I'm trying to put together an aesthetically pleasing thought experiment / narrative, and am struggling to come up with a way of framing it that won't attract nitpickers.

In a nutshell, the premise is "what similarities and differences are there between real-world human history and culture, and those of a different human history and culture that diverged from ours at some prehistoric point but developed to a similar level of cultural and technological sophistication?"

As such, I need some semi-plausible way for the human population to be physically divided ~10,000 years ago with no cross-cultural contamination, and for both sides of the divide to each develop into a "global" culture, with one being very much like ours, and speculation of the nature of the other being the point of the thought experiment.

Current contenders are:

Aliens build a really huge impenetrable wall round the equator

Nitpicks are "what about air and space travel?" I could set the narrative in the early 20th century, when we're only just developing means of circumventing the wall, which also frames the "what is the world like on the other side of the wall?" speculation. The trouble is that a lot of interesting cultural, scientific and technological developments have happened in the past century, and it's hard to speculate if they've also occurred on the other side of the wall if they haven't occurred on this one. It also smacks a little too much of alternative histories, with it being a tremendous strain on credulity to claim "this side" of the wall developed in line with real-world history in spite of a hemisphere being missing.

Aliens (who clearly have nothing better to do than fuck around with prehistoric humans) take a bunch of humans and put them on a Counter-Earth

Of course, they have to then replicate the biosphere of earth, and somehow retro-engineer the existence of fossil fuels, and populate it with a bio-diverse ecology of other earth life for the humans to eat, and really they may as well have started a several hundred million years earlier, by which point whatever life is on the Counter-Earth is just going to be classical aliens, rather than human beings, because transplanting them into an ecological niche they're not optimised for is just asking for trouble.

So yes, a separate 10,000 years of human history, completely unrelated to ours, that we haven't interacted with until now. How do I frame it in a narrative that won't let people pick it to pieces without addressing what it's trying to get them to think about?