Did you ever wonder what the earth was like long ago?  What strange
   organisms roamed the earth?  What the climate was like?   

     Studying earth's history is not an easy task. Earth is 4.6 billion years old, and
    humans did not appear until 140,000 years ago. For much of earth's history
    there were no people around to record the events of earth's past.

    So what do scientists use to learn about earth's history? Scientists study
    rock layers and fossils of dead organisms to learn about the past.  By
    studying the appearance and disappearance of organisms, scientists have
    been able to create the geologic time scale.  Earth's history has been
    divided into time units based on the life-forms (index fossils) that lived only
    during that specific period.