The Collaborative International Dictionary of English v.0.48:

Precursor \Pre*cur"sor\, n. [L. praecursor, fr. praecurrere to
   run before; prae before + currere to run. See Course.]
   One who, or that which, precedes an event, and indicates its
   approach; a forerunner; a harbinger.
   [1913 Webster]

         Evil thoughts are the invisible, airy precursors of all
         the storms and tempests of the soul.     --Buckminster.
   [1913 Webster]

   Syn: Predecessor; forerunner; harbinger; messenger; omen;
        sign.
        [1913 Webster]

WordNet (r) 3.0 (2006):

precursor
    n 1: a substance from which another substance is formed
         (especially by a metabolic reaction)
    2: a person who goes before or announces the coming of another
       [syn: precursor, forerunner]
    3: something that precedes and indicates the approach of
       something or someone [syn: harbinger, forerunner,
       predecessor, herald, precursor]