Radiometric evidence from old Earth and solar-system materials supports the age estimate. The Paleozoic, Mesozoic and Cenozoic eras follow one another; the Cenozoic begins around the end-Cretaceous extinction about 66 million years ago.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• Geologic time organizes Earth’s history. Eras contain periods; Earth is about 4.6 billion years old.
• Do not confuse Earth’s age with the much more recent start of the Cenozoic.