Scientists from the RIKEN Center for Biosystems Dynamics Research, European Molecular Biology Laboratory (EMBL) Barcelona, Universitat Pompeu Fabra, and Kyoto University have found that the 'segmentation clock'—a genetic network that governs the body pattern formation of embryos—progresses more slowly in humans than in mice because the biochemical reactions are slower in human cells. The differences in the speeds of biochemical reactions may underlie differences between species in the tempo of development.