Tempus fugit: How time flies during development

“Fugit irreparabile tempus,” wrote Virgil, a reminder that our lives are defined by the irreversible flow of time. As soon as the egg is fertilized, embryonic cells follow a developmental program strictly organized in time. The sequence typically is conserved throughout evolution, but individual events can occur over species-specific time scales. Such differences can have marked effects. For instance, it takes 3 months to generate cerebral cortex neurons in a human but only 1 week in a mouse. This prolonged neurogenesis likely contributes to evolutionary expansion of the human brain (1). But the mechanisms underlying developmental time scales remain largely unknown. On pages 1449 and 1450 of this issue, Rayon et al. (2) and Matsuda et al. (3), respectively, report an association between species-specific developmental time scales and the speed of biochemical reactions that support protein turnover.