Brown algae, the only group of heterokont protists with complex multicellularity, have evolved diverse thallus morphologies and gamete types. The study by Choi et al. addresses the timing and mechanisms of these evolutionary transitions using plastid genome data from 138 species. The results show that brown algae first emerged with true multicellularity during the late Ordovician Period (ca. 450 Ma), coinciding with a major diversification of marine fauna. The major diversification of brown algae occurred during the Mesozoic Era, coincident with Pangea rifting and the diversification of other heterokont algae. The transition from ancestral isogamy to oogamy was followed by three independent reappearances of isogamy during the Cretaceous Period, which are correlated with parthenogenesis. The study provides a robust phylogenetic framework and new interpretations of algal fossils, shedding light on the intertwined evolution of multicellularity and sexual reproduction in brown algae.Brown algae, the only group of heterokont protists with complex multicellularity, have evolved diverse thallus morphologies and gamete types. The study by Choi et al. addresses the timing and mechanisms of these evolutionary transitions using plastid genome data from 138 species. The results show that brown algae first emerged with true multicellularity during the late Ordovician Period (ca. 450 Ma), coinciding with a major diversification of marine fauna. The major diversification of brown algae occurred during the Mesozoic Era, coincident with Pangea rifting and the diversification of other heterokont algae. The transition from ancestral isogamy to oogamy was followed by three independent reappearances of isogamy during the Cretaceous Period, which are correlated with parthenogenesis. The study provides a robust phylogenetic framework and new interpretations of algal fossils, shedding light on the intertwined evolution of multicellularity and sexual reproduction in brown algae.