A declarative is an utterance about something that is detached from its physical presence. More specifically, it is the prolongation of an interrogative until a possible or hypothetical new ostensive is presented.
See first - Interrogative, Negative Ostensive
After the origin of the interrogative, the recipient of what is now a question faces a new problem of maintaining linguistic presence. Because the "stakes" of the situation have been lowered and it's clear that physical confrontation is no longer imminent, he now has the opportunity to "inform" the interlocator that that the requested object is not available. But he cannot simply "want" to "offer information," because the speech form in which such a desire could be formulated is precisely what is about to be invented. The name of the object (which will become the "subject") is repeated, and conjoined with the operator of negation (which will be the "comment" or predicate), a more open-ended imperative forbidding some action. Our proto-declarative is the negative ostensive: the repetition of the object demanded, along with the operator of negation, something like "Don't..."
Intuitively, this proto-declarative would be taken as a command addressed to the other to refrain from persisting in the request. The declarative is only born when this is counter-intuitively taken as a command issued towards the object, demanding it to refrain from making itself present. This command, while uttered by the target of the request, could not originate with him - after all, if he was able to order the object off the scene, he presumably could have supplied it in the first place. Thus, this command is assumed to have come from the central object, or the sacred Being, establishing a "reality" immune to the desire of both interlocutors.
While the recipient of the command (interrogative) may be repeating the ostensive-imperative pair as an act of deferral (to maintain linguistic presence), to the other, who has now "asked a question," the order given to the object would provide an "answer." The declarative has been born, and only makes sense in terms of the question(s) we take it to be answering. An "answer" is the prolongation of the question until a possible or hypothetical new ostensive is presented, which places both interlocutors under authority of an imperative from the center including them in a space with the world of new things.
The declarative can commonly be distinguished by "talking about" something, where it's not "present" in that there's no imperatives currently being issued or obeyed as it pertains to the ostensive in question.
Gans, E. L., Katz, A. L. (2019). The Origin of Language: A New Edition
Katz, A. (2020). Anthropomorphics.