Why was this? The researchers hypothesise that "disgust" contorts the face in such a way as to minimize the chances of the disgust-provoking object getting into our bodies. The nose wrinkles, the mouth seals tightly and the eyes narrow significantly - all as if to keep the noxious stimulus out. If it's something you've just bitten that you're finding disgusting, the tongue will often shoot out, as if to eject the food.
"Fear", on the other hand, engenders just the opposite response, in an attempt to maximise the
amount of sensory information getting to the brain. In a fearful situation, you require as much knowledge as possible! Consistent with this parallel hypothesis, the eyes widen (expanding the visual fields) and flicker more effectively between targets. The nostrils and nasal passages also enlarge, allowing us to inhale larger volumes of air than usual. (Is it also coincidence that you may even take in a sudden involuntary gasp of air - perhaps to 'smell out' the situation even better?)
I think that this study conclusively shows that these two facial expressions, at least, were chosen for their adaptive evolutionary value (even if they no longer perform them much today). It answers one of the questions about our facial expressions: why does this constellation of muscle movements read "fear" (or "disgust") to us, and not that set of facial expressions? There is doubtless much more to be said on the broad topic though. Have a look here for a more extensive write-up of this interesting topic.