Evolution of cetaceans
Hippo fossils are not found until the Miocene, but whale ancestors have been found from the Eocene. This leaves a gap of nearly 30 million years where no hippo ancestors are found.  The most recent hypothesis is that hippos and whales shared a common semi-aquatic ancestor which branched off from other Artiodactyls around 60 million years ago (mya).  This ancestral group probably split into two branches around 54 mya. One branch evolved into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 mya.  These early cetaceans gradually became adapted to life in the sea. They became the completely aquatic cetaceans.