Mammals are a group of animals (of the class Mammalia) found throughout the world. Even in regions where the most extreme climatic conditions exist, there are likely to be mammals. Seals, walruses, and whales survive in the cold Arctic and Antarctic. Small pikas live near the tops of high mountain ranges. Arid deserts are home to a wide variety of rodents and their coyote predators. Camels and their amazing ability to survive for long periods without drinking water are an almost universal symbol of the desert. Mammals have successfully survived in so many environments that it is no wonder scientists call this the Age of Mammals.

Mammals, however, did not always exist over such a wide geographical range. They have an evolutionary history going back to the dinosaurs. The history of mammals may surprise people, because mammal fossils have been found in rock deposits similar in age to...