Primate is an achievement in Halo: The Master Chief Collection. It is unlocked by finding and viewing the "family" of four Cavemen, or "missing links", on the level Sierra 117. The achievement is worth 5 gamerpoints.
To find the cavemen family, go past the point where Johnson says his Pelican was shot down. Then, enter the jungle and stick to the left wall. You will have to climb over some rocks, jump over a giant rock in the way, and then crouch under a tight ledge to find them.
Despite the achievement's description, the Caveman family are not the first missing links found on the level.