For the last 2 decades, the origin of cetaceans (whales, dolphins, and porpoises) has become one of the best case studies in mammalian evolution. A rich collection of fossil cetaceans, mostly from Asia, Africa, and North America, traces the transition of cetaceans from a terrestrial to a completely aquatic environment. Cetaceans initially took to the seas about 50 million years ago in an ancient shallow seaway, the Tethys Sea, which was located between the Indian subcontinent and Asia. Within about 10 million years in the aquatic environment, cetaceans evolved specialized ears for hearing underwater, reduced the size of their hindlimbs, and evolved tail flukes for propulsion. The cetacean body plan has so radically evolved from terrestrial artiodactyls (even-toed hoofed mammals) that modern cetaceans are no longer able to bear their weight on land and are obligatorily aquatic.