa is thus the sister group of hippos.
caption: Hippos are a geologically young group, which raises questions about their origin.
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