missing link

• 1. 

  N-COUNT The missing link in a situation is the piece of information or evidence that you need in order to make your knowledge or understanding of something complete. 缺少的环节; 指为完全理解某事尚需知道的一则信息或证据 [usu sing]

  例：

  We're dealing with probably the biggest missing link in what we know about human evolution.

  我们正在解决的可能是我们所知的人类进化过程中缺失最大的一环。