A cross-kingdom history

B etween two and three billion years ago, living things evolved the ability to use oxygen to increase the amount of energy that could be released from carbon-rich molecules. However, these reactions produce harmful molecules called ‘reactive oxygen species’ as byproducts, so antioxidant systems had to be developed in parallel to detoxify them (Noctor and Foyer, 1998; Gest et al., 2013). A compound called ascorbate is an excellent antioxidant that is made by almost all living organisms. However, certain animals—including guinea pigs (Nishikimi et al., 1992), most bats, some fish and bird species, as well as humans and many other primates—have all have lost the ability to synthesise this compound for themselves (Mandl et al., 2009). Instead, these animals must obtain ascorbate (which is also known as ‘vitamin C’) from their diets. Moreover, all of these animals are unable to make ascorbate for the same reason: they have all lost a gene that encodes an enzyme called gulonolactone oxidase (or GULO). This enzyme catalyses the final step in the metabolic pathway that produces ascorbate in other animals.