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Scientists divide the history of Earth into eras. Different types of animal life help distinguish these eras. We live in the Cenozoic era, which began roughly sixty-six million years ago. The Cenozoic era is known as the Age of Mammals because mammals, like humans, are the largest land animals. During the previous era, called the Mesozoic era, big reptiles, like dinosaurs, were the dominant animal species—it’s nicknamed the Age of Reptiles. Some scientists believe that, at the end of the Mesozoic era, a large object from space collided with Earth, helping cause the mass extinction of the dinosaurs and ushering in the Age of Mammals.