This timeline of natural history summarizes significant geological and biological events from the formation of the Earth to the arrival of modern humans.
Radiometric dating measures the steady decay of radioactive elements in an object to determine its age.
The ages of more recent layers are calculated primarily by the study of fossils, which are remains of ancient life preserved in the rock.
Most of the boundaries in recent geologic time coincide with extinctions (e.g., the dinosaurs) and with the appearances of new species (e.g., hominids).
The inner Solar System aggregated more slowly than the outer, so the terrestrial planets were not yet formed, including Earth and Moon.