Libby introduces radiocarbon dating
In 1940 Martin Kamen discovered radioactive carbon-14 (an isotope of carbon) and found that it had a half-life of about 5,700 years. Scientists had also found that some of the nitrogen in the atmosphere was turned into carbon-14 when hit with cosmic rays. Thus, an equilibrium was reached, the newly formed carbon-14 replacing the carbon-14 that decayed, so that there was always a small amount in the atmosphere.
In 1947 American chemist Willard Libby (1908-1980) 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. With this technique scientists could determine the age of plant-based artifacts -- wood, parchment, textiles -- up to 45,000 years old. This has allowed estimates of the age of Egyptian mummies, prehistoric dwellings, and so forth.
For his work on carbon-14 dating, Libby received the Nobel Prize in chemistry in 1960.