Consciousness Blog 16 May 2010
Darwin (The Expression of the Emotions in Man and Animals) believed that facial expression, used to express emotion, had its origin in non-human animals, and that such displays emerged from the process of natural selection. Well, that much at least seems obvious nowadays; everything about us emerges from natural selection. And the purpose of facial expression, again to state the obvious, is to communicate with conspecifics. The highly complex assemblage of facial musclature in humans, an order of magnitude more complex and expressive than that found in earlier mammals, or even in other primates, is a direct reflection of the equivalent degree of social complexity in human groups.
Turning the argument on its head, one can say that any animal physiologically capable of facial expression must have social relationships in which that expression can be useful. So if mice can express the feeling of pain, there must be other mice that can respond. Recognition of the expression of pain may be helpful as a warning or may evoke a helpful response. Rats and mice certainly are social animals, it goes without saying.
Darwin, in his published work at least, didn't go beyond mammals in describing facial expression, but the question obviously arises as to whether there may have been precursor mechanisms in pre-mammal species. There is plenty of anecdotal evidence that birds have facial expressions (see eg Some Remarks on the Facial Expression of Birds, by George M. Sutton, 1922, the Wilson Ornithological Society). Of course they lack the evolved expressive musclature of even the earliest forms of mammal, but they do for instance have muscular control over the eyelid, the behaviour of the beak, and in some species the behaviour of feathers around the eyes, and it is tempting to associate these muscular abilities with the observed expressiveness of birds' faces. Unfortunately there doesn't seem to have been any research that could throw light on whether the behaviour of birds' facial muscles is under the control of the animals' hedonic system.
From a purely theoretical perspective, it is a certainty that in a highly social species, means of communication will evolve alongside sociality. It follows that among birds, which are indeed highly social, there must exist techniques of interpersonal communication. There is song, of course, and body language; the presence of a degree of facial musclature makes it entirely possible that facial expression is playing a role as well.
Read more in Chapter Three of Agent Human by Michael Bell, The Evolution of Social Consciousness in Humans.