While there are arguments on either side of the debate, there isn’t a tonne of evidence that speaks directly to it. We don’t have time travel to go back and observe the early development of humanity, and our tools for objectively testing either claim on present-day humans are still too blunt to be of any use. What we can do is compare language to an unambiguously communication-first system: physical gestures. We certainly use gestures to augment language, but we certainly can use language without gestures and gestures without language. When we gesture without language we are certainly communicating but we aren’t using a language.
So, If you’re on board with me so far (language and gesture are distinct systems, and gesture is communicative), we can get some evidence that language is not inherently communicative. If it were communicative, we would expect it to operate like a communicative system (i.e., gesture) in a context where communication is not called for.
The test is as follows: First, be alone (the next time you find yourself alone in a room would be a good time to try it out). Second, don’t be reading, watching TV/Movies, or listening to anything (Isolate yourself from virtual people). Once you’re isolated, say something aloud. It doesn’t matter what, whatever comes naturally. Next, gesture. Point at something, shush the room, make an obscene gesture. But make sure it’s not accompanied with any language. Consider how unnatural one of these tasks felt compared to the other. If you’re like me, one will feel a bit odd, and the other will feel like one of the strangest things you’ve ever done.