When did the first whales evolve from land mammals?

Whales evolved from land-dwelling mammals during the early Eocene, about 50 million years ago. Transitional fossils such as Pakicetus, Ambulocetus, and later forms show a gradual shift from four-legged land animals to fully aquatic whales. These fossils document changes in ears, limbs, and tail propulsion over millions of years. Whale evolution is one of the clearest examples of major anatomical transition in the fossil record.