An interjection starts the sentence right

Fair warning: half-baked hypotheses and uninformed drivel are sure to follow. I'm writing it anyway, because I can.

I was thinking once about interjections, about the role they serve in language. The interesting thing about interjections is that they have sort of an atomic meaning: if I say "ow," you know I've been hurt. You know that just from one little inarticulate utterance, without my having to get multiple words and syntax involved. The word has its own complete meaning, without the need for nouns, verbs, adverbs, or what have you.

For the most part, other parts of speech can't be used that way, to themselves convey all the meaning you need. Other parts of speech mostly depend on other words and syntax to get the meaning across. But not interjections.

So why does that interest me? Well, communication interests me. In particular, human language. So far as we know, humans are the only creatures on the planet that use syntax in their communication.

But that isn't to say that other creatures don't communicate. Far from it. They just don't use syntax in their communication.

So how do the other creatures communicate? Lots of ways, including chemical, physiological, and behavioral, but I'm interested mostly in vocal here. How do other creatures communicate vocally without syntax? Well, they make sounds that others of their species will understand to have a particular meaning. So they make a particular sound, and the sound has a complete meaning, by itself and without the need for syntax: "I see a predator, we should run away," or "I'm upset," or "I've been hurt," or whatnot.

And if you asked me, what's the closest equivalent to those sounds in human language, then I'd say it's the interjection. In fact, I'd go so far as to say that the usage is exactly the same.

Now let's assume that humans developed over time from other creatures. (I do so unapologetically.) This means that, at some point, we had ancestors that didn't use syntax in their vocal cummunication. Their decendants must have added syntax, since we have it now. How did they do that? We don't know exactly, of course, but the reasonable path for that to happen is over top of what existed previously. Nouns and verbs and adjectives got added on to what they had, which was the inarticulate utterances with their own complete meaning, the interjections.

I would conclude that, in human language, interjections are very likely a sort of vestigial remanant of pre-syntactic speech.