The evolutionary mystery of why our faces contort when we are scared has been solved by a team of Canadian neuroscientists.

When our facial expression shifts to one of eye-bulging, nostril-flaring fear, our ability to sense attackers or other imminent danger improves dramatically, researchers found.

The findings lend support to an idea first laid out by Charles Darwin in one of his less well-known tomes, The Expression of the Emotions in Man and Animals, published in 1872. Darwin noted that facial expressions of emotion were often remarkably similar across human cultures, and even the animal kingdom, implying they may have a common evolutionary benefit.

"Most people think expressions are social signals, that they are intended to communicate what someone's feeling. We're saying they probably evolved as a sensory function first, even if they do help convey our feelings to others," said Adam Anderson, a cognitive neuroscientist who led the study at the University of Toronto.

Writing in the journal Nature Neuroscience, Anderson and his colleague Joshua Susskind reveal how the classic expression of fear increases our range of vision, speeds up eye movement and improves air flow through the nose. All of these reactions boost our ability to see or smell threats and prepare ourselves for the "fight or flight" response, where we either battle it out with our attacker or take to our heels.

In the study, Susskind developed computer models for the facial expressions of fear and disgust. He then trained volunteers to pull each face. A fearful expression required participants to widen their eyes, raise their eyebrows and flare their nostrils, while a disgusted face was the opposite: a lowered brow, closed eyes and scrunched-up nose.

Measurements from video footage revealed those pulling fearful faces were not only better at spotting objects either side of them, but scanned their eyes faster, suggesting they could see danger coming more quickly.

In another round of tests, volunteers pulling disgusted faces were found to have a reduced field of vision and slower air flow through the nose.

"Fear expressions open up the face and expose the sensory surfaces, whereas disgust does the opposite, it's a protective wincing. Fear is about vigilance and disgust is rejection," said Anderson.

The team confirmed their findings by asking volunteers to pull different expressions while inside a magnetic resonance imaging scanner. The images reveal that fear expressions open up the nasal tubes, allowing air to be breathed in twice as fast as someone pulling a disgusted expression.

"What we're doing is psychological archaeology. We're unearthing the residues of the functions of these expressions. Facial expressions might be more important as social signals, but that doesn't explain where they came from. This work explains why these expressions are common across cultures," said Anderson.