The document summarizes key aspects of the geologic time scale including its structure from present to past and major divisions of Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. It notes that Precambrian time covers around 88% of Earth's history and ended 544 million years ago. The eras are further divided into periods ranging from tens to less than 2 million years, with the Cenozoic periods divided into epochs due to its more complete fossil record.