They used pottery and other materials in sites to date 'relatively'.
They thought that sites which had the same kinds of pots and tools would be the same age.
The half-life refers to the amount of time it takes for half the radiocarbon in a sample of bone or shell or any carbon sample to disappear.
Libby found that it took 5568 years for half the radiocarbon to decay.
Today, there are over 130 radiocarbon dating laboratories around the world producing radiocarbon dates for the scientific community.
During the period of a plant's life, the plant is taking in carbon dioxide through photosynthesis, which is how the plant makes energy and grows.
Animals eat plants, and some eat other animals in the food chain.
After twice that time (about 11000 years), another half of that remaining amount will have disappeared.
After another 5568 years, again another half will have disappeared.