Both hippos and whales evolved from four-legged, even-toed, hoofed (ungulate) ancestors that lived on land about 50 million years ago. ... The theory is that some land-living ungulates favoured munching on plants at the water's edge which had the added advantage of allowing them to easily hide from danger in shallow water.

Where do cetaceans evolve from?

Cetaceans originated from land mammals (Thewissen and Williams 2002; Fordyce and Muizon 2001). Many features that are common in land mammals have changed in the evolutionary process that led to cetaceans. The presence of hair or fur, for instance, is characteristic of mammals.

What evidence is there for the evolution of cetaceans?

Archaeocetes had a “double-pulley” astragalus, confirming that cetaceans had evolved from artiodactyls. Mesonychids were not the ancestors of whales, and hippos are now known to be the closest living relatives to whales. Recently scientists determined which group of prehistoric artiodactyls gave rise to whales.