precursor

That which precurses: a forerunner, predecessor, or indicator of approaching events.

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 critical component of the photosynthetic system is the “water-oxidizing complex”, made up of manganese atoms and a calcium atom.

One of the compounds that participates in the chemical reaction that produces another compound.

Caused by the following symbol.