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]

Moby Thesaurus II by Grady Ward, 1.0:

166 Moby Thesaurus words for "precursor":
   advance guard, agent, airhead, alterant, alterative, alterer,
   ancestor, ancestors, announcer, antecedence, antecedency,
   antecedent, anteposition, anteriority, apprentice, architect,
   artificer, artist, author, avant-garde, battle line, beachhead,
   begetter, beginner, bridgehead, buccinator novi temporis, builder,
   catalyst, catalytic agent, colonial, colonist, colonizer,
   conceiver, constructor, craftsman, creator, danger sign, designer,
   deviser, discoverer, dominion, early bird, early comer,
   early riser, early symptom, effector, engenderer, engineer,
   executor, executrix, falling barometer, farthest outpost, father,
   ferment, first arrival, first line, forefront, foregoer, foregoing,
   forerunner, foreshadower, founder, front, front line,
   front position, front rank, front-runner, gathering clouds,
   generator, grower, harbinger, heading, herald, high sign,
   homesteader, immigrant, inaugurator, industrialist, initiator,
   innovationist, innovator, instigator, institutor, introducer,
   inventor, journeyman, le pas, leading, leaven, line, maker,
   manufacturer, master, master craftsman, messenger, modificator,
   modifier, mother, nester, omen, organizer, originator, outguard,
   outpost, outrider, past master, pioneer, planner, planter, point,
   precedence, precedency, precedent, preceding, precession,
   predecessor, preference, prefixation, preliminary sign, prelude,
   premise, premonitor, premonitory sign, premonitory symptom,
   presager, prime mover, priority, prodroma, prodrome, producer,
   prothesis, prototype, quarantine flag, railhead, raiser, realizer,
   red flag, red light, scout, settler, shaper, sire,
   skull and crossbones, smith, sooner, spearhead, squatter,
   storm petrel, stormy petrel, superiority, symptom, the lead,
   thundercloud, thunderhead, top priority, transformer,
   transmogrifier, urgency, van, vanguard, warning sign,
   warning signal, wright, yeast, yellow flag, yellow jack