Laughter:

David Sloan Wilson:

"Why should a merry mood enhance survival and reproduction? ... You can't learn new things if you're scrabbling for your next meal or fearing for your life. Our ancestors spent a lot of time scrabbling for food and fearing for their lives as they descended from the trees and ventures out onto the African plains in their wobbly two legs. Periods of safety and satiety were few and far between. Human laughter probably initially evolved as a signal for identifying these periods ..."