By Eliza Strickland, Discover Magazine, June 16, 2008

When confronted with something truly terrifying (say, for example, an irritated grizzly bear), most human faces assume the same expression, with bulging eyes and flaring nostrils. Researchers have long suspected that those facial adjustments serve some evolutionary purpose, but the mechanism has been unclear for over a century.

Now, a study presents an answer that seems rather obvious in retrospect. Those wide-open eyes and flared nostrils take in more sensory information, which helps when you’re trying to figure out how to evade swiping bear claws. 

Curiosity about the purpose of facial expressions goes back to Charles Darwin. In 1872, Darwin published The Expression of the Emotions in Man and Animals, which discussed the similar facial expressions found across human cultures and in some animal populations, and theorized that the expressions must have some evolutionary benefit. He guessed that the advantage lay in the ability to communicate emotions, which could reduce misunderstandings and help a group function efficiently.

Later scientists followed Darwin’s train of thought and discovered that the expression of emotions is strikingly similar across cultures – horror and disgust look pretty much the same on the face of a New Yorker as they do on a Nigerian, and people from different cultures can recognize emotions such as happiness, anger and surprise on others’ faces, even if they don’t share a language. The fact that emotional expressions seem to be universal led scientists to believe they weren’t used only for communication and social purposes, but also served an additional adaptive biological function.

More here.