Anger is one of the four (or six) basic emotions found independent of location, culture, or age, among humans. Anger, along with sadness, happiness, and fear/surprise, make up the basic pallet of feelings which we internally mix together to form complex emotions. There are also self-reflective emotions like shame and pride, which are far more variable. 
Anger is a very clear emotion, and expressed identically all over the world and is even displayed identically by congenitally blind children who have never seen the emotion. Marked by a lowered brow, thin lips, and flared nose, the expression is controlled by seven distinct muscle groups. The researchers at Santa Barbara wanted to understand what the purpose of the expression actually is.
One of the co-authors of the study, John Tooby, explained the context of this research,
“In earlier work we were able to confirm the predictions that stronger men anger more easily, fight more often, feel entitled to more unequal treatment, resolve conflicts more in their own favor and are even more in favor of military solutions than are physically weak men.”
Looking into the use of anger as a “bargaining emotion,” the present study indicates that,
“The anger face not only signals the onset of a conflict, any distinctive facial display could do that. We hypothesized that the anger face evolved its specific form because it delivers something more for the expresser: Each element is designed to help intimidate others by making the angry individual appear more capable of delivering harm if not appeased.” lead researcher Aaron Sell stated in the University of Santa Barbara press release.
They came to this conclusion by showing volunteers computer-generated faces displaying different components of the “anger face.” They found that each of the components actually increased the perceived physical strength of the model, for instance pictures with a digitally lowered brow were perceived to come from physically stronger men than those without the lowered brow. This was repeated (with identical results) for all of the facial cues for anger.
Thus the angry face serves to make the bearer appear stronger and more willing to fight. It evolved to help communicate an escalation of emotional conflict, and to warn of physical conflict before it occurs. Anger signalizes the refusal to accept whatever is currently “on the table,” and signals the negotiating partner that they need to change the terms or risk direct conflict.