In the history of life, the most transformative symbiosis was the ‘primary endosymbiosis,’ wherein a cyanobacterium was engulfed by a eukaryote and became genetically integrated as a heritable photosynthetic organelle, or plastid. Since we cannot travel back in time to witness these evolutionary junctures, Gregory S Gavelis and Gillian H Gile draw on examples of unicellular eukaryotes (protists) spanning diverse modes of photosymbiosis. They also review experimental approaches that could be used to recreate aspects of early primary endosymbiosis on a human timescale.