Until 1985, naturalists grouped hippos with domestic pigs by suggesting the structure of their teeth. However, data obtained from the study of blood proteins, molecular phylogeny (path of ancestral development, origin, and evolutionary changes), DNA, and fossils indicate that their closest living relatives are cetaceans - whales, porpoises, dolphins, and others. The common ancestor of whales and hippos separated from other even-toed ungulates about 60 million years ago.
