Over 5,000 species of living animals are classified as mammals. These animals share common features such as warm-bloodedness, characteristic skull shapes, and mammary glands with which the females nourish their young. Although they also had these features, the earliest mammals were very different from modern mammals. Fossils have shown that mammals that existed over 200 million years ago were small and shrew- or rodent-like. Over time, these mammals evolved, becoming more like modern mammals.

Fossil evidence shows that during the Cretaceous period — from about 145 million years ago to about 65 million years ago — significant evolution in the variety of life on Earth occurred. Flowering plants appeared, and land and sea animals diversified. However, dinosaurs were the dominant land animals, and other animals — such as mammals — were constrained to limited ecological niches. These mammals were small, probably nocturnal, and based underground. Not until the dinosaurs became extinct did mammals evolve significantly and diversify.

The fossil record clearly shows that about 65 million years ago, Earth underwent a mass extinction. Scientists are not sure exactly what caused the extinction, but after that time, about two-thirds of all species on the planet ceased to exist. The most famous victims of this extinction event are the dinosaurs that once dominated Earth's landscape; the most famous survivors are the mammals. Scientists do not know why some species were able to survive the extinction while others could not, but the fossil record shows that mammals not only survived, they went on to flourish.

Without competition from the dinosaurs, mammals had the opportunity to adapt to many more environments. As mammals evolved, they diversified, grew bigger, and adapted to new niches. Today, mammals come in all sizes — from the very small to the very large — and can be seen on land, in water, and even in the air. The era in which we live is often called the "age of mammals" because of the explosion in their diversity and because the largest land animals are mammals. However, in the absence of the dinosaurs, many more habitats and ecological niches became options for all surviving life — not just mammals.

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