During hominid evolution an increasing number of complex facial muscles appeared to support an array of emotional expressions that are now universal across modern cultures. The amygdala plays a central role in both interpreting and orchestrating the response to these expressions.
A New York Times article today discusses a very practical use of noting subtle changes in this evolved facial musclulature. It is a key element of behavioral profiling increasingly being used at airports to discern potential terrorists. Work over many years by Paul Ekman at UCSF has generated a detailed catalog of these muscles and how they change in different contexts. He has developed a Facial Action Coding System (FACS) that is now widely used.
Here are some photos provided by Ekman showing several expressions. See if you can recognize them before looking at the captions.