Todays knowledge of fossil ages comes primarily from radiometric dating, also known as radioactive dating. Radiometric dating relies on the properties of isotopes. These are chemical elements, like carbon or uranium, that are identical except for one key feature -- the number of neutrons in their nucleus.

How far back can we date fossils?

Scientists called geochronologists are experts in dating rocks and fossils, and can often date fossils younger than around 50,000 years old using radiocarbon dating.