BEFORE 1940s, scientists had no accurate way of determining the age of fossils or other ancient objects. They had to rely on relative dating techniques, which were far from accurate.
In 1947, Willard Frank Libby first proposed his radiocarbon dating theories. Soon after, he presented evidence of its efficiency in various field experiments.
Libby figured that plants would absorb some of this trace carbon-14 while they absorbed ordinary carbon in photosynthesis. Once the plant died, of course, it couldn’t absorb any more carbon of any kind, and the carbon-14 it contained would decay at its usual rate without being replaced. By finding the concentration of carbon-14 left in the remains of a plant, you could calculate the amount of time since the plant had died.