At present, the largest animal in the world is a marine mammal whale. It has a streamlined body like a fish, keeps a constant temperature and breathes with its lungs. Its offspring are conceived in the womb.

The recent ancestors of whales and hippos were semi-aquatic omnivores that differentiated from ruminants 60 million years ago. One of them entered the water from South Asia and other places in Eocene 54 million years ago, and gradually evolved into a complete aquatic whale, which spread to all major sea areas in the world, and some species also entered inland rivers and lakes.

Molecular phylogenetic studies show that cetaceans originated from Artiodactyla and are sister groups with hippopotamus, so there are no cetaceans in the same group (that is, some evolutionary branches are classified into other groups). Most of the current classifications combine Artiodactyla and Cetacea into one line? Even-hoofed animals, but generally still use the name of even-hoofed animals.

In the evolutionary history of species, all vertebrates evolved from fish in the ocean 300 million years ago, and the ancestors of cetaceans returned to the ocean after leaving the ocean for 250 million years. This is called "secondary entry into the water", which is a rare major turning point in evolutionary history.