 The oldest play signals in humans are smiling and laughing. According to ethologists, these evolved from similar play signals in pre-human apes. The apes that evolved into Homo sapiens split off from the apes that evolved into chimpanzees and gorillas about six million years ago. In chimps and gorillas, as in other mammals, play usually takes the form of mock-aggression such as chasing, wrestling, biting, and tickling. According to many ethologists, mock-aggression was the earliest form of play, from which all other play developed (Aldis 1975, 139; Panksepp 1993, 150). In mock-aggressive play, it is critical that all participants are aware that the activity is not real aggression. Without a way to distinguish between being chased or bitten playfully and being attacked in earnest, an animal might respond with deadly force. In the anthropoid apes, play signals are visual and auditory. Jan van Hooff (1972, 212–213) and others speculate that the first play signals in humans evolved from two facial displays in an ancestor of both humans and the great apes that are still found in gorillas and chimps. One was the “grin face” or “social grimace”: the corners of the mouth and the lips are retracted to expose the gums, the jaws are closed, there is no vocalization, body movement is inhibited, and the eyes are directed toward an interacting partner. This “silent bared-teeth display,” according to van Hooff (1972, 217), evolved into the human social smile of appeasement. 