ParticleGirl writes "New Scientist has an article examining 10 human features (bugs?) that we still don't understand, like blushing, laughing, and nose-picking. There are some interesting, speculative evolutionary explanations listed for each. '[Psychologist Robert R. Provine] thinks laughing began in our pre-human ancestors as a physiological response to tickling. Modern apes maintain the ancestral 'pant-pant' laugh when they are tickled during play, and this evolved into the human 'ha-ha.' Then, he argues, as our brains got bigger, laughter acquired a powerful social function — to bond people. Indeed, Robin Dunbar at the University of Oxford has found that laughing increases levels of endorphins, our body's natural opiates, which he believes helps to strengthen social relationships.'"