Excluding the biological polymers proteins, lipids and nucleic acids, modified tetrapyrroles are the biological molecules that have had the greatest impact on the evolution of life over the past 4 billion years. They are involved in a wide variety of fundamental processes that underpin central primary metabolism in all kingdoms of life, from photosynthesis to methanogenesis. Moreover, they bring colour into the world and it is for this reason that these compounds have been appropriately dubbed the pigments of life . To understand how and why these molecules have been so universally...