The invention of the human mental self would extend back to our protohuman ancestors. Chimps have individual characters, too. Their mentality is largely locked in their skull because of their inability to express it with complex languages and with an opposable thumb.

So the invention of human mentality could have taken hundreds of thousands of years, as it happened mostly internally, like a sealed hose with only a few pinpricks in its side to let the water out slowly (via rough communication and the accumulation of knowledge). Cultural evidence of behavioural modernity within the last 100,000 years represents the unsealing of the hose, as circumstances evidently clicked to release the pent-up mentality that would have been bubbling up internally for a long time.

The timing of these things, though, isn't the crucial point for me. What matters is whether the human self should be construed as the brain's main tool.

Certainly, other species have some tools, and our species may even have developed similar rudimentary tools before the advent of symbolic thinking. Evidently, clever animals can learn to shape their environment, to some extent, and can even adapt themselves to rely on external phenotypes. This would happen more by trial and error and by synergy with the genes and natural selection.

That kind of tool use differs dramatically from the elaborate, systematic kind that was made possible in our species by the development of our mentality. What I'm saying here is mainly that that inner development might be construed as yet more ingenuity and tool-use. The brain used its thoughts and feelings as ethereal tools, as software to program its body's behaviour.

And I'm saying that no one is aware of the brain, the true inner self, unless you're a neurologist who dissects a skull and puzzles out what all the neurons do. I know you think our true self is Consciousness. I'm just assuming naturalism here, in which case the brain is our true, approximate self. This self is hidden within our skull, and over a long period in prehistory, it devised minds and cultures, which we can construe as tools that therefore have functions (purposes).