There's another one with an ape, a human, and an overwatch soldier. That one was put by the Combine, and portrays them as the next step in human evolution.
theres no way that was put there by the combine, its much more likely a resistance members critisism of the route that human evolution WILL take if the combine are not resisted against.
I can't believe you're all missing the blindingly obvious. Look at the head shape! The ape has a low skull, an elongated snout and a ridge above the eye. Then it evolves into a human. Then the next step has been to devolve - compare the ape with the Combine. See the similarities? The whole point is that becoming the slaves of the Combine represents a step backwards for the human race; they may as well have become mindless apes once again.
Sorry for dredging up an old thread, but I can't believe nobody got it!