Modern whales are fully aquatic mammals, i.e. they cannot live outside of water. Like other mammals they breathe air, give birth to live young that they nurse, and have hair in early stages of their development. Cetaceans arose in the Eocene, approximately 50 million years ago and became fully aquatic in a few million years.
Whale origins are well known from fossils.
They document the transition from land to water and tell one of the most exciting stories in evolution. All pictures on these pages are public access, although the source must be identified in publication.