The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses.