I toyed with some ideas for Neolithic D&D back in 3.x. (And maybe even in 2e? I don't even remember, lol.) More recently I've been tinkering with a Bronze Age setting idea, which actually brought up some of the same questions and concerns.
I'm going to reiterate what others have said and recommend checking out Planegea as a starting point. It addresses a lot of the things brought up in this thread, and is basically a fully-fantastical D&D world projected back into the Stone Age. It would be trivial to tweak or ignore certain elements to bring it up to a Neolithic level. At any rate, it would serve as a better foundation than standard 5e, IMO.
On the non-fantastical end of the spectrum, there's Born from Ice, which has also been mentioned. I'm not as familiar with it, but if you want no magic or spells, classes based on each Ability Score for a "d20 Pleistocene" feel, then starting with that and building towards what you want seems like a good bet.