Final answer:
Cetaceans, including whales, dolphins, and porpoises, evolved from land-dwelling mammals, and the fossil record shows a transition from land to water with species like Pakicetus and Rodhocetus. Their closest living relatives on land are the hippopotamuses.
Step-by-step explanation:
DNA evidence and the fossil record indicate that the ancestors of living cetaceans, which include whales, dolphins, and porpoises, are land-dwelling mammals. Specifically, fossil record analysis has identified ancient species such as Pakicetus and Rodhocetus as ancestors of modern whales. These ancient cetaceans display a gradual transition from land to water-adapted creatures.
Rodhocetus, for example, is known to have had a type of ankle bone that is otherwise unique to a group of land animals, confirming their terrestrial origins.
Additionally, cetaceans are understood to be descended from even-toed ungulates, sharing a common ancestry with hippopotamuses, which are their closest living relatives among land-dwelling species.
Mammals are unique in having evolved to transition from land back to the ocean, and cetaceans present a clear example of this remarkable evolutionary journey. Their skeletal structures, including vestigial hindlimbs, show traces of their terrestrial past.
Phylogenetic trees also support this connection, revealing that cetaceans are more closely related to even-toed ungulates like the hippopotamus than any other group of animals. Therefore, among the options provided, the correct answer indicating the ancestors of living cetaceans is land-dwelling mammals.