WordNet (r) 3.0 (2006):

missing link
    n 1: hypothetical organism formerly thought to be intermediate
         between apes and human beings [syn: missing link, ape-
         man]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "missing link":
   Bronze Age man, Hominidae, Iron Age man, Stone Age man, aboriginal,
   aborigine, ancient, antediluvian, anthropoid, ape-man, arrearage,
   autochthon, break, bushman, cave dweller, caveman, defalcation,
   defect, deficiency, deficit, discontinuity, fossil man, gap,
   hiatus, hominid, humanoid, interval, lack, lacuna, man of old,
   need, omission, outage, preadamite, prehistoric man, prehuman,
   primate, primitive, protohuman, shortage, troglodyte, ullage, want,
   wantage