Return to the Sea portrays the life and evolutionary times of marine mammals--from giant whales and sea cows that originated 55 million years ago to the deep-diving elephant seals and clam-eating walruses of modern times. This fascinating account of the origin of various marine-mammal lineages--some extinct, others extant but threatened--is for the nonspecialist. Against a backdrop of geologic time and changing climates and geography, this volume takes evolution as its unifying principle to help us to understand today's diversity of marine mammals and their responses to environmental challenges. Annalisa Berta explains current controversies and explores patterns of change now taking place, such as shifting food webs and predator-prey relationships, habitat degradation, global warming, and the effects of humans on marine-mammal communities.