Along with sealions and walruses, seals belong to a group called pinnipeds, which evolved from land animals. Researchers pinned down concrete evidence of this transition in 2007 with the discovery of Puijila darwini. Alive 20-24 million years ago, this amphibious freshwater carnivore had the muscular legs characteristic of a land creature but the webbed feet, streamlined body and tail of an aquatic animal. The theory goes that P darwini lived in once temperate Arctic lakes and rivers before gradually switching to an ocean lifestyle as these lakes began to freeze over.
Answered by Alexandra Cheung