Consider an utterance of sentence like 'Isabel slept and Lily cried'.
What is the structure of this utterance?
First we can break the sentence down into two sentences and a connective.
Then we break these into words.
And the words break down into phonemes.
Which are eventually realised in continous bodily movements and the sounds these produce.
This is a very simplified picture.
But already we can see that comprehending an utterance will involve several steps.
Consider someone experiencing linguistic communication for the first time.
She is experiences continuous bodily movements and their acoustic consequences.
From these she has somehow to extract the phonetic gestures.
And then she has to group the phonetic gestures into words.
And finally she has to work out the syntactic structure of the words.
It turns out that the abilitues to make these different steps involves largely different
mechanisms, and that the steps can be made independently of each other (not that there aren’t
bottom-up and top-down effects, just that these appear to be inessential in some cases).
Our question can be broken down accordingly.
Our question, How do humans first come to communicate with words?
We can break the process of language comprehension into a series of transitions, from bodily
movements and their acoustic effects to phonemes, etc.
And we can think of language production as involving the same transitions, but in reverse.
And then our question can be broken down accordingly.
We don't have to ask how humans come to have abilities to communicate by language all in one go.
Instead we can ask how humans come to be able to identify phonemes in continous bodily movements and their acoustic effects, and how they come to identify words from uninterrupted sequences of phonemes, and so on.
In this way, our question becomes tractable.