Pure instinct of the species. Once upon a time a great Neanderthal named Sabertooth Jones saw his child, Little Tadpole, crying from a piece of walrus meat stuck in his tooth. Jones took him to the Neanderthal Dentist, Dr. Tyranasaurus Humphrey, who told Little Tadpole to sit in his chair which also served as a table for cleaning pre-historic swamp rats. The doctor could not find the cause of all the pain and, therefore, decided to try a proper diet. Humphrey, then presented Tadpole with a monstrous ancient chocolate bar. After that, Humphrey's mouth remained in such position ready for that chocolate. The family, too, were introduced with the chocolate bar and were equally enthused at it's wonderful taste. It also served to tune their killer insticts when on the hunt for indigenous amphibian whales. So the family started the mutation of mankind's great trek into yawning, the secret gene that has it's beginnings into the great delight in chocolate. The End!
It's a medical fact that when you suck in an overextended ammount of air, it creates a vacuum in the room causing the normal atmospheric pressure to be reduced, therefore upsetting the natural balance of the nearest victim's diaphram. Thus causing the need for more oxygen to fill the space that the body is used to. Further, if enough people yawned at the same time in a small enough space, everyones internal organs would spew out of their mouths and they would all die. Can I get the nurse to confirm this?