The Mesozoic and Cenozoic Eras are two of the three major divisions of geologic time. The Mesozoic ERA, which began about 252 million years ago and ended about 66 million years ago, is often referred to as the Age of Reptiles. During this period, dinosaurs were the dominant land animals, and the first birds and mammals appeared. The Cenozoic ERA, which began about 66 million years ago and continues today, is often referred to as the Age of Mammals. During this period, mammals became the dominant land animals, and humans evolved.