Haplorrhini
From Halopedia, the Halo wiki
Haplorrhini is an achievement in Halo: The Master Chief Collection. It is obtained by finding the third, "lonely" Caveman near the end of the level Sierra 117. The achievement is worth 5 Gamerscore.
The third Caveman can be found in the area where you rescue Sergeant Johnson.
Immediately after the cutscene where the brute chieftan throws Johnson into a cell, a phantom is seen departing the area. The caveman is seated among some foliage in a small alcove where the faces of the cliff meet, just above and behind the phantom as it is taking off.
It is possible to reach the lonely caveman in co-operative play by exploiting a glitch. However, zooming in on the Caveman with a scoped weapon (repeatedly if necessary) from the bridge below the phantom is sufficient to unlock the achievement.
Trivia
Haplorrhini (or Haplorhini) is the suborder of primates which includes apes and, conversely, modern humans.