Oddly, genetic study and the fossil record show that whales and hippos are evolutionary cousins, according to a report published in the science journal Proceedings of the National Academy of Sciences. Their shared ancestor, an aquatic mammal, lived 50 to 60 million years ago. From Reuters:
“If you look at the general shape of the [hippo] it could be related to horses, as the ancient Greeks thought, or pigs, as modern scientists thought, while molecular phylogeny shows a close relationship with whales,” Dr. (Jean-Renaud) Boisserie explained. “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.”