missing link 
noun
1A thing that is needed in order to complete a series, provide continuity, or gain complete knowledge: she is the missing link between the European ballad tradition and Anglo-American white soul...
- The police know there are people who could provide those vital missing links which would bring the driver to justice.
- While minor and house league hockey is providing the missing link for many NHL fans, there is a dedicated bunch of Montrealers whose team's slow rise to the throne of hockey greatness is also filling the gap.
- But it is something else that provides the missing link… they are all veterans of the Armed Forces and are now members of the Rochdale Royal British Legion.
1.1A hypothetical fossil form intermediate between two living forms, especially between humans and apes.The rest of his arguments, such as the lack of missing links in the fossil record and the evidence for a young Earth, were verifiable....
- He claimed it was found in a peat bog and was a vital missing link between modern humans and Neanderthals.
- So he was particularly keen on suggesting to people that he'd found missing links not just between humans and animals but between widely different types of animals.