They probably began by imitating the songs of birds, the howling of coyotes and wolves and the sounds that other animals, like monkey troops and insects, create. it may have started as ways to communicate over distances while hunting and then became more common while the people were in camps. Many early humans occasionally occupied caves, which have natural resonant echos. No doubt that helped inspire people to chant and make other rhythmic sounds. Humans seem to have a natural prediliction for rhythmic sound patterns and resonance -- even babies tend to 'sing" by making variable sound patterns.