From Halopedia, the Halo wiki
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The first Caveman can be found in the area where Sergeant Johnson's Pelican is shot down. Across the water, above and to the right where a Kig-Yar Sniper will spawn on Heroic and Legendary difficulties, the first Caveman can be seen sitting in a tree. Unlike the other Cavemen, there is no way to reach him. Zooming in on the Caveman with a battle rifle (repeatedly if necessary) will unlock the achievement.