For most of our evolutionary history, we did indeed live in Africa - but not just the eastern savannahs, as previously thought: our biological ancestors were distributed everywhere from Morocco to the Cape. Some of these populations remained isolated from each other for tens or even hundreds of thousands of years, cut off from their nearest relatives by deserts and rainforests. Strong regional traits developed. The result probably would have struck a modern observer as something more akin to a world inhabited by hobbits, giants and elves than anything we have direct experience of today.
"The Dawn of Everything", David Graeber and David Wengrow
What environments might cause a larger variety of human, on the scale of 8 to 10, or possibly even 12 feet tall, to evolve?
I presume that humans haven't evolved to average heights of more than about 6 feet because A) humans can adapt to different climatic conditions by developing new tools and strategies, rather than through physical adaptations like growing very large or very hairy, or B) simply because enough time hasn't elapsed for Homo sapiens to evolve that way. That being said, I know that some populations have adaptations for high-altitude life, and that Inuit have genetic mutations which make them more adapted to cold and a high-fat diet, which same mutations also affect height. Nevertheless, aside from some outliers, humans on average don't get much bigger than about 6 feet. How could sizes beyond this evolve?
Could very large, stocky humans evolve in a cold climate, yeti-style? Or would adaptation to a cold climate be more likely to have the opposite effect?
Or, would large size be more likely to evolve in hotter climates? Could a population of humans populating a hot savannah evolve to 8-10 foot height, in a similar way to how African megafauna evolved great size? Perhaps humans could attain great sizes in hot, humid environments where biodiversity is high and food is plentiful? I believe I read that average human height has increased in the past century because of better nutrition in the modernized world--but for the sake of this question, let's assume our hypothetical megahumans don't live in this kind of world--rather, these populations of larger humans may spend more time on the move, settling in one place for a few months out of the year at most.