Here's why:

* Fossil Record: Fossils provide a record of life on Earth over millions of years. The types of fossils found in rock layers indicate the types of organisms that lived during that time.

* Changes in Life: Evolutionary changes over long periods led to the extinction of some species and the emergence of new ones. These shifts in the types of organisms are reflected in the fossil record.

* Geologic Periods: Geologists have divided Earth's history into distinct periods based on major changes observed in the fossil record. These periods represent significant shifts in the dominant life forms and environmental conditions.

Examples of Geologic Periods:

* Paleozoic Era: Characterized by the rise of marine invertebrates, fish, amphibians, and early reptiles.

* Mesozoic Era: Known for dinosaurs and the rise of flowering plants.

* Cenozoic Era: Marked by the diversification of mammals and the appearance of humans.

By studying the changes in the fossil record across different geologic periods, scientists can gain valuable insights into the history of life on Earth and the processes of evolution and extinction.