Some mammal had started to evolve into aquatic animal from the Early Eocene Period, about 50 to 40 million years ago. They were back to water from land. Then gradually they had become the ancient of cetacean family.

Pakicetus probably inhabited water nearby around 50 million years ago. They have water functional ears of which structure is very similar to that of whale-ears. The special ears provide them good direction hearing in the water.

Ambulocetus were more adapted to life in the ocean during the Early Eocene Period, around 49 million years ago. They have a webbed foot and amphibious life as of otters do now.

Dorudon swam in the ocean during the Late Eocene Period, around 40 to 37 million years ago. They have long, streamed body with short neck to become a fully aquatic mammal. Instead of developing vision and smell they acquired keen sense of hearing in the water. Also they have round shape vertebrate and this feature indicates that they probably have tale like that of whales.

Dorudon has special teeth to crash clams.