An interview from The Economist with Satoshi Kanazawa on intelligence and evolution:
What, if any, evolutionary advantage does intelligence give us?
Actually, less intelligent people are better at doing most things. In the ancestral environment general intelligence was helpful only for solving a handful of evolutionarily novel problems.
You mean our ancestors did not really have to reason?
Evolution equipped humans with solutions for a whole range of problems of survival and reproduction. All they had to do was to behave in the ways in which evolution had designed them to behave—eat food that tastes good, have sex with the most attractive mates. However, for a few evolutionarily novel problems, evolution equipped us with general intelligence so that our ancestors could reason in order to solve them. These evolutionarily novel problems were few and far between. Basically, dealing with any type of major natural disaster that is very infrequent in occurrence would require general intelligence.