Researchers have now determined that hippos were likely some of the first large animals to migrate from Asia to Africa, swimming from one continent to the other roughly 35 million years ago. However, they certainly weren't the semi-aquatic giants they are today. Fossil evidence indicates that ancient hippos were no larger than modern sheep.

That's at least according to a study recently published in the journal Nature Communications, which describes the migration and evolution of Epirigenys lokonensis - a species paleontologists and evolutionary experts are calling a "gap-closer" for a gaping hole in the fossil record.

"The origins of the hippopotamus have been a mystery until now," Fabrice Lihoreau, a paleontologist at France's University of Montpellier and co-author of the study, recently told the Agence France-Presse (AFP). "Now we can say that hippos came from anthracotheres," a group of plant-eating, semi-aquatic mammals with toed-hooves that is long-gone from the Earth.

So what was the discrepancy? Taxonomists have long grouped hippos with pigs, as younger pig and hippo ancestors (at most 20 million years old) shared striking similarities in skeletal design. However, this flies in the face of genetic evidence first identified in 2004, which has since been expanded on.

"Molecular phylogeny (tracing DNA sequences) shows a close relationship with whales," Jean-Renaud Boisserie, a researcher who led the 2004 study, explained in a statement. "But cetaceans - whales, porpoises and dolphins - don't look anything like hippos. There is a 40-million-year gap between fossils of early cetaceans and early hippos."

Enter E. lokonensis reamins, found in Kenya, that are some 28 million years old. Not only do they indicate that hippos' small ancestors were one-twentieth the size of modern sub-Saharan hippopotamuses, but its shape indicates an "extreme affinity for water" - making the cetacean theory slightly more believable.

"They are slender hippos, very thin hippos," Lihoreau told Live Science.

Closely analyzing teeth from this 'original hippo' (the first true hippopotamidea emerged in the fossil record only 15 million years ago), the researcher and his colleagues were able to conclude that the essential evolution of these beasts occurred entirely in Africa long before the continent's other iconic mammals arrived.

"They are not really African mammals," Lihoreau added, speaking of giraffes, rhinos, and lions. "This [the hippopotamus] is really an African mammal."

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