The above link briefly covers nothing relevant to what I am going to tell you. Scientists have named the protein that determines how the human face develops after good old Sonic the Hedgehog. Apparently, too much of the Sonic Protein and you develop an extra face, too little and your foetal face collapses to form a cyclops-esque visage.
So next time you are playing with the little blue fella (Sonic, not anything rude!), just stop a moment and give thanks to the person(?) responsible for your appearance!