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When scientists want to find out how old a fossil is, they often look at the amount of carbon 14 it contains. This radioactive isotope was present in the atmosphere in a steady ratio with its more stable counterpart, carbon 12, for a long time.
Scientists can cross-check this ratio with knowledge of the half-life of carbon 14 to determine the era a given fossil comes from. That ratio changed in the mid-20th century, when above-ground nuclear testing threw it off by filling the atmosphere with carbon 14, which in turn incorporated itself into the DNA of every plant and animal on the planet.
Though this poses a problem for scientists in future millennia (they may no longer have a reliable comparison of one type of carbon to the other), it's a windfall for scientists in the modern day. This glut of carbon 14 has fallen at a steady and predictable rate since testing stopped in 1963, so the amount of the isotope present in an organism alive since mid-century can tell us the exact year it came into being. But this doesn't just go for whole organisms. If scientists want to know when an individual cell was born, they can find out how much carbon 14 it contains, then figure out the year when that much carbon 14 was in the atmosphere. That's why we now know how long each cell in your body lives, from your skin to your heart to your brain.
