Mammalian nephrons, the functional unit of the kidney, arise from a limited nephron pool of progenitor cells. The pool gives rise to around 14,000 nephrons over an 11-12 period in the mouse and a million nephrons over a 30-week period in man. In this study, we demonstrate a relationship between the time a nephron progenitor embarks on a program of nephron formation and the cell types it generates in the forming nephron in the mouse and human kidney. Progressive recruitment predicted from high-resolution image analysis and three-dimensional reconstruction of human nephrogenesis was confirmed through direct analysis of mouse kidneys developing in the tissue culture dish. Examining gene activity in single cells predicts pathways to the development of regional cell diversity essential for normal nephron function in the kidney.