Sentence:  Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis:.... The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The <WSD>critical</WSD> component of the photosynthetic system is the "water-oxidizing complex", made up of manganese atoms and a calcium atom. 
Meaning of the word marked with <WSD> tags:  Extremely important.