Mitochondria are small organelles that provide the energy critical for each cell in our body, in particular in the high fuel-consuming brain. In this week’s edition of Science, a Belgian team of researchers led by Pierre Vanderhaeghen (VIB-KU Leuven, ULB) finds that mitochondria also regulate a key event during brain development: how neural stem cells become nerve cells. Mitochondria influence this cell fate switch during a precise period that is twice as long in humans compared to mice. The seminal findings highlight an unexpected function for mitochondria that may help explain how humans developed a bigger brain during evolution, and how mitochondrial defects lead to neurodevelopmental diseases.