In ever other species showing the teeth is a sign of anger. why have humans evolved to smile, but yet its a sign of happiness?
my dog opens his mouth and shows his teeth when he is happy.
Humans evolved from a species with large canine teeth, fangs, in fact. When they bared their fangs others were frightened, and gave in to dominance.
After we evolved our present physiognomy, whenever someone bared their fangs it cracked everyone up. No fangs, ha ha ha ha ha. All the fang bearers had to develop a sense of humor about it.
Tris
Can’t remember where I read this, but it was probably Jared Diamond’s The Third Chimpanzee. The theory was that smiling actually did evolve from baring teeth as a ritualized way of reducing physical aggression, similar to the way cats do. From there, teeth baring became a way to relieve any social tension–the nervous smile. Soon we were using it as a part of greetings (a potentially tense social situation), and finally it evolved into what we see today.
Of course, this is all speculation. Personally, I like Triskadecamus’ theory better.