They have specialized skin glands, including the most important ones called the mammary glands that produce milk to nourish the young. They breathe air with lungs . Their bodies are typically covered with hair . They reproduce through internal fertilization and gestation, and producing live young .
Mammals also have teeth that are specialized to the most advanced degree of all animals.
Although these animals seem to be very different today, they share a common ancestor that lived on earth fifty million years ago. The ancient animal is called mesonychid, and looked something like a wolf with hooves. It lived on land, walked on four legs, and ate meat. Some of the mesonychids evolved into fish-eating animals with nostrils that were positioned on top of their snouts. These were the first ancient whales, called archeocetes.