Cetaceans descend from land animals that returned to the sea about 50 million years ago. They share common ancestors with the even-toed ungulates (Order Artiodactyla), and scientists now often classify the cetaceans in that Order, rather than the separate Order Cetacea. More specifically, the cetaceans are most closely related to the hippopotamus. Both the cetaceans and the hippopotamus have a unique arrangement of their vocal cords designed to enhance the projection of sound underwater.

Today there are about 95 species of whales and dolphins. These are divided into the toothed whales (80 species) and the baleen whales (15 species). In all of these, their limbs have evolved into fins.

The killer whale (orca), false killer whale (pseudorca), and beluga whales are examples of toothed whales. Technically, the dolphins are also toothed whales. That includes bottlenose, common, dusky, and Amazon river dolphins.

The humpback and southern right whales are examples of the baleen whales. About 34 million years ago, the baleen whales evolved plates of stiff bristles in their mouths in place of teeth. The baleen bristles are modified skin cells made of the same keratin proteins as fingernails and horns. The whales use these to strain food from mouthfuls of seawater.