This study reports the discovery of a natural protein, citrate synthase (CS) from the cyanobacterium *Synechococcus elongatus*, which self-assembles into Sierpiński triangles. Using cryo-electron microscopy, the researchers reveal how the fractal assembles from a hexameric building block. The fractal assembly is modulated by different stimuli and can regulate the enzymatic activity of citrate synthase in vitro, but it may not serve a physiological function in vivo. Ancestral sequence reconstruction shows that the fractal assembly evolved from non-fractal precursors, suggesting it may have emerged as a harmless evolutionary accident. The findings expand the possibilities for protein complexes and demonstrate that intricate and regulatable assemblies can evolve through a single substitution.This study reports the discovery of a natural protein, citrate synthase (CS) from the cyanobacterium *Synechococcus elongatus*, which self-assembles into Sierpiński triangles. Using cryo-electron microscopy, the researchers reveal how the fractal assembles from a hexameric building block. The fractal assembly is modulated by different stimuli and can regulate the enzymatic activity of citrate synthase in vitro, but it may not serve a physiological function in vivo. Ancestral sequence reconstruction shows that the fractal assembly evolved from non-fractal precursors, suggesting it may have emerged as a harmless evolutionary accident. The findings expand the possibilities for protein complexes and demonstrate that intricate and regulatable assemblies can evolve through a single substitution.