Haplorrhini Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in the mission, Sierra 117.

On the first mission of the Campaign, there are unique hidden Cavemen that can be located in three distinct locations within the African jungle. The first of these Caveman can be located high above the tree line that runs parallel to the river where Johnson's Pelican is shot down. Utilizing a Battle Rifle to zoom in and scan the treetops will unlock "Missing Link" once you spot the Caveman.

The second location in the jungle ahead of this area. After clearing out the Covenant forces around the pump station, enter the jungle ahead and stick to the left. Continue jumping on the rocks along the path and you will eventually reach a large plateau where a family of Cavemen can be found. This family of Cavemen will unlock the Primate achievement.

The final Caveman can be located on a cliff just outside the boarders of the level in the area where Sergeant Johnson is being held prisoner. In order to reach this location, you will need to perform a complicated glitch to exit the mission’s boarders, however the Cavemen is still visible via a Sniper Rifle while standing on the bridge where Hocus picks you up. Whether you decide to reach this Caveman’s location via the glitch or simply by zooming in on his location, the "Haplorrhini" achievement will unlock once you spot him.