A paper just published in the March 25th issue of Nature (1) suggests that a mutation in a single gene (MYH16) may have been responsible for many of the cranial difference between Homo and other primates. "A gene responsible for a majority of jaw musculature was lost from human ancestors, presumably 2.4 million years ago, according to the study. Drastic reductions in these muscles may have lifted significant physical constraints on braincase volume, allowing primates with weak jaws and big brains to eventually think about their origins." (link) While support from genetics researchers seems forthcoming (link), the anthropological community seems less convinced by the results. Owen Lovejoy states "[s]uch a claim is counter to the fundamentals of evolution, [t]hese kinds of mutations probably are of little consequence." The mutation would have reduced the Darwinian fitness of those individuals," said Bernard Wood. "It only would've become fixed if it coincided with mutations that reduced tooth size, jaw size and increased brain size. What are the chances of that?" (link).
Whether or not these results pan out in the long run, they are a powerful indication of the possible effects of single mutations and should spur further comparisons of human & ape genomes.
(1) H.H. Stedman et al., “Myosin gene mutation correlates with anatomical changes in the human lineage,” Nature, 428:415-418