"The geologic time scale is like a calendar of evolutionary history that is based on a study of Earth's rocks and the fossils they contain. Because rock layers appear in the same order in which they were formed, each layer represents a different period of time. Lower layers are older than upper layers if they have not been disturbed. Therefore, fossils found in lower layers are older than fossils found in upper layers. The order in which types of fossils appear shows the way life changed over time"