 The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even @-@ toed ungulates 50 million years ago , over a period of at least 15 million years , however a jawbone discovered in Antarctica may reduce this to 5 million years . Archaeoceti is an extinct parvorder of Cetacea containing ancient whales . The traditional theory of cetacean evolution , first proposed by Van Valen in 1966 , was that whales were related to the mesonychids , an extinct order of carnivorous ungulates ( hoofed animals ) that resembled wolves with hooves and were a sister group of the artiodactyls ( even @-@ toed ungulates ) . This theory arose due to similarities between the unusual triangular teeth of the mesonychids and those of early whales . However , molecular phylogeny data indicates that whales are very closely related to the artiodactyls , with hippopotamuses as their closest living relative . Because of this , cetaceans and hippopotamuses are placed in the same suborder , Whippomorpha . Cetartiodactyla ( formed from the words Cetacea and Artiodactyla ) is a proposed name for an order containing both cetaceans and artiodactyls . However , the earliest anthracotheres , the ancestors of hippos , do not appear in the fossil record until the Middle Eocene , millions of years after Pakicetus , the first known whale ancestor , appeared during the Early Eocene , implying the two groups diverged well before the Eocene . Since molecular analysis identifies artiodactyls as being very closely related to cetaceans , mesonychids are probably an offshoot from Artiodactyla , and cetaceans did not derive directly from them . 
