Transitions between marine and freshwater environments provide new clues about the origins of multicellular plants and algae

Marine–freshwater and freshwater–marine transitions have been key events in the evolution of life, and most major groups of organisms have independently undergone such events at least once in their history. Here, we first compile an inventory of bidirectional freshwater and marine transitions in multicellular photosynthetic eukaryotes. While green and red algae have mastered multiple transitions in both directions, brown algae have colonized...