Return to Book List Grade 2   Clues from the Past   Life Long Ago     Earth's layers show how Earth has changed over time.   Time on Earth is divided into eras, which are long periods of time.   The Paleozoic era is the oldest time period.   As Earth changed, many living things could not live and became extinct.   Extinct means to die out.   The Mesozoic era followed the Paleozoic era and is called the "Age of Reptiles."   During the Mesozoic era, the Earth became warmer. Dinosaurs lived at that time.   We live in the Cenozoic era, which is called the "Age of Mammals." It started after the Mesozoic era.   The Cenozoic era started a long time ago.