missing link

noun2

1. •
   (informal) A hypothetical primate once thought necessary to explain a perceived evolutionary gap between apes and humans.

   • "In the latter case, he may well approximate to what the vulgar have called the 'missing link'." — 1912
2. •
   (figurative) Any sought-after or valuable intermediary figure or position.

   • What we need is an artist who is also a scientist; or a scientist who thinks like an artist. And at last I can unveil that missing link of art and science! — 2010