Sentence:  Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into <WSD>carbohydrates</WSD> and waste oxygen using solar energy. 
Meaning of the word marked with <WSD> tags:  A sugar, starch, or cellulose that is a food source of energy for an animal or plant.