It is still not known what caused our ancestors to leap-frog this obstacle so spectacularly and became far more intelligent than was actually needed for survival.
Well surely the size of our brains is what was needed for survival? Nature rarely produces redundancy in any form.
Actually, human intelligence is significantly above what is needed to prosper even for an "intelligence niche" species (all all other such species demonstrate).
And yes, nature almost always only evolves an attribute until it is "good enough". Hence the mystery of our unnecessarily (from a survival perspective) large brains.