Among whale fossils that date from later than about fifty million years ago, none are fossils of whale species that drank only fresh water. The earliest whales that drank salt water differed from fresh-water-drinking whales only in their possession of specialized kidneys. Fossils of whale species dating from between sixty million and fifty million years ago will not be found on continents that were at the time separated from ancient Asia by wide expanses of ocean. Between sixty million and fifty million years ago, the freshwater lakes and rivers in which whales originated were gradually invaded by salt water.
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.
Which of the following is most strongly supported by the information given?