In a series of experiments, Fernando Nottebohm and his research team showed that the numbers of neurons in the forebrains (areas controlling complex behaviors) of male canaries dramatically increased during the mating season, when the birds learn new songs to attract females. Fortunately, in the early 1980s, a scientist trying to understand how birds learn to sing began to see how neurogenesis in the adult brain might make sense. These discoveries about neurogenesis in the adult brain were surprising to other researchers who thought they were not true in humans. In 1979, another scientist, Michael Kaplan, confirmed Altman’s findings in the rat brain and in 1983, he found special kinds of cells-called neural precursor cells-with the ability to become brain cells like neurons, in adult monkeys. He later reported that newborn neurons traveled from their birthplace in the hippocampus to other parts of the brain.

In 1962, scientist Joseph Altman challenged this belief when he saw evidence of neurogenesis (the birth of neurons) in a region of the adult rat brain called the hippocampus. However, scientists believed that once a neural circuit was in place, adding any new neurons would change the flow of information and break the brain’s communication system As children, we might grow some new neurons to help build the pathways-called neural circuits-that act as information highways between diﬀerent areas of the brain. Until recently, most neuroscientists (scientists who study the brain) thought we were born with all the neurons we were ever going to have. Neurons are nerve cells that send messages all over your body to allow you to do everything from breathing to talking, eating, walking, and thinking.

How many neurons in human brain