Precambrian Time Notice that the detail of the geologic time scale does not begin until about 512 million years ago, the date for the beginning of the Cambrian period The more than 4 billion years prior to the Cambrian is divided into three eons, the Hadean, the Archean, and the Proterozoic. These in tum, are divided into seven eras. It is also common for this vast expanse of time to simply be referred to Precambrian Although it represents about 88 percent of Earth history, the Precambrian is not divided into nearly as many smaller time units as is the Phanerozoic eon. The quantity of information geologists have deciphered about Earth's past is somewhat analogous to the detail of human history. The further back we go, the less we know. Certainly more data and information exist about the past 10 years than for the first decade of the 20th century; the events of the 19th century have been documented much better than the events of the first century An, and so on Thus it is with Earth history. The more recent past has the freshest, least disturbed, and most observable record. The further back in time the geologist goes, the more frag mented the record and clues become.