Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Mysticeti (baleen whales) and Odontoceti (toothed whales) – are thought to have separated from each other around 28-33 million years ago.