The mechanism by which adult neural stem cells (NSCs) are established during development is unclear. In this study, analysis of cell cycle progression by examining retention of a histone 2B (H2B)-GFP fusion protein revealed that, in a subset of mouse embryonic neural progenitor cells (NPCs), the cell cycle slows between embryonic day (E) 13.5 and E15.5 while other embryonic NPCs continue to divide rapidly. By allowing H2B-GFP expressed at E9.5 to become diluted in dividing cells until the young adult stage, we determined that a majority of NSCs in the young adult subependymal zone (SEZ) originated from these slowly dividing embryonic NPCs. The cyclin-dependent kinase inhibitor p57 is highly expressed in this embryonic subpopulation, and the deletion of p57 impairs the emergence of adult NSCs. Our results suggest that a substantial fraction of adult SEZ NSCs is derived from a slowly dividing subpopulation of embryonic NPCs and identify p57 as a key factor in generating this embryonic origin of adult SEZ NSCs.