Yawn
Charles Darwin argued, in The Expression of the Emotions in Man and Animals, that if a behaviour was present in many cultures, then it must be inherited (wholly or partly). All traits which appear in a wide range of animals must have some function which is supported (or was supported) by natural selection. In most cases the function is obvious, but with yawning we do not know what that function is. Suggestions have been made, but they do not seem to account for its widespread occurrence in mammals, and possibly in other vertebrates.