On the Sierra 117 mission, the Pelicans will crash. After that, you should clear the area like you normall would. As the area narrows as you proceed, there are some rocks on your left that you can jump on. Continue along a ledge thereafter. Once it heads down, you'll pass a large boulder on your left. After you do, turn around and take a look where the wall and the boulder intersect. You'll see a family of primitive cavemen-like people!