File under "That's Freaky":
It was a dispute over whether the cortex ever makes any new cells that got Dr. Frisen looking for a new way of figuring out how old human cells really are. Existing techniques depend on tagging DNA with chemicals but are far from perfect. Wondering if some natural tag might already be in place, Dr. Frisen recalled that the nuclear weapons tested above ground until 1963 had injected a pulse of radioactive carbon 14 into the atmosphere.
Breathed in by plants worldwide and eaten by animals and people, the carbon 14 gets incorporated into the DNA of cells each time the cell divides and the DNA is duplicated.
So a variety of nuclear tests 40-some years ago blasted enough radioactive carbon into the atmosphere that we all carry bits of it in our DNA.
It's really a wonder that humanity hasn't destroyed itself yet.
