Sentence:  <WSD>Oxygen</WSD> levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste <WSD>oxygen</WSD> using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. 
Meaning of the word marked with <WSD> tags:  Molecular oxygen (O<sub>2</sub>), a colorless, odorless gas at room temperature, also called dioxygen.