Each layer, laid down during succeeding geological periods, contain a distinct assemblage of fossils. When we know that a particular organism lived only during a certain period, rocks that contain fossils of those organisms must be from that same period. In this way, certain fossils provide an index to the relative ages of the rock. It provides evidence to show that the Earth is 4.5 billion years old. Absolute dates are established through radiometric dating techniques.
Some elements are radioactive.
They break down spontaneously at a constant rate into other elements. They can be used to establish the time of events in Earth's history and the ages of its fossils. The marvelous invention that is life on Earth has survived for more than 3.5 billion years. The planet itself is over a billion years older. It has taken a long time for the Earth and its life to develop into what we know today.