 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 . Being mammals , they surface to breathe air ; they have finger bones in their fins ; they nurse their young ; and , despite their fully aquatic life style , they retained many skeletal features from their terrestrial , mammalian ancestors . These mammalian characteristics all point to their terrestrial origin , also the first mammals were terrestrial , so cetaceans , being mammals , must have come from the land . Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea . 
