Steps for sentience

That's assuming cavemen existed.

I like this, thank-you for starting this discussion.

Of course I have not an answer, I wonder who may have that, but it is intriguing to sketch some hypothesis.

First problem, what is sentience?
I fear to get tangled into some definition. It is one of those subjects supposed to be sufficiently clear or "self-evident", only that later it comes out that nobody really agree on what is actually so self-evident.
At most, and maybe some day someone should do that, we might agree on some (minimal) check-list that would divide what is "sentient' from what is not. Nevertheless, this would be a sort of AI approach to the problem and probably we would miss that insightful dimension maybe necessary to discuss this issue.
Well done that you gave 2 questions as metaphorical begin and end of the journey, this makes things easier.
Btw, as you put your questions, I guess that by “sentience” you mean what most people would call (self-)consciousness, and I shall refer to that meaning rather than to the mere capability to experience sensations.

Assuming that the 1st question was not actually expressed using a language and assuming that cavemen have indeed existed, I believe that our "great ancestors" (and the great-great ancestors) had always been worried about being eaten.
This fear is probably the lowest degree of self-consciousness and it seems to me quite possible that our species has that in common with about all living beings, also on a unicellular level.

What I quite like in your question is that I gather that you assume that consciousness has evolved with the species. I agree with that.
So how this very initial consciousness required for self-defence has become that over -expanded and suffocating subject that we call “ I “ ?
As I said, I have not a definite answer. Anyway, my view is that consciousness is more historical than physiological, it's software and not hardware.
My hypothesis – to be honest, it is not really mine - is that the forces that have driven to building the self-conscious subject that we know – that we “are” - have been language and society. One may say society tout court, as it is doubtful that there would be a language with no society at all.
By society I mean a group of people that acknowledge and practice a common economy and law.
Being accountable, stipulate contracts, the need of a reputation in business and politics, all this made for the need of a “point of presence” in men available 24/7, available to everybody – including ourselves.
If we consider this shift to the subject as a process function of the economic (and normative) activity, that would also explain how come the subject (“ I “) has become so hyper-trophic in our times.

Societies with different economies and laws end up with different notions about the subject. Take Castaneda's Don Juan as an example.

Adopting this view implies that, finally, the question “why am I here” would receive an answer like “'I' am here because 'they' need me to be here (but I am not really ' I ' and, however, I am not sure about what ' I ' is).”
At the same time, the Yaquis (Don Juan's ethnic group) would not ask such questions, or would come with different answers, derived/influenced by the fact that their economy (and laws) are linked to a very different system of production, where a sort of intimate knowledge of the natural environment is more required and valuable. Therefore a member of such a group should be capable (occasionally) not to be himself, and to become somebody else: the game, the sky, the land, the trees, the eagle, the wolf...
(Sounds a bit Marxist, but it is the quickest and clearest way I found to express my point of view).

Of course, there is also a competing theory stating that conscience is exactly alike for all men in all times, and hybrid theories like Hegel's – where there is only one road to one conscience (“ein Volk, ein Ja”).

cavemen in the light, perhaps? well, Julian Jaynes thought consciousness developed through hearing voices that told you what to do (that were actually coming from your other hemisphere)

When an Atom asks, who am I, gravity is the "news" bringer...

"Sentience" is a matter of layers and degrees .