missing link

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hypothetical primate that is postulated to be the common ancestor from which both humans and nonhuman apes evolved. Not in scientific use.
• noun Something lacking that is needed to complete a series.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun colloquial A hypothetical primate once thought necessary to explain a perceived evolutionary gap between apes and humans.
• noun figuratively Any sought-after or valuable intermediary figure or position.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun hypothetical organism formerly thought to be intermediate between apes and human beings