It’s one of the most fundamental things that humans do. Smile. Newborns can manage it spontaneously, as a reflex, and this is sometimes misinterpreted by new parents as a reaction to their presence, although it’s not until six to eight weeks of age that babies smile in a social way. That new parents optimistically interpret the first reflex smiles reflects the complexity of smiling: there is the physical act and then the interpretation society gives to it – the smile and what the smile means.

On a physical level, a smile is clear enough. There are 17 pairs of muscles controlling expression in the human face, plus the orbicularis oris, a ring that goes around the mouth. When the brain decides to smile, a message is sent out over the sixth and seventh cranial nerves. These branch across each side of the face from the eyebrows to the chin, connecting to a combination of muscles controlling the lips, nose, eyes and forehead.

Culturally, smiling resonates across the arc of human history, from the grinning Greek kouros sculptures of 2,500 years ago right up to emojis. Emojis with smiling faces are by far the most prevalent in online messages. The most popular emoji of all – the face with tears of joy – was picked as the 2015 Word of the Year by Oxford Dictionaries. Just as this emoji expresses more than mere happiness – tears adding the ironic twist – smiles themselves convey so much more, too.

A 2016 study published in the Journal of Nonverbal Behavior questioned thousands of people in 44 cultures about sets of photographs of eight faces – four smiling, four not. Most people deemed the smiling faces to be more honest than the non-smiling ones. This difference was huge in some countries, such as Switzerland, Australia and the Philippines, but small in others, such as Pakistan, Russia and France. In a few countries, such as Iran, India and Zimbabwe, there was no trustworthy benefit to smiling at all. The researchers concluded that where trust was low, smiling was less likely to influence the respondent. “Greater corruption levels decreased trust granted toward smiling individuals,” the authors concluded. If anything it could arouse suspicions.

There is exactly one smile in the Old Testament – Job, ironically, in the book of suffering – though in many passages faces are said to “shine”, which could mean smiling or could mean heavenly radiance. Eastern religions, however, often use the smile to denote enlightenment. The Buddha and various religious figures were depicted with serene smiles, though the original Buddhist texts are as devoid of smiling as scripture. Jesus weeps but never smiles.

There are various medical conditions that can disable us from smiling. A common one is facial paralysis caused by a stroke. Rarer is Moebius syndrome, a congenital facial paralysis caused by missing or stunted cranial nerves, where you can’t smile, frown or move your eyes from side to side. “You essentially have a mask on your face,” says Roland Bienvenu, 67, who has Moebius syndrome. Without being able to smile, others “can get an incorrect impression of you”, he says. “You can almost read their thoughts. They wonder: ‘Is something wrong with him? Has he had an accident?’ They question your intellectual ability, think maybe he’s got some intellectual disability since he’s got this blank look on his face.”

A lopsided smile can be as problematic as no smile at all. “I have half a smile, so even with that I am able to successfully convey emotion,” writes Dawn Shawn, born with a teratoma – a fast-growing tumour that was interfering with her windpipe. “The hardest part for me was seeing photos of myself smiling, because smiling exaggerates the fact that half my face doesn’t move very much. But eventually I learned to own it. That is me. That is how I look.”

While losing a smile is a serious blow at any age, it can have a particular impact on younger people who are forming the bonds that will carry them through the rest of their lives – or trying to. “It’s a huge problem,” says Tami Konieczny, supervisor of occupational therapy at the Children’s Hospital of Philadelphia. “When you look at somebody, the first thing you see is their face, their ability to smile or not smile, or an asymmetrical smile. It’s your social world. If someone can’t read your facial expressions, then it’s difficult to be socially accepted. It’s hugely devastating for kids. I had kids Photoshopping their pictures. They are taking mirror images of their good side and copying it, Photoshopping their own pictures before posting them to social media.”

The father of modern plastic surgery, Harold Gillies, reported in 1934 that restoring the ability to smile made patients’ faces “feel much more comfortable”. In addition, Gillies observed, “The psychological effect is also one of considerable value.” On the interpretive side, Charles Darwin discusses the meaning and value of smiles in his 1872 landmark study The Expression of the Emotions in Man and Animals. Like many, Darwin sees a smile as the first part of a continuum.

“A smile, therefore, may be said to be the first stage in the development of a laugh,” he writes, then reverses course, musing that perhaps the smile is instead the remnant of laughter.

He observes his own infants closely, detecting in two their first smiles at six weeks, and earlier in the third. He comments how smiles do more than merely convey happiness, mentioning the “derisive or sardonic smile” and the “unnatural or false smile”, and showing photos to see if his associates can read what they mean.

The scientific study of smiles finds differences in gender (generally, women smile more) and culture. Smiles are definitely communicative – people smile more when in public than they do when alone, and more when interacting with others than when not.

Scientists have shown that smiles are far easier to recognise than other expressions. What they don’t know is why. “We can do really well recognising smiles,” says Aleix Martinez, a professor of electrical and computer engineering at Ohio State University. “Why is that true? Nobody can answer that right now. We don’t know. We really do not know. We have a classical experiment, where we showed images of facial expressions to people, but we showed them very rapidly… 10 milliseconds, 20 milliseconds. I can show you an image for just 10 milliseconds and you can tell me it’s a smile. It does not work with any other expression.”

Fear takes an exposure time of 250 milliseconds to recognise – 25 times as long as a smile, “which makes absolutely no sense, evolutionarily speaking”, Martinez says. “Recognising fear is fundamental to survival, while a smile… But that’s how we are wired.”

Studies have shown that smiling faces are judged as more familiar than neutral ones. And it’s not just us that can recognise smiles more easily. “This is true both for humans and for machines,” says Martinez. Although scientists have been studying smiles for about 150 years, they are still at the stage of trying to categorise types of smile among the millions of possible facial expressions. “One of the fundamental questions in the scientific literature right now is, how many facial expressions do we actually produce?” says Martinez. “Nobody knows.”

Scientists such as Martinez theorise that smiles – as well as frowns and other facial expressions – are remnants of humanity’s distant pre-linguistic heritage. Human language started developing as far back as 100,000 years ago, but our expressions reach back further still, even to before our origins as human beings.

“Before we could communicate verbally, we had to communicate with our faces,” Martinez says. “Which brings us to a very interesting, very fundamental question in science: where does language come from?” One of the hypotheses is that it evolved through the facial expression of emotion, he says. “First we learned to move our facial muscles – ‘I’m happy. I feel positive with you! I’m angry. I feel disgust.’” Then a grammar of facial expressions developed, and over time that evolved into what we call language. So when we wonder how something as complex as language evolved from nothingness, the answer is it almost certainly started with a smile.

This is an edited version of a story first published on mosaicscience.com by Wellcome. It is republished here under a Creative Commons licence. Sign up to the newsletter at mosaicscience.com#newsletter