Evolving from a group of mammal-like reptiles, the first true mammals were shrew-like animals that lived 195 million years ago. Over the nest 130 million years, dinosaurs ruled the Earth and mammals remained small. It was not until dinosaurs went extinct around 65 million ago that mammals could evolve and diversify to the extent seen today. There are 4,785 species of mammal on Earth ranging from the thimble-sized field mouse to the blue whale, which weighs 1,750 times more than the average man. Mammals are warm blooded, usually possess body hair, and nurse young with milk produced by mammary glands.

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