Why did We Evolve such Big Brains?

Mauricio González-Forero set out to answer the question of why we have big brains using a mathematical model. He started from the assumption that we got smart largely for social reasons, that is, our ancestors used their brains mainly for manipulating each other, leading to an arms race for bigger brains. But that isn't what his model showed. In fact he found that just ecological challenges could lead to big brains, and that social factors did not model very well:

To our surprise, we found that the intuitively appealing social hypotheses did not materialize in the model. Social cooperation and competition, possibly involving cognitive arms races, failed to lead to stable, human-sized brain sizes and may actually decrease brain size. The reasons are that (1) cooperation enables individuals to rely on social partners’ brains and to reduce their allocation to costly brain production, and (2) that competition leads to arms races where outcompeting social partners requires an exceedingly large brain and so it becomes a better strategy to reproduce early in life. In contrast, we found that the human brain size could evolve with a combination of ecological and social challenges, where ecological challenges are the drivers of brain expansion. Additionally, we found indirect evidence that for this to occur, ecological challenges must be coupled with what seems to be culture, suggesting an explanation for why ecological challenges may have failed to lead to brain expansion in other taxa.

Of course this is just one modeling exercise, but it makes sense to me that big brains would be connected to culture at a fundamental level. After all we don't rely on our own brains to solve problems; we use our brains to access the stored wisdom of our people, that is, our culture. Hence our obsession with genealogy, with storytelling, with using cultural clues to distinguish between friends and others. And with language, which we use to manipulate each other but which we use at the most sophisticated level to engage with our cultures.