The most recent theory into the origins of hippopotamidae suggests that hippos and whales shared a common semi-aquatic ancestor that branched off from other Artiodactyls around 60 million years ago.This hypothesized ancestral group likely split into two branches around 54 million years ago. One branch would evolve into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 million years ago with other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans. The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of whom in the Late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants. A rough evolutionary lineage can be traced, however, from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene. Merycopotamus, Libycosaurus and all hippopotamids can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about 20 million years ago. Hippopotamids are therefore deeply nested within the family Anthracotheriidae. The Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus which lived in Africa from 16 to 8 million years ago. While hippoptamid species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas, though, various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to 1.8 million years ago an ancestor to the modern hippopotamus, the Archaeopotamus lived in Africa and the Middle East. While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as 8 million years ago. Taxonomists disagree whether or not the modern Pygmy Hippopotamus is a member of Hexaprotodon—an apparently paraphyletic genus also embracing many extinct Asian Hippopotamuses that is more-closely related to Hippopotamus; or Choeropsis—an older and basal genus.
