Apparently, faces of fear and disgust are polar opposite expressions and seemed to have been the first few facial expressions to have evolved. They may have evolved for the purpose of moderating sensations and external physiological experiences in the environment. In an article in Nature magazine in 2008, researchers state that fear can be used to monitor and scrutinize the surrounding - flared nostrils and widened, terrified eyes may be able to take in more of the environment in both the visual and olfactory sense. In contrast, when feeling disgust, the nose crinkles naturally to impede nasty odors that may be harmful. Eyes squint so that less of a disgusting scene is taken in. This is very interesting to me as I never thought of fear and disgust as polar opposites; they often come hand-in-hand in everyday expression that I have assumed they complemented each other. There was also a contrast in air intake, which increased for fear and decreased for disgust. Air intake was interestingly measured with MRI. MRI images show that fear opens nasal passages while disgust closes them!
The above study was performed by Susskind and Anderson at the University of Toronto. They had participants feign expressions of fear and disgust to gauge relative eye opening sizes and visual fields. However, artificially and forcibly expressing these emotions for this experiment may not be the most ecologically valid approach. It's difficult to disentangle real fear/disgust being voluntarily experienced from the participants simply putting on an expression that they have been encultured/conditioned to produce as corresponding to those emotions.
This idea is definitely not new as Darwin himself discussed in the Expression of Emotions in Man and Animal that facial expressions actually were like shields to protect the person. They were not simply used for communication and conversation purposes. Modern researchers second this idea - expressions did not originate from language as expressions are not as variable as language, i.e. there are different words for anger but the expression of anger is shared. However, expression as language also helped with survival purposes as understanding one another's facial expressions likely cohered social groups and implicit communication could be exchanged. For instance, understanding and copying a terrified gaze on someone else's face can instigate another individual to do the same and become more vigilant of his/her surroundings to ensure survival. Then I wonder if different extents of making a face, i.e. if you made a huge facial expression of disgust, proffers any advantages... Also, I wonder why there would a need to copy expressions and why that's such a prevalent theme in adaptation when physiologically and physically, we're all more or less equally equipped to make the same expressions? If this ability is hard-wired to some extent, wouldn't it be somewhat out of voluntary and conscious control and arise spontaneously?