The older a once-living artifact is, the less 14C it contains
Before many of the scientific techniques that are widely available today were developed, understanding world history used to rely on two main principles. First, the belief that older artifacts are almost always found buried beneath younger ones. And second, the knowledge that people with cultural ties make similar appearing objects, such as stone tools.
In the early 19th Century historians suspected that much of human history—widely thought at that time to be only 2,000 years old—was older than believed.
This was because many artifacts and object, like the Great Pyramids of Giza, had the appearance of being older than 2,000 years. But there was no way to know for sure.
That changed in the middle of the 20th Century with the development of radio-carbon dating.
American chemist Wilfrid Libby showed how the remains of all living things—from trees and plants to animal and human bones—contain a chemical clock that starts ticking when they die.
The principle of radio-carbon dating is simple. All living things contain carbon (C) atoms, and carbon comes in two main forms: 14C and 12C. The ratio of these two forms of carbon remains constant in living things. For example, when a person is alive every time he or she eats food, fresh supplies of carbon are absorbed. But after death with no more carbon being absorbed, the balance between 14C and 12C begins to change. This is because 14C is a naturally unstable and over time decays, becoming 12C.
This process of decay is so regular, it means that the less 14C an important specimen—be it charcoal, bone or tooth—contains, the older it must be. When used in archaeology, radio-carbon dating is accurate to between 40,000 and 50,000 years ago.