a person or thing that precedes, as in a job, a method, etc.; predecessor.

a person, animal, or thing that goes before and indicates the approach of someone or something else; harbinger

The first robin is a precursor of spring.

Chemistry, Biochemistry . a chemical that is transformed into another compound, as in the course of a chemical reaction, and therefore precedes that compound in the synthetic pathway

Cholesterol is a precursor of testosterone.

Biology . a cell or tissue that gives rise to a variant, specialized, or more mature form.

noun

forerunner , harbinger , herald , messenger , outrider , usher , vanguard , ancestor , antecedent , antecessor , forebear , foregoer , original , originator , parent , pioneer , predecessor , prototype , foreshadower , presager , progenitor , trailblazer