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.
We're dealing with probably the biggest missing link in what we know about humanevolution.
missing link in British English
noun
1. the missing link
2.
any missing section or part in an otherwise complete series
missing link in American English
something necessary for completing a series; specif., an animal not yet found in the fossil record, that is an intermediate form of two animals, as such an animal coming between an ape and a human
Examples of 'missing link' in a sentence
missing link
Chanel This is the missing link that will make an outfit.