|MadSci Network: Evolution|
To best answer your question I think it is best to quote the paragraph at
the top of page 161 in The Cambride
Encyclopedia of Human Evolution: "All
primates, including humans, use their facial features and muscles to
generate expressions. Our own perception tends to label facial expressions
as outward manifestations of an emotive state; a smile shows happiness.
Like many kinds of behaviour, facial expressions help to regulate social
relationships. At this level, the smile (or its simian equivalent) is a
display that disarms aggression and eases social interactions."
Try the links in the MadSci Library for more information on Evolution.