Evolution of Facial Expression

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Abstract
Facial expressions in primates are caused neither by specific conditions or drives nor by pleasure or unpleasant feelings, although they do convey information about the motivation of the animal which shows them. They appear to have evolved from such sources as responses through which vulnerable areas are protected, responses associated with vigorous respiration, and grooming responses, and they retain something of the causation of the early responses from which they stem. Similar facial displays have evolved in the four lines of primates in which monkey-like forms have developed. The resemblances and the differences between the displays of our own line and those of the others offer new sources of information concerning the evolution of human behavior.