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Without nuclear explosions producing new 14 C, existing 14 C began to mix with other carbon sinks, diluting its concentration in the air.Radioactive 14 C is incorporated into all living things: by plants that use carbon dioxide for photosynthesis, by the animals that eat the plants, by the animals that eat other animals that have eaten plants.Other cells, like those in the adult brain and nervous system, have been viewed as more like the Mona Lisa. Spalding, once a postdoc at the Karolinska Institute in Stockholm, Sweden and now a professor there, knew there were tantalizing hints that the adult hippocampus—a seahorse-shaped region deep in the brain that is important for memory and learning—could regenerate neurons.But without knowing exactly when each neuron was created, scientists couldn’t say with any certainty that this was true.
“It’s an amazingly powerful tool, whether you want to look at a fat cell or a brain cell,” Spalding says.Most aboveground nuclear bomb testing happened between 19, and those detonations released untold numbers of neutrons into the atmosphere.These slammed into nitrogen atoms, causing their nuclei to eject a proton.The bomb pulse has been declining since the 1963 above-ground test ban treaty, creating a sort of clock they could exploit.By determining how many radioactive carbon atoms a cell contained, Spalding and Frisén hoped they could calculate its birthdate. Spalding’s curiosity eventually leading her to a slaughterhouse on the outskirts of Stockholm.
Spalding would then spend hours chipping away to extract the necessary cells, a grisly procedure that was just the first in a decade-long stretch of hurdles she had to surmount.