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Over 50 years ago, above ground nuclear bomb tests released heavy carbon-14, a nonradioactive form of carbon into the atmosphere. When the nuclear test bomb treaty occurred in 1963, which prohibited above ground tests, it allowed researchers to measure the steady rate of decline in carbon-14 that existed in the atmosphere. Using this form of carbon dating, researchers discovered that neurons in the hippocampus of the brain develop everyday during adulthood.

"It was thought for a long time that we are born with a certain number of neurons, and that it is not possible to get new neurons after birth," said senior author, Jonas Frisén of the Karolinska Institute according to Medical Xpress. "We provide the first evidence that there is substantial neurogenesis in the human hippocampus throughout life, suggesting that the new neurons may contribute to human brain function."

The research team used carbon dating on the brains of deceased people. The researchers know that when people consume plants and animals, they inevitably absorb carbon and heavy carbon. Depending on when these foods are eaten, the atmospheric concentration of carbon in that particular time is stamped in the DNA. This ratio of carbons helps scientists determine how old the neurons are in the brain. Utilizing this carbon dating method, the researchers discovered that over one third of the cells in the deceased participants' brains were renewed throughout their lifetime. The researchers concluded that nearly 1,400 new neurons are created daily and that this number steadily decreases as one ages.

This finding suggests that cognitive decline or mental illnesses could be attributed to poor hippocampal neurogenesis. By studying how neurons develop or fail to develop, researchers could possible understand why cognitive decline and illnesses, such as depression, occur.

The findings were published in Cell.