Carbon

Every living thing inhales carbon from the atmosphere. Most is stable C-12, but a tiny fraction is radioactive C-14. While we live, we replenish it. When we die, the intake stops, and the C-14 begins to decay into nitrogen. By measuring the ratio of C-14 to C-12, we can calculate precisely how long ago that breath ceased. This was the first time humanity could ask a physical object 'when?' and get an honest answer.

Carbon dating broke the 'time barrier' of history. Suddenly, we knew the age of Stonehenge, the timing of the Ice Ages, and the migrations of early humans. It transformed archaeology from a study of sequences into a study of absolute chronology. Time was no longer a guess; it was a measurable quantity trapped in the debris of the world.