The use of radioisotopes has radically altered previous concepts of geological history. Early estimates placed Earth's age at less than 40 million years. Later calculations using the radioactive decay of natural uranium suggested an age more than 10 times this number. More recent radiological calculations set the age of the Earth at 460 million years old!
Archeologists can also determine dates and ages of fossils by a technique known as "carbon dating." Carbon-14 is a naturally occurring, long-lived radioisotope present in all living things. Carbon-12 is present in the atmosphere, and the ratio of carbon-14 to carbon-12 has been relatively constant throughout history.
When an animal or plant dies, it stops taking in carbon. The amount of carbon-14 in its tissue begins to decrease through the process of radioactive decay. Comparing the carbon-14 to carbon-12 ratio in the dead material with the "living ratio" enables us to calculate how long ago the plant or animal lived. This comparison permits us to date fossils that are found in archaeological explorations as belong to a certain period in history.
