New neurons continue to be born in the subgranular zone (SGZ) in the dentate gyrus (DG) of the adult mammalian hippocampus1–5. This process has been linked to learning and memory, stress and exercise, and is thought to be altered in neurological disease6–10. In humans, some studies suggest that hundreds of new neurons are added to the adult DG every day11, while other studies find many fewer putative new neurons12–14. Despite these discrepancies, it is generally believed that the adult human hippocampus continues to generate new neurons. Here we show that a defined population of progenitor cells does not coalesce in the SGZ during human fetal or postnatal development. We also find that proliferating progenitors and young neurons in the DG sharply decline in the first year of life and only a few isolated young neurons are observed by 7 and 13 years of age. In adult normal and epileptic patients(18–77 years; n=17 postmortem; n=12 epilepsy), young neurons were not detected in the DG. In the monkey (M. mulatta) hippocampus, a proliferative SGZ was present in early postnatal life, but diminished during juvenile development as neurogenesis declined. We conclude that recruitment of young neurons to the primate hippocampus declines rapidly during the first years of life, and that DG neurogenesis does not continue, or is extremely rare, in the adult human. The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved.

The first few months after birth, when a child begins to interact with the environment, are critical to human brain development. The human frontal lobe is important for social behavior and executive function; it has increased in size and complexity relative to other species, but the processes that have contributed to this expansion are unknown. Our studies of postmortem infant human brains revealed a collection of neurons that migrate and integrate widely into the frontal lobe during infancy. Chains of young neurons move tangentially close to the walls of the lateral ventricles and along blood vessels. These cells then individually disperse long distances to reach cortical tissue, where they differentiate and contribute to inhibitory circuits. Late-arriving interneurons could contribute to developmental plasticity, and the disruption of their postnatal migration or differentiation may underlie neurodevelopmental disorders.