The Paleozoic and Mesozoic Eras are two of the three major divisions of geologic time. The Paleozoic ERA, which began about 541 million years ago, is characterized by the emergence of the first complex life forms on land and in the oceans. The Mesozoic ERA, which began about 252 million years ago, is known as the Age of Reptiles and saw the evolution of dinosaurs. Both eras are part of the Phanerozoic Eon, which is the current eon in Earth's history.