(expand) The geologic timescale is used by geologists and other scientists to describe the timing and relationships between events that have occured during the history of the Earth. ... The Precambrian or Cryptozoic is the period of the geologic timescale from the formation of Earth around 4500 million years before the present (BP) to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, some 542 million years BP. Remarkably little is known about...