The BBC reports on a study in which a gene sequence that is unique to humans has been found. Speculation is that it is at least partially responsible for the brain expansion in humans. The story notes:
The area, called HAR1, has undergone accelerated evolutionary change in humans and is active during a critical stage in brain development.
Evidently, this gene is very different in humans than in other mammals, showing 18 base pair differences from Chimpanzees. As the authors notes, that is a lot in a few million years. Interesting.