This is old news to some, but I just watched a National Geographic television show about hippos and was stuck again by how wierd and wonderful is evolution. Hippos have long been grouped with pigs because of certain ridges on their molar teeth. But, on an evolutionary scale, teeth change rapidly in response to environmental pressure. Fifty years ago, immunological tests suggested a relationship between whales and the cloven-hoofed mamals (even-toed ungulates), the Artiodactyla. Twenty years ago, DNA analysis of certain proteins suggested that whales are closely related to them. But there was no other evidence so the hypothesis was put on hold. There was no explanation—no theory.
Then, in 2001, fossils of ancient whales called Basilosaurus were discovered. They still had tiny back legs. Basilosaurus skeletons had been discovered before, but the smaller bones were missing. Finally, one was discovered with the small bones intact. And in its legs were Artiodactyl ankle-joints.
Another fossil, Dorodon atrax, was discovered in 1998, in Egypt, by Philip Gingerich. That fossil is on display at the University of Michigan. And in its tiny legs Professor Gingerich found the double-pulley ankle bones of a sheep or an antelope.
The show went on with more oddities of the hippo, some of which can now evoke the “Why didn’t I think of that?” reaction. Hippopotamus can roar or grunt above water, but click, whine, and whistle below, sounding remarkably like whales. They can call and listen both above and below water at the same time, making them unique. Just as in whales, their noses have flaps that close automatically when the nostrils sink below water, even if the animals are asleep. Fascinating creatures!
The long and the short of it is that hippos are more closely related to whales than to any land mammal.