I will define a "tone" as a steady periodic sound. As an example, I consider a sinusoidal wave to be a tone. By "tonal processing", I mean ratio relationships between the notes. For instance, if I play 220Hz sine wave then a 440Hz wave will be perceived as an octave higher.
I don't understand why humans have the ability to perceive frequency relationships between tones. I have read several evolutionary anthropology books like The Singing Neanderthal and Finding Our Tongues and, between these two books, they suggest that the obvious evolutionary purpose of music is for communication. Each author takes a slightly different approach but the basic underlying point remains that tonal processing exists because it gives us advantageous communication abilities. But even accepting this hypothesis, it doesn't explain why humans have a tonal processing mechanism that is specific enough to perceive ratio differences between notes. The tonotopic organization of both the basilar membrane and the precortical auditory pathways might partially account for the machinery used for tonal processing and why the advanced forms in music could have developed. But this still leaves us with the question as to why would such information be useful?
I remember reading somewhere one person hypothesized that the purpose of perceiving these differences was to identify "relative pitch", i.e. the ability to identify or re-create a given musical note by comparing it to a reference note and identifying the interval between those two notes. But this hypothesis doesn't really make complete sense to me because it doesn't explain why the brin can have such specificity when identifying the ratio between two notes.
Another phenomena that seems pertinent is the idea of a "drifting reference point" which Glimcher discusses in Foundations of Neuroeconomic Analysis. In Chapter 12, he explains how our perception of color isn't fixed but depends on context and in this sense drifts. IIRC, there is a literature that discusses possible neural mechanisms for this. Perhaps something similar would be relevant here for explaining why humans can process tones so precisely.
What are the accepted arguments for why would ratios between tones would be be useful information for social communication?
Just to clarify, I am expecting the answer to this question to involve a discussion of the physical properties of tones and why a hominid brain would find this useful information for communication purposes.