Time periods on the geological time scale are not necessarily even in length. The division of time periods is primarily based on significant geological and paleontological events rather than equal time intervals. Some time periods may be longer or shorter depending on the geologic events that occurred during that period. For example, the Precambrian period, which covers about 88% of Earth's history, is much longer than the Cenozoic era, which covers the last 66 million years.
