Tempus fugit: How time flies during development

Developmental-timing differences between species are linked to protein turnover “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.