It wasn't a mutation they talk about in this paper but rather a gene that is different expressed in mice and Human. This little difference leads to remarkable different brain size. It is a big step towards understanding why our brain has this huge cortex (which is folded) and it is this cortex where all our higher brain functions are located, Math, language, etc. Of course a big brain is not necessary more intelligent than a smaller brain, but given the right input while its develops it is far superior to every other brain. For prove see history...