precursor
//prɪˈkɜːsər//
Noun
precursorSingular
precursorsPlural
1
A person or thing that comes before another of the same kind; a forerunner or predecessor.
The telegraph was a precursor to the telephone.
2
A substance from which another is formed, especially by metabolic reaction.
Tryptophan is a precursor of serotonin in the brain.