We now know the timing of important events that have happened during the history of Earth. The Earth is about 4.567 billion (4,567 million) years old. The geological or "deep time" of Earth's past has been organized into various units. Boundaries on the time scale are usually marked by major geological or palaeontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the Cretaceous–Tertiary extinction event. This marked the end of the dinosaurs and of many marine species.