One place to start is this thread (Human Gene for Big Brain), and all the papers it references. We have been learning an immense amount about how human brains are different then chimps.
Another thread of equal interest, even though it is not chimps considered, is this paper on Neanderthal minibrains, a technique being developed to understand the functional significance of Neanderthal and Sapiens genetic differences (Growing Neanderthal Minibrains).This one raises some interesting ethical issues too.