Origins and evolution of hypoxia response In our current oxygen-rich atmosphere, the ability of eukaryotic cells to sense variation in oxygen concentrations is essential for adapting to low-oxygen conditions. However, Earth's atmosphere has not always contained such high oxygen concentrations. Hammarlund et al. discuss oxygen-sensing systems across both plants and animals and argue that the systems are functionally convergent and that their emergence in an initially hypoxic environment shaped how they operate today. Science , this issue p. eaba3512