How does anyone know what prehistoric people universally believed? Have they interviewed them? From the wikipedia flat-earth link above we read:
The belief that the Earth was flat was almost universal until about the 4th century BC, when the Ancient Greek scientists and philosophers proposed the idea…
Maybe they did, but there’s no way to know without universal interviews! Universal! What epistemic arrogance. The people that built the pyramids, and those astronomers of ancient meso-america seem to have had an excellent grasp of the heavens. They didn’t seem to be using clumsy flat-earth geometry. Why would one assume they thought the earth was flat? Because years later, Herodotus wrote down that they did? Because some drawings in holy sites in India show a flat earth atop a stack of turtles? As if, in the past, everyone has always believed what they teach in sunday school, and no one thought of it as a metaphor, or a social thing, and only now are we sophisticated enough to tell the difference.
FYI, the earth is, indeed, resting on a turtle. You can’t see it, because it’s in its stealth/invisi-sheild shell. When it shifts, that’s what causes the northern lights.