Why do we have the brain more developed than the monkeys ? The assumptions are numerous. And the most original and more recent (in April 2004), published a few times ago in Nature, does not reach us by a cranium study and fossils, but by a protein called MYH16. An important protein since it is responsible for the development of the muscles of the jaw.
2.4 billion years ago, a genetic change relating to gene coding MYH16 would have returned to us unable to produce protein. Consequently, our muscles of the less being useful jaw, the peaks of cranium (being used to fix the muscles) would have become useless, and would have been atrophied, leaving place with the brain !
Rather logical assumption, more especially as the date of this change coincides with the transition to the fossils.
However, of other arguments contradict this theory. Because it is not proven that the presence of the peaks does not prevent the development of the brain. On the contrary : the chimpanzee shows us that both coexist. As for Neandertal, the enigma of the evolution (see The first genocide ?), it extremely muscular but was equipped with a brain-pan larger than our !
What one will retain of this history, until there is new experiments affirming or cancelling the theory, it is that even research on the muscular diseases can derive on the genetics...