Nonverbal signaling is an interesting component of interpersonal communication because it is extraordinarily significant while being ambiguous and idiosyncratic.
While we are capable of improving our ability to understand nonverbal communication through training, the ability itself is innate. It is contingent upon hereditary components; autistics generally have to undergo extensive training to comfortably interact with the neurologically typical. Since it contains a hereditary component, it must have historically conferred differential reproductive success. To me it seems initially counterintuitive that we would have an evolved capacity for interpreting such ambiguous signalling. If communication is important for reproductive success in other species, they generally adopt rather unambiguous modes of communication, like the clear, distinct tones of birdsong, the croak of a frog, or the vivid extravagance of a peacock’s tail.
Our forbears, with their machiavellian intelligence, obviously had some hereditary capacity for interpreting gross motor movements like threat displays and teeth-baring. In addition to neurological adaptations, we have several physiological adaptations whose primary function is to improve non-verbal signaling.
The prominent chin and pronounced cheeks of the movie star highlights the attractiveness of the capacity to make more nuanced facial expressions. The number of muscles in the modern human face and our degree of control over them far exceeds that of our cousins, and presumably our ancestors as well.
Modern research suggests that we make facial expressions preconsciously; we express them and then we suppress them, making microexpressions that last 1/5 of a second. Why is this? It seems like we should either be unable to make an expression or in control of the expression. This tension leads me to infer that expressions came first, and control over expressions later. Experiments to see if any of our cousins (with similar, though simpler, repertoires of facial expression) also have microexpressions would help test this thesis. Another piece of supporting evidence is that people who are lying tend to reduce expressiveness, something that may indicate that controlling facial expression carries a heavy cognitive burden.
The emotional system is present in all mammals to a greater or lesser degree, so it had to serve a purpose before simians arose. Emotions probably developed as a sort of neurological system for categorising experience. In a social situation, both the recognition of emotion’s behavioural correlates in others and the amplification of them in yourself would be selected for.
As soon as you live in a social situation where everyone is signalling their internal state, it becomes helpful to manipulate your signalling. Not just for deception but to let decisions based on social perceptions mediate emotional responses based on direct perception.