Graziano suggests that our smile originated in the defensive reaction of monkeys to other monkeys moving into their personal space. He then proceeds to make just-so stories about simian origins of our laughing and crying. To begin, imagine Monkey B steps into the personal space of Monkey A.
Monkey A squints, protecting his eyes. His upper lip pulls up. This does expose the teeth, but only as a side-effect: in a defensive reaction, the point of the curled lip is not to prepare for a biting attack so much as it is to bunch the facial skin upward, further padding the eyes in folds of skin...The head pulls down and the shoulders pull up to protect the vulnerable throat and jugular....The torso curves forward to protect the abdomen...Monkey B can learn a lot by watching the reaction of Monkey A...And so the stage is set for a social signal to evolve: natural selection will favour monkeys that can read the cringe reactions of their peers and adjust their behaviour accordingly...If Monkey B can glean useful information by watching Monkey A, then it’s useful for Monkey A to manipulate that information and influence Monkey B. Evolution therefore favours monkeys that can, in the right circumstances, pantomime a defensive reaction. It helps to convince others that you’re non-threatening. Finally we see the origin of the smile: a briefly flashed imitation of a defensive stance.
In people, the smile has been pared down to little more than its facial components — the lifting of the upper lip, the upward bunching of the cheeks, the squint. These days we use it mainly to communicate a friendly lack of aggression rather than outright subservience...We can’t help feeling warmer towards someone who beams that Duchenne smile.
On laughing:
...chimps have something like laughter: they open their mouths and make short exhalations during play fights, or if someone tickles them. Gorillas and orangutans do the same. The psychologist Marina Ross compared the noises made by different species of ape and found that it was the sound of bonobos at play that comes closest to human laughter, again, when play-fighting or tickling. All of which makes it seem quite likely that the original type of human laughter also emerged from, yes, play-fighting and tickling.
On crying:
My best guess, strange as it might sound, is that our ancestors were in the habit of punching each other on the nose. Such injuries would have resulted in copious tear production...According to recent analysis by David Carrier and Michael Morgan from Utah University, the shape of human facial bones might well have evolved to withstand the physical trauma of frequent punching. Thickly buttressed facial bones are first seen in fossils of Australopithecus, which appeared following our split with chimpanzees...the reason we weep now may well be that our ancestors discussed their differences by hitting each other in the face. Some of us still do, I suppose.
In any event, the entire behavioural display that we call crying – the tear production, the squinting, the raised upper lip, the repeated alarm calls – makes for a useful signifier. Evolution would have favoured animals that reacted to it with an emotional desire to dispense comfort.
Graziano's speculative summary:
An age-old defensive mechanism, a mechanism that monitors bubbles of space around the body and organises protective movements, suddenly takes flight in the hyper-social world of primates, spinning into smiles and laughter and crying and cringeing. Each one of those behaviours then splits further, branching into a whole codebook of signals for use in different social circumstances. Not all of human expression can be explained in this way, but much of it can. A Duchenne smile, a cold smile, laughter at a joke, laughter that acknowledges a clever witticism, cruel laughter, a cringe to show servility, standing straight to show confidence, the arms-crossed expression of suspicion, the arms-open expression of welcome, tilting your head as a sign of surrender to a lover, the fleeting crinkling of the face that hints at crying as we show sympathy for some sad story, or a full blown sobbing jag: this whole vast range of expression could well have emerged from a protective sensory-motor loop that has nothing to do with communication. Evolution is bizarre.
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