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]

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。