Imo to have a notion (a concept, if you want to use that term instead) of an external, would already require some mental work (obviously also - or even mostly or entirely- of the non-conscious type). But if one goes by (which can be tricky, but at least is based on experience, something rare in mental examinations) the progressions in the earliest of childhood years (including pre-language ones), then it seems rather evident that at some point some mental creations/concepts rise without being that stable to begin with. For example, i do recall that many concepts i had (including about the external world) took time to 'settle in', which would mean that they aren't already pre-made and then used as-is but have to rise out of more fundamental or innate abilities.
Now that is just childhood. Prehistory is something we can't know empirically in this way. Yet one has to assume than humans/hominids didn't just start having a language at some point. It likely was a long and chaotic progression. This doesn't lend credence to the idea that a number of concepts which are used typically today were always there, which in turn (their non-being there) would seem to allude to a likely fleeting nature of those concepts both in the way they appeared and their continuous re-emergence in humans as they start (as children) to identify the world as something external.