This pressure on the shore line and the animal probably forced them to begin diving for plants growing in deeper water. Diving for food became a practical way of making a living, and from this beginning, the largest animals to live on the earth began to evolve. These animals are called marine mammals. Marine mammals are now found in all of the world's oceans and seas. The evolutionary development of marine mammals is probably the most remarkable in the animal kingdom.
All mammals give birth to live young.
The story of the evolution of land mammals into marine mammals can be told by how much the original terrestrial features have changed. Evolution has transformed the basic land mammals into a creatures beautifully adapted to life in the ocean. Their adaptations enable them to swim, dive, breathe, keep warm, and find food in a cold, dark, hostile saltwater environment. The torpedo shape is the latest form for swimming. This spindle-shaped body is circular and thickest near the center of its length like the body of a dolphin.