Apes play a key role in human evolution. But our facial muscles also have a story to tell, says Roger Highfield
It is a familiar scene from countless horror films: the heroine, eyes wide and mouth gaping, prepares to scream as the killer approaches. But now scientists have discovered that, in doing so, she is obeying not the dictates of her director but the laws of evolution.
The reason for this is that our facial expressions have a purpose. Even if there is nothing scary around, pulling a scared face will make you more alert. Similarly, the disgusted face we make when encountering a bad smell is designed to cut out the offensive odour.
The idea that the faces we make at times of high emotion did not evolve randomly, but have some evolutionary benefit, was first proposed by Charles Darwin. If they boosted our chances of surviving, he thought, they would be selected for in the gene pool.
This explains why everyone, from a City trader in London to a hunter-gatherer in Africa, uses the same expression when frightened. "Whether they're from Toronto or Papua New Guinea, people raise their eyebrows and eyelids during fear, or raise their upper lips and wrinkle their noses during disgust," says Dr Adam Anderson of the University of Toronto.
His study, written with Joshua Susskind and other colleagues, has been published in the journal Nature Neuroscience. As we reported yesterday, it found that people asked to make frightened expressions had a wider range of vision, faster eye movements and an increased sense of smell as they breathed more rapidly through their nostrils.
Those making the opposite expression, of disgust - with eyes and mouth scrunched up, rather than widened - had a smaller range of vision and a decrease in nasal volume, meaning that they saw and smelt less of what had offended them.
The discovery comes in the wake of the suggestion by William James, the pioneering philosopher and psychologist, that making a particular face contributes to feeling the related emotion. Moving your face into certain configurations changes the blood flow to the brain and the way you feel - so smiling, for example, helps to make you happy.
But in another recent study, a scientist at the University of Portsmouth questioned the idea that all facial expressions are universal. Strangely, Dr Bridget Waller and anatomists at two universities in Pittsburgh reported in the American Psychological Association journal that the facial muscles that control our expressions are not common to everyone.
We all have a core set of five facial muscles that control our ability to produce standard expressions which convey anger, happiness, surprise, fear, sadness and disgust. But there are up to 19 muscles present in the face, and many people do not possess all of them.
The risorius muscle, which controls expressions of extreme fear, is found in only two thirds of people.
"Everyone communicates using a set of common signals," says Dr Waller, "so we would expect to find that the muscles do not vary. The results are surprising. Some less common facial expressions may be unique to certain people."
In other words, while we all know instinctively how to look afraid, not all of us can do it quite as expressively as in the movies.