Evidence of humans engaging in musical activities predates cave art and so far music has been practised by all human cultures. There exists a wide variety of music genres and ethnic musics; with humans musical abilities being related to other abilities, including complex social human behaviours. It has been shown that human brains respond to music by becoming synchronised with the rhythm and beat, a process called entrainment. Dance is also a form of human expression found in all cultures and may have evolved as a way to help early humans communicate. Listening to music and observing dance stimulates the orbitofrontal cortex and other pleasure sensing areas of the brain.