An Etymological Dictionary of Astronomy and Astrophysics

1. A person or thing that comes before another of the same kind; a forerunner.
   

   2. A substance from which another is formed, especially by metabolic reaction (OxforddDctionaries.com)

Etymology (EN): M.E., from L. præcursor “forerunner,” from præcursus, p.p. of præcurrere, from præ- “before,” → pre-, + currere “to run,” → current.

Etymology (PE): Pišgâm, from piš- “before,” → pre-, + gâm “step, pace,”
→ stage.