Etimologically, neurogenesis means "birth of neurons". It is the process by which neurons are generated, especially during embryological and fetal development. Neurogenesis is responsible for populating the growing brain of a fetus with neurons. Although it was thought to occur only in developing organism, the first evidence of adult mammalian neurogenesis in the cerebral cortex was presented by Joseph Altman in 1962, followed by a demonstration of adult neurogenesis in the dentate gyrus of the hippocampus in 1963. Also in the early 1990s hippocampal neurogenesis was demonstrated in non-human primates and humans. However, some researchers have questioned the scientific evidence of these findings, arguing that the new cells may be of glial origin (new astrocytes and oligodendrocytes).
It is believed that, through the process of mitossis, new cells are generated from existing nerve cells. These new stem cells are born without function. Stimulation from their physical environment causes these new cells to differentiate and specialize into neurons. Then these new nerve cells migrate from their original place, either adapting and developing into mature neurons, or not adapting at all and dying. Many scientists think that on going neurogenesis is an important mechanism underlying neuronal plasticity, enabling organisms to adapt to environmental changes and influencing learning and memory throughout life.