The answer is 'carbon dating'. The technique depends on a form of radioactive carbon which is formed by the interaction between cosmic rays and atmospheric carbon. This radioactive carbon, as a constituent of carbon dioxide, is taken up by plants through photosynthesis. This radioactive carbon can then move up the food chain. Once an animal dies, it stops exchanging carbon with its environment, and the amount of this radioactive carbon begins to decrease through decay according to a specific half life. So by measuring the amount of radioactive carbon in animal remains, scientists can estimate the age when the animal died. The half life of radioactive carbon is approximately 5,730 years, so the oldest dates that can be reliably measured by this process are around 50,000 years ago.