Researchers from the Max Planck Institute for Biology Tübingen broke new ground by demonstrating that an HMG-box gene in brown algae is crucial for determining male sex. This breakthrough significantly expands our understanding of sex-determination mechanisms in eukaryotic organisms. Until now, master sex-determination genes had been identified in only a select number of vertebrates and plants. Published in Science, this study illuminates the evolutionary parallels in developmental pathways between animals and seaweeds, despite their millions of years of independent evolution. It highlights the use of a shared genetic ‘toolkit’ across lineages and explores distant evolutionary convergences.