As I understand it, during the most recent ice age, there were small populations living in the so-called refugia. I don't think there is good evidence yet from northern/high altitude Asia during the most recent ice age, so maybe there were significant populations living in heavily glaciated areas.
My general understanding is that tool using hominids have migrated into Northern Europe during warmer spells for over a million years, but have either died out or migrated back to more southerly regions when the ice ages have lowered temperatures.
So, I'm not really definitely answering your question, more asserting that I don't think that humans - until the last 10k years or so - have had the right tool kit to survive in very cold regions. Perhaps the Neanderthals or Denisovans did. Or, you know, I could just be generally wrong.
I agree that our early ancestors likely sequestered into geographic pockets where they had some refuge from glaciated areas, but it's also plausible that others adapted to harsher conditions to survive. Some archeologists suggest that glacial periods may have forced early humans to develop tools to support survival such as bone needles to make warm clothing and spears to hunt animals which may have been a primary food source when nothing else was available.
Some make a powerful argument that early man was primarily carnivorous, and only resorted to plant food in times when animal food was scarce, as a poor second choice.
"Early man" spans millions of years and a vast geographic region with shifting climate. I agree that some were very likely primarily carnivorous out of necessity (during glacial periods) or even preferentially when meat was available. My guess is that there were also periods when meat wasn't
available and other foods (berries, honey, grasses, leaves, bark, insects, etc.) were eaten. Here's an interesting article
that discusses how climate may have altered dietary options and shifted human evolution.
The one thing that came out of the Circ recommended podcast is none of the groups ate ultra processed foods . This paper suggests "acellular carbohydrates promote an inflammatory microbiota" and "Due to being made up of cells, virtually all “ancestral foods” have markedly lower carbohydrate densities than flour- and sugar-containing foods, a property quite independent of glycemic index. Thus the “forgotten organ” of the gastrointestinal microbiota is a prime candidate to be influenced by evolutionarily unprecedented postprandial luminal carbohydrate concentrations."
Also, in the Circ podcast, it was noted that most of the hunter/gatherers don't eat all the time (nod to Satchin Panda)
YES, that was my biggest take away as well. Modern hunter gatherers have extraordinarily diverse diets but none eat processed food and they all eat in a time restricted window. In terms of diet, this may well be one of our most important lessons in terms of overcoming our genetic mismatch issue.