The Geologic Time Scale: Reading Earth's History in Rock Layers

0:00How do we know what lived on Earth long before there were people to see it?

0:05Rock forms in layers, one on top of another, over millions of years.

0:11The lowest layers formed first, so they are the oldest. The top layers are the youngest.

0:18Each layer holds fossils of the living things from its time.

0:22Rocks all over the world show the same order of fossils, from bottom to top.

0:28So scientists divided Earth's history into blocks of time. This is the geologic time scale.

0:35The Precambrian is most of Earth's history, from 4.6 billion to 540 million years ago. Its fossils are mostly tiny and simple.

0:46The Paleozoic means ancient life. Its fossils include shells, fish and the first land plants.

0:54The Mesozoic means middle life. This is the age of the dinosaurs, and the first birds.

1:01The Cenozoic means recent life. Mammals and birds spread, and humans appear near the very end.

1:09Each boundary marks a big change in the fossils, such as when the dinosaurs died out.

1:15So the fossil record shows that the kinds of living things on Earth have changed over time.

1:21Rock layers and their fossils let us read Earth's history, from oldest to youngest.