Intellect is extremely useful. The ability to abstract and "role play" possible futures is extremely important as a developmental tool for all the more complex species.
Ants have no sense of imagination, and no rational ability to predict into even the reltively close future. An ant might try to cross the same leaf in the wind many times before finding that the wind has died down, as it always does, and he can now cross easily, but tired for his useless excersions.
An ape uses more advanced statistical analysis, and knows that by thrusting a stick into a termite mound the termites will attack the stick and can be drawn out as food. However, he cannot extrapolate from this stick use to, say, fire use or tactical warfare.
Then comes primitive man, and he is even cleverer than the ape, and develops more tools, better tools, better visions and more abstract conceptualizations. He even abstracts a spirit out of himself, a vision so complete that it seems to stand apart from the physical reality that first created it.
The next step? Why, obviously man will develop into a bigger and smarter version of himself, with more abstract understanding and abilities beyond our imagining.
However, a step was ignored because it does not fit the pattern:
Neanderthal man, the smartest creature to ever walk the earth. Neanderthal had a larger brain size, and there are indications of cultural complexity not found among primitive sapien sapien societies.
Man reached a plateau.
Neanderthal may have died out because his reflexes were too slow, his considered rational responses coming at the expense of survival when faced with hungry predators. Or, perhaps, (having exhausted calculus, religion, physics, quantum and temporal mechanics) he simply realized that life had no purpose, was a game of struggle and torment for the foolish, and chose nirvana.
The meek inherited the earth.
Those not intelligent enough to know that evolution had run its course and was already finished, continued building toys and quibbling over their games.
God is dead, He let us kill Him.