"To evolutionary psychologists, the noise made by gorillas, chimps and bonobos when you tickle their feet is no laughing matter. These distinctive vocalisations suggest that rather than evolving separately, laughter evolved in a shared common ancestor before becoming tailored in each primate species, including humans." "Through the differentiation of human social interaction over time this 'simple' form of laughter may have diversified to become a spectrum of different laughter variants in order to accommodate increased complexity of human social interaction."