The vast majority of music worldwide uses melodies with discrete pitches that form a set, which we call a scale [1,2]. Humans have theorized about the origin and evolution of scales over millennia [3,4], with recent analyses reaffirming the longstanding emphasis on harmonic origins [5,6]. However, recent decades have seen the reemergence of competing hypotheses of scale origins based on melody rather than harmony [7,8], and the predictions of these competing hypotheses have yet to be directly compared against systematic, cross-cultural data. Therefore, we rigorously compare several influential theories on scale evolution using a set of 1,316 scales, encompassing instrumental, vocal and music-theoretic scales [1,9,10]. We find almost universal support for melodic (interval spacing, motor constraint) theories, which predict scale step sizes in the range of 1-3 semitones. Harmony models are
 needed to explain the prevalence of harmonic intervals (e.g. octaves, fifths), but this varies widely across geographic regions and scale types.  Harmony models perform exceptionally well for the few Silk-Road societies that use music-theoretic scales, which explains the historical fixation with harmony amongst Western scholars. Our findings suggest that the traditional emphasis on harmony is short-sighted, whereas a global view illuminates the primacy of melody in the evolution of scale structure. These results reaffirm the need to overcome entrenched Western biases [11,12], and demonstrates that the solution lies not in traditional aural examination of music [1,2], but in modern computational methods.

John McBride is an interdisciplinary scientist working in both evolutionary biophysics and music evolution and cognition. His primary musical focus is on how biological constraints shape the use of pitch (scales, notes, melodies) across cultures.

Citations:
[1] Savage et al, Statistical universals reveal the structures and functions of human music, PNAS, 2015
[2] Mehr et al, Universality and diversity in human song, Science, 2019
[3] Aristoxenus, Aristoxenou Harmonika Stoicheia: The Harmonics of Aristoxenus, Translation by Henry Macran, Oxford University Press: Oxford, 1902 (original work published ca. 300 C.E.)
[4] Rameau . Book One, Chapter Five.  In: Treatise on harmony. Dover Publications: New York.
[5] Bowling et al, Vocal similarity predicts the relative attraction of musical chords, PNAS, 2017
[6] Marjieh et al., Timbral effects on consonance disentangle psychoacoustic mechanisms and suggest perceptual origins for musical scales, Nat Comm, 2024
[7] Pfordresher & Brown, Vocal mistuning reveals the origin of musical scales,  J Cog Psych, 2017
[8] Tierney et al, The motor origins of human and avian song structure, PNAS, 2011
[9] McBride et al, Convergent evolution in a large cross-cultural database of musical scales, PLOS ONE 2023
[10] Brown et al, Musical scales optimize pitch spacing: A global analysis of traditional vocal music, under review at Humanities and Social Sciences Communications
[11] McDermott et al, Indifference to dissonance in native Amazonians reveals cultural variation in music perception, Nature, 2016
[12] Jacoby et al, Universal and Non-universal Features of Musical Pitch Perception Revealed by Singing, Current Biology, 2019