Cells, like planets, gain complexity as they develop. Zamai et al. report on how isolated stem cells gain character and complexity on their way to mature NK cells (see the article on page 893). Maturation is followed by aging on the river of life. Noppe et al. show that human fibroblasts cultivated from young and old donors, although phenotypically similar, differ significantly in their enzyme activity (see the article on page 910).