Biological evidence of the age of the earth includes radioactive dating of rocks, fossil evidence, comparative anatomy and molecular clocks. Most scientists think there is lots of evidence to prove that the earth is billions of years old. They use the above techniques to prove that the earth is as old as it is. Molecular clocks are molecules in the earth that act as "age indicators" or an indicator of how old the earth is. There are however, some scientists who believe that the earth is much younger and there is no evidence to prove the age of the earth