Proto-signals
A basic question in the evolutionary theory of animal communication is: Which came first, the sending or the receiving?
On the one hand, if no one is sending a signal, then who is going to know that they want to receive it?
On the other hand, if no one is receiving the signal, then why bother sending it?
Given that many different forms of animal communication have evolved, both inter-specific and intra-specific, there must be some general answer to this question.
For any given type of signal that might be sent or received, there must have been some first occasion on which the signal was produced.
Also, prior to that first occasion, there was no reason for any species (whether it's the same species as the sender or not) to evolve an ability to receive that signal.
So the very first signal had to be sent, even though there was no a priori system in place for receiving and interpreting it.
These questions lead to the concept of the proto-signal, which is a signal which is sent, for some reason, and which the receivers successfully receive and interpret, without having evolved the ability or tendency to receive the signal based on a context where that signal is sent. (Reference: see Evolution of signals)
In practice animals can learn to interpret a signal that has been sent, so it is not necessary for there to be any prior evolution of the ability to receive and interpret that particular signal.
A system of sending and receiving information only really counts as communication if there is some benefit to both sender and receiver.
In the proto-signal scenario, the sender initially produces the signal for some reason un-related the reception of that signal by the receiver.
We can consider scenarios where the reception of such a proto-signal benefits only the sender, or only the receiver, or both.
If it benefits only the sender, then there is no reason for the receiver to make any effort to interpret the signal or respond to it in anyway, so it will not become a system of communication.
If the reception of the proto-signal benefits the receiver, at the expense of the sender, then there will be evolutionary pressure for the sender not to send the proto-signal.
And the end result of that will not be a system of communication.
It is only when the reception and interpretation of the signal benefits both the sender and the receiver that the proto-signal will evolve into a full "signal" that is part of a system of communication.
Proto-musical Pre-language
According to my Revelation Hypothesis, Proto-Music, the ancestor of music, was a pre-language, which expressed the occurrence of revelation of information to the speaker of that proto-musical pre-language.
This pre-language was a form of communication, because it communicated to listeners that the "speaker" had information revealed to them, ie they saw something, or heard something, or realised something, and this information had emotional significance.
So if the pre-language existed as a form of communication, then, following the argument given above, that pre-language must have evolved from some corresponding proto-signal.
So, investigating further, we can ask, what did the proto-signal for the revelatory pre-language sound like?
How and why would an individual utter such a proto-signal?
Revelatory Interjections
Such a proto-signal would have been similar in form and function to what we would consider an interjection. Interjections can be regarded as special words or sounds that have self-contained meanings. That is, they are not used as components used to construct a full utterance, rather they are the whole utterance. When you've said the interjection, you have said the thing that you were going to say.
There are indeed interjections which are de facto statements of revelation. That is, I utter one of these interjections in response to perceiving or discovering or realising something. And if someone else hears me utter one those interjections, then that person will reasonably suppose that I just perceived or discovered or realised something.
Different relevatory interjections can have specific emotional colouration attached to them.
For example, a sad "oh ooooh". Or an excited "wow!". Or a now-the-mystery-is-solved "ooooooooooh".
Repetition followed by change
Despite the existence of revelatory interjections, both as part of and existing alongside modern human word-based spoken languages, there is reason to believe that the original proto-signal for proto-musical pre-language may have been something quite different.
One thing we can observe about music is that often the most emotional point in a musical item occurs when there is a change, and often this change occurs within the context of a prior repetition.
Or there may be a musical phrase, which repeats two or three times, but the last time it occurs, the start of that last occurrence matches the previous repetitions, but a change occurs within the final part of that last occurrence.
(This pattern of repeating the beginning of a previous component but introducing change towards the end is also observed with song lyrics.)
This suggests an alternative scenario for a proto-signal, which is a repeating vocalisation which changes at the moment that a revelation occurs.
Given that revelation is very often something that happens when you are not particularly expecting it, the implication is that the individual is generating repeated vocalisation of some sort, for some reason unrelated to any possible revelation, and when the revelation happens, the repeated vocalistion continues, but with a change that is caused by the change in state of mind caused by said revelation.
On the one hand this alternative scenario presents a more expensive proto-signal, because it requires continous vocalisation, whether or not a revelation is expected to occur, whereas a revelatory interjection that only happens when the revelation occurs necessarily requires less effort (and no effort at all if no revelation occurs). Not to mention wear and tear of the vocal apparatus.
Nevertheless this scenario of repetition and change is a better match for the manner in which emotion is observed to be expressed within musical items.
So, given the hypothesis that this pre-language expressing the occurrence of revelation was the ancestor of modern human music, I am inclined to believe that it may indeed be the correct scenario.
Final Note: Repetition and the Glial Illusion Hypothesis
I have previously surmised that the intensity of musical emotion is determined by a Glial Illusion, where the glial illusion is caused by the exact repetition of individual aspects of musical sound.
This hypothesis about the role that repetition plays in the perception of music is quite different to the role proposed in the above hypothesis about a proto-signal for pre-language.
So in effect I am proposing that repetition in music plays two quite distinct roles in the perception of musical emotion.
In the case of the proto-signal, the perception of repitition is neuronal, not glial (ie it is just normal "perception"). Also the effect does not have any strong dependence on the exactness of the repetition. The hypothetical hominid ancestor uttering the proto-signal form of the pre-language engaged in endless repetitive vocalisation for no particular reason, but there was not any musical precision in their repetition. (Whereas, for the glial illusion, the required level of precision plays a critical role in the intensification caused by that illusion.)