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. 缺少的環節; 指爲完全理解某事尚需知道的一則信息或証據[usu sing]

We're dealing with probably the biggest missing link in what we know about human evolution. 我們正在解決的可能是我們所知的人類進化過程中缺失最大的一環。