missing link

1. A theoretical primate postulated to bridge the evolutionary gap between the anthropoid apes and humans. Not in scientific use.
2. Something lacking that is needed to complete a series.

A supposed animal midway in evolution between apes and humans. The term is based on a misunderstanding about the theory of evolution, which does not state that humans are descended from apes, but rather maintains that both humans and apes descended from a common ancestor. Modern evolutionary scientists do not search for a “missing link.”

The noun has one meaning:

Meaning #1: hypothetical organism formerly thought to be intermediate between apes and human beings
  Synonym: ape-man