By studying the types of fossils in different rock strata and determining their age by radioactive dating method, geologists have constructed a geological time scale or stratographical scale.
The scale is the calendar of the earth's past history indicating the evolution of life through time recorded in the sequence of rocks.
The geological time was developed by Giovanni Arduiana (1760), which estimated that the earth was formed about 4.6 or 5 billion years ago and life on earth originated about 3.6 billion years ago.