For many years scientists believed that you were born with all the neurons you would ever get. The evidence for this dogma seemed strong: neuroanatomists in the early 20th century had identified immature neurons under the microscope but only in the brains of mammalian embryos and fetuses, never after birth.
We now know that the truth is not quite so simple. By radioactively labeling DNA, researchers gradually began to find exceptions to the rule against new neurons in the adult brain. Today scientists have identified two small regions where neurogenesis, or the birth of new neurons, continues throughout life: the olfactory bulb and the hippocampus. The former area is part of the brain's odor-discrimination system, so neurons there likely participate in this process. But the hippocampus has a much broader function. It gives us memory.