Complex sentence: They are known to have evolved from cyanobacteria through endosymbiosis, i.e. after the engulfment of a cell by another-in this case, a photosynthesizing cyanobacteria that was engulfed in some ancient eukaryotic cell.
Split into simpler sentences: They are known to have evolved from cyanobacteria through endosymbiosis, i.e. after the engulfment of a cell by another. In this case, a photosynthesizing cyanobacteria that was engulfed in some ancient eukaryotic cell.