From Halopedia, the Halo wiki
Haplorrhini is an achievement in Halo: The Master Chief Collection. It is unlocked when a player finds the lonely caveman easter egg near the end of the level Sierra 117. The achievement is worth 5 Gamerscore.
The lonely caveman is in the area where you rescue Sergeant Johnson, and can be found sitting on a cliff across the water. He can be clearly seen by using a scoped weapon, such as a sniper, or the binoculars. Although it is possible to reach the lonely caveman in co-operative play by exploiting a glitch, it is only necessary look at him to unlock the achievement.
Haplorrhini (or Haplorhini) is the suborder of primates which includes apes and, conversely, modern humans.