A new hypothesis says that humans are so clever because our brain cells are the result of a copying error, or, really two copying errors:

The second, more recent, duplication seems to be incomplete, with only part of the gene being duplicated. The researchers think this partially duplicated gene is able to interfere with the actions of the original, ancestral copy of SRGAP2. When the researchers added the partially duplicated gene copy to the mouse genome (mice don’t normally have it) it seemed to speed the migration of brain cells during development, which makes brain organization more efficient.