Adult mammalian brains cannot produce new neurons.

For much of the twentieth century, the mature brain was treated as essentially finished. Neurons lost during adulthood might be compensated for by changes in other cells or connections, but they were not expected to be replaced by newly born neurons.

The difficulty was that new-neuron formation is rare and hard to see directly. Beginning in the 1960s, researchers used cell-labeling methods that mark cells when they divide. They found newly generated neurons in adult mammalian brains.

Later microscopy and molecular markers made the evidence stronger. In several birds and mammals, new neurons formed after development can survive and become part of existing neural circuits. This process is called adult neurogenesis.

The human case is less settled. Studies disagree about how much neurogenesis continues in the adult hippocampus, a brain region important for memory, and how sharply it declines with age. The old absolute claim—that adult brains cannot make new neurons at all—has failed. The amount of new-neuron formation in adult humans remains an open question.