Figure 1.

Retrospective birth dating of cells. Schematic depiction of the strategy to retrospectively birth date cells. Nuclear bomb tests during the Cold War resulted in a large addition of ¹⁴C to the atmosphere. ¹⁴C in the atmosphere reacts with oxygen to form ¹⁴CO₂, which is taken up by plants in photosynthesis. Eating plants or animals living off plants results in the atmospheric ¹⁴C concentrations at any given time being mirrored in the human body. When a cell divides and duplicates its genome, it will integrate ¹⁴C in DNA with a concentration corresponding to that in the atmosphere at any given time. Measuring the ¹⁴C concentration in genomic DNA allows determination of when cells were born.