You infer it from the existence of 10,000 year old musical instruments. See http://en.wikipedia.org/wiki/Diatonic_scale#Prehistory http://en.wikipedia.org/wiki

You infer it from the existence of 10,000 year old musical instruments. See http://en.wikipedia.org/wiki/Diatonic_scale#Prehistory http://en.wikipedia.org/wiki/Diatonic_scale#Prehistory - which also says that the circle of fifths was described much earlier than I thought.