How can scientists tell how old a bone is?

Scientists date old bones, wood and other once-living things by measuring a radioactive form of carbon, carbon-14. High in the atmosphere, cosmic rays constantly turn nitrogen into carbon-14. It ends up in carbon dioxide, plants breathe it in through photosynthesis, and animals eat the plants, so every living thing carries a little carbon-14. When a plant or animal dies, it stops taking in new carbon, and the carbon-14 it contains slowly decays away. The older the sample, the less is left.

The decay runs like a clock. Carbon-14 has a half-life of about 5,730 years: after that time, half of it is gone; after another 5,730 years, half of what remained, and so on. Measure how much is left in a fragment of bone or a piece of wood and you can calculate when it died. Beyond about 50,000 years, too little remains to measure reliably.

The method was developed in the late 1940s by Willard Libby at the University of Chicago, and it won him the Nobel Prize in Chemistry in 1960. One of his first tests used methane from a sewage works in Baltimore, which contained carbon-14, while methane from ancient petroleum contained none. Then he checked the clock against history, dating samples from the tombs of Egyptian kings of known age.