The answer is carbon dating. This method is based on the fact that a radioactive isotope of carbon is constantly being created in the atmosphere due to the interaction of cosmic rays with atmospheric nitrogen. This radioactive carbon combines with oxygen to form radioactive carbon dioxide. This carbon dioxide enters the food web, first through plants through photosynthesis, and then to the primary and secondary consumer levels. Therefore, animal and plant material will contain a portion of this radioactive carbon. This particular form of carbon has a half life of 5,730 years due to radioactive decay. So this is how scientists are able to date the age of biological material.
