The theoretical species
which would, if discovered, bridge the gap between the oldest human
specimen and the simian
most nearly related to us.
Anthropology no longer searches for a missing link, realizing that it is more useful to seek our last common ancestor.
In popular culture, a particularly stupid, thoughtless, or offensive person is sometimes referred to as The Missing Link -- implying, none too subtly, that the person is less than human. Usually, however, such a person is being quite human; the apes might take offense at having such behavior ascribed to them, however.
A snarky bumper sticker says: "I've found the missing link between apes and humans. It's us."