What Explains the Big Brains of Homo Sapiens?

"...If having a large brain were all that advantageous, it seems that every animal would have one. And yet most species have been content, evolutionarily speaking, with relatively small ones. The brain of our closest living cousin, the chimpanzee, is less than a third the volume of ours, even though chimps weigh almost as much as humans."


http://www.slate.com/articles/healthandscience/humanevolution/2012/10/humanbrainsizesocialgroupsledtotheevolutionoflargebrains.htm