One method that scientists use to date ancient fossils and artifacts is called radiocarbon dating. All living things on Earth are made up of a high percentage of an element called carbon. Carbon combines with other elements in complex ways to form the molecules that make up our bodies. Most carbon on Earth is not radioactive, but a very small percentage is. Thus, as living things take in carbon, they inevitably will take up a small amount of radioactive carbon into their bodies.
When these lifeforms die, they stop taking in new carbon. The carbon in their bodies at the time of their death will remain in their bodies until they decompose, or if they become fossilized, then forever.
Radioactive carbon decays at a known rate. This allows scientists to look at the amount of decay in a fossil’s radioactive carbon and determine a relative date.
Radiocarbon dating is only effective for objects and fossils that are less than 50,000 years old. However, scientists can look at the decay of other elements in these objects allowing them to date them up to 2.2 billion years.
These dating techniques are by no means perfect, but they are always improving, and they are the best methods that we have at this time.