Whales originated in the freshwater lakes and rivers of ancient Asia about sixty million years ago  Not until about ten million years later did species of whales develop specialized kidneys enabling them to drink salt water
Although fossil evidence shows that some early whale species that lacked such kidneys sometimes swam in the Earth' s saltwater oceans, these species must have had to return frequently to freshwater rivers to drink
Among whale fossils that date from later than about fifty million years ago, none are fossils of whale species that drank only fresh water.