How could early group singing produce forms of polyphony?

Early group singing likely evolved into polyphony through several spontaneous and practical methods during the early Middle Ages. Because the origins of this transition from monophony are not fully documented, historians suggest several plausible theories for how these forms emerged:1

• Natural Vocal Ranges: Singers with different natural vocal ranges may have simply sung a melody at their most comfortable pitch levels, inadvertently creating simultaneous melodic lines.1
• Improvisational Doubling: It is believed that singers spontaneously added a second voice, known as the vox organalis, to a principal melody (vox principalis). This practice, known as organum, often involved doubling the chant at the interval of a fourth or fifth.21
• Instrumental Influence: The practice may have been adopted by singers from organists, or it could have arisen when a melody was repeated at a different pitch level simultaneously with the original statement.1

While early attempts likely relied on parallel motion, these techniques eventually evolved into more complex forms involving contrary movement and rhythmic independence.1