In this mission you can find three "missing links" for three separate achievements.1:
You'll come to an area with two small shelters near the water where your allies ship is attacked by covenant ships. Take out the enemies and stand next to the river. Aim across the river with a scoped weapon and look into the middle tree, on on of the lower branches to spot this missing link.2:
After clearing out a section of enemies, you will proceed to a dense jungle where jackals with carbines will be on the sides of the area. Kill the jackals and proceed down this area, holding on to the left hand side of the walls that will it. At the end, before the turn, you should see a rock that you can jump on. Jump one step higher onto a ledge, and then crouch walk past the rocky path to spot this family of cavemen in the corner. Zoom and shoot them to unlock the achievement.3:
Near the end of the level after you'll approach a large base that is connected via a bridge. Take a scoped weapon, go to the middle of the bridge, and look off to the right hand side. There are two cliffs that meet each other just off the side, and you should be able to see a lonely caveman on the left cliff next to spot the cliffs meet. Zoom in on him for a few seconds for this achievement to unlock.
Following the above method will also unlock: