Researchers have used the radioactive fallout from atomic bomb tests to show that new neurons are produced in one part of the human brain throughout life. Studies have shown that rats can grow new neurons, but there was little definitive evidence that it happens in humans too.

When atomic bombs were tested between 1945 and 1963, radioactive particles were released into the Earth’s atmosphere. Among the isotopes created was carbon-14, which is commonly used in radio carbon dating.

As cells divide, they incorporate carbon from the environment, and some of that carbon comes from the atmosphere. That is why carbon-14 released by the atomic bombs found its way into the DNA of multiplying cells. The amount of carbon-14 in this DNA corresponded to its concentration in the atmosphere at the time the new cells were born.

This essentially meant carbon-14 in DNA can be used as a measure of the age of cells, such as neurons in adult brains. A team led by Jonas Frisén at the Karolinska Institute, used brain cells obtained from 120 people who had consented to have their cells used for experiments after their death. Of the cells analysed, some had much higher levels of carbon-14 than others. This meant that the cells with lower levels were produced after 1963, when bomb testing ceased, and therefore showed that new cells can be produced later in life.