Kevin, chimps also bare their teeth when they are frightened, uncertain, or uncomfortable. If it were any true comparison to humans, I think people would be 'smiling' a lot more than they do nowadays. (big grin with REAL humour attached)

Another 'smiling' observation is that a LOT of time and money seems to be being spent on things to make your teeth 'perfectly white'--if you never smile, how's anyone ever going to know?

~Elle