Answer: The basic theme of whale evolution is the development of large animals from much smaller ancestors, and nowhere is this more evident than in the case of multi-ton sperm and gray whales, whose ultimate forebears were small, dog-sized prehistoric mammals that prowled the riverbeds of central Asia 50 million years ago. Perhaps more intriguingly, whales are also a case study in the gradual evolution of mammals from fully terrestrial to fully marine lifestyles, with corresponding adaptations (elongated bodies, webbed feet, blowholes, etc.) at various key intervals along the way.
