Before the 1940s, scientists wanting to determine the age of fossils and other organic materials relied on relative dating techniques, grouping objects by the estimated date of surrounding rock strata. This method could produce large inaccuracies.
Strangely, the ability to date objects more accurately was to originate from outer space. High- energy cosmic rays are constantly bombarding our atmosphere; when they hit, they break up atoms in the stratosphere. A consequence of this is the production of an unstable radioactive form of carbon called Carbon-14 (C-14) from atmospheric nitrogen.
American scientist William Frank Libby (1908-1980) reasoned that, as all living creatures are made up of carbon and are constantly replacing this carbon from the atmosphere (plants take it in as carbon dioxide and animals absorb it from plants), all living organisms should have an equal amount of C-14 in their bodies in relation to the proportions occurring in the atmosphere. However, when an organism dies, it stops replacing this C-14. Being unstable, its C-14 breaks down and is converted back to normal nitrogen at a fixed rate (approximately half every 5,700 years). The time since the death of an organic sample can be worked out by comparing the proportions of C-14 in the atmosphere to what is in the sample. The method should be accurate up to about 50,000 years ago (after which all the C-14 will have broken down).
Libby presented his radiocarbon dating method in 1947 after testing it on fir trees (where the age had already been ascertained by counting their rings). It was soon widely adopted and has been used to date such diverse objects as the Dead Sea Scrolls and relics from the end of the last Ice Age.