The geologic time scale has divided the Earth's history into units that are based upon rock units and changes in fossil forms. Scientists have determined the Earth to be approximately 4.6 billion years old. The vast majority of the Earth's history (88%) lies in the Precambrian time span. In this early part of Earth's history life was sparse and much of this rock is buried deep within the ground. Therefore not much information is known about this time period.
Some other time periods that are relevant to our study is the Paleozoic era "ancient life". During this era life became abundant around the globe. The current geologic period is the "Quaternary". This is the time period in which we reside- the current time period.