Congratulations, your adult brain isn't meekly withering into a useless mass after all—it is instead generating new neurons all the while. Scientists have long debated whether the brain keeps growing into adulthood, and new research in Cell seems to have settled the mystery. What's more, the Swedish researchers who accomplished the feat have an unusual factor to thank—nuclear fallout from the Cold War. As Science explains, all those above-ground atomic tests in the 1940s and '50s left a lot of carbon-14 in the atmosphere that ended up in humans.
Scientists looked for that isotope in autopsied brains and—because it decays at a known rate and because those atomic tests occurred in a known span of years—were able to conclude that the brain does indeed produce new cells as it ages. In fact, this happens in the hippocampus, which plays a key role in memory, reports Ars Technica. So why do you keep forgetting where you left your keys? "Unfortunately, the rate of renewal doesn’t make up for the rate of cell death, which is also going on constantly—and the new cells live for a far shorter time than our original hippocampal neurons," writes Michael D. Lemonick at Time. (Read more atomic bomb stories.)