A friend of mine taught young children in Africa for a while. If she tried to teach them a tune, they harmonised spontaneously, and found it impossible to sing in unison. They were so used to hearing harmonies in their traditional music that the ides of a single-line melody was foreign to them.

This does not prove that harmony is a part of every musical tradition in the world , but it does suggest that it arises naturally and spontaneously when singing is part of a communal culture.

This contrasts with the view that somehow the common people of Europe wouldn't have used harmony until it was developed in church music or noted down by the musically literate. Church music may at times lead the trend in certain musical fashions and developments, but in other ways the Church has, over the centuries, imposed strict controls and limitations on the ways in which music has been used for religious purposes. It's unlikely, I'd have thought, for the whole range of musical expression in a given culture to be reflected in the church music of the time - I don't suppose medieval peasants working in the fields used to chant plainsong as they worked.