The hypothesis that laughter evolved as a play signal is appealing in several ways. Unlike the Superiority and Incongruity Theories, it explains the link between humor and the facial expression, body language, and sound of laughter. It also explains why laughter is overwhelmingly a social experience, as those theories do not. According to one estimate, we are thirty times more likely to laugh with other people than when we are alone (Provine 2000, 45). Tracing laughter to a play signal in early humans also accords with the fact that young children today laugh during the same activities—chasing, wrestling, and tickling—in which chimps and gorillas show their play face and laugh-like vocalizations. The idea that laughter and humor evolved from mock-aggression, furthermore, helps explain why so much humor today, especially in males, is playfully aggressive.
— Read on plato.stanford.edu/entries/humor/