Carbon-14, or radiocarbon—which is now widely used to date organic material—was discovered 75 years ago on February 27 by Martin Kamen and Sam Rubin at the UC-Berkeley Radiation Lab. Franz Kurie had previously theorized the existence of this isotope of carbon, which has 6 protons and 8 neutrons. It’s one of three naturally occurring carbon isotopes on the planet, and is found in just trace amounts (the vast majority of carbon found on Earth is carbon-12). But those trace amounts were key to transforming our conceptions of the past.
In 1949, William Libby and colleagues discovered that the measurement of carbon-14’s decay over time could be used as kind of clock for the dating of organic material. Bone, wood (including charcoal), and other plant remains could be dated to approximately 50,000 years ago. Libby was awarded the 1960 Nobel Prize in Chemistry for this.
An expert on radiation, Libby gets to the heart of the matter in this article on radiocarbon dating: “(1) Cosmic rays make living things radioactive to a certain level fixed by the environment through the food eaten. (2) At death the intake of food stops so no replenishment of the immutable radioactive decay of 14C can occur and the degree to which decay is observed to have occurred gives the time lapse since death (radiocarbon age.)”
The dating of archaeological evidence before radiocarbon was based on historical records, stratigraphy (the study of layers of rock), and educated guesswork. Anthropologist R.E. Taylor, who calls radiocarbon dating revolutionary, summarized a half-century of advances made in radiocarbon dating. Taylor wrote, “It is difficult for many people to appreciate just how dramatically the advent of radiocarbon dating transformed archaeology. Libby’s invention made the study of the world prehistory truly possible.”
Radiocarbon dating only works for organic material and only reaches a relatively short distance into the past (at least in geological terms). The technique is still a potent one; the modern, recalibrated version makes adjustments for the historical fluctuations of carbon in the atmosphere and other conditions.
Today, it’s one of numerous dating methods used, including those for inorganic material. Thermoluminescence dating, potassium-argon dating, amino acid dating, and archaeomagnetic dating can all help give us a window into a much wider span of time.