Ekman and friesen (1975) analyzed hundreds of films and photographs of people experiencing various emotions. on the basis of their analysis, they concluded that

I believe the answer is: 
- there are six primary facial expressions of emotion (those six primary facial expressions are: Anger. Disgust. Fear. Happiness.Sadness. Surprise)

- all facial expressions of emotion are mixtures of six primary expressions.
(and the ability for us to show these mixtures are inherent/does not have to be learned)

To prevent the population from overgrowing