Whales themselves were never land animals, but their ancestors were. The first mammals (animals that give milk and have hair) lived on land. Some of the species that evolved from these first mammals were carnivores and hunted other mammals. Some of these evolved into predators that lived in water. Some of their descendants became even more adapted to living in water. There’s a good whale “family tree” at: whale family tree.

One interesting thing to notice is that even the whales that have no teeth are the descendant of carnivores. Another interesting thing is that whales are more closely related to hippos than to other marine mammals, like seals, walruses, and sea lions.

The ancestors of the whales are long gone, so how do we know that some of them lived in land and some lived in water? Scientists study the skeletons (or fossils of the skeletons) of animals. They look for clues like flippers instead of paws. Another big clue is whether a skull has nostrils on the nose, or whether the nostrils are on the top of the head—a blowhole. No land mammals have a blowhole.

Some mammals spend all of their lives in water, some of them live in water most of the time, but also spend time on land. Take a look at pictures of seals and sea lions. How are they like whales? How are they different? Remember that I said that mammals have hair? Do whales have hair? Do whale fetuses have hair before they are born?

If you are interested in questions like these, you may want to study marine biology, anatomy, or evolution.

Whales are mammals (a specific kind of warm-blooded, backboned animal), and indeed, the ancestors of whales lived on land. Interestingly, several different fossils document the transition from land-living (terrestrial) to water-living (aquatic), including the famous animal illustrated below, Ambulocetus, the name of which means "walking whale."

50 million years ago, after the dinosaurs but still long before the ice age, the ancestors of whales lived on land. Today's whales are born in the ocean and spend their whole lives there.

Yes, all modern whales evolved from an early land mammal called a Pakicetus. See this nice video from the Smithsonian Museum of Natural History for an animation of the evolution of modern day whales:

The animals from which whales descended lived on land. The skeleton of modern whales gives scientists clues on how the whale evolved. It is apparent from comparing the modern whale skeleton to fossils and land animals that the key features of the skeleton for land animals are present in the whale.