Today, archeologists and paleontologists employ this technique to determine the age of organic materials (bones, teeth, wood, etc.) that are less than fifty thousand years in age. The theory is simple: Cosmic particles coming from outer space continuously collide with stable carbon-12 atoms in CO2 molecules, which are widespread in the atmosphere.Each carbon-12 atom takes up two neutrons and is converted into a radioactive carbon-14 atom.
In the biosphere, both carbon-14 and carbon-12 atoms are added to the food chain via assimilation; first by plants through photosynthesis and then by animals through consumption of the plants.
For an animal or a plant, a carbon-14 atom is no different from a carbon-12 atom in assimilation.
Some Neolithic (later stone age) remains were dated back to fifty thousand years in Russia and Africa.
The city of Eriha in Palestine was dated back to eleven thousand years, and was designated as the first permanent human settlement.
After this point, although the amount of carbon-12 remains constant, carbon-14 continues to disintegrate and the ratio starts to decrease after the body dies.