Wow, I'd never heard that the glaciation period had more unstable climates than the interglacial we live in now. I'd have guessed the opposite since interglacials are short compared to glaciations; it isn't clear to me why they'd have more stable climates than during glaciations. If I read the charts posted a page back correctly I suppose the point is made. That certainly would seem to have a bearing on why no precocious civilization has been found before the continental glaciers started their retreat.
There's a recent ASB thread I've been participating in that asks what if our species had somehow arisen in the Americas instead of Africa; there I've been pretty foursquare for the idea that no one founds any civs until after the glacial retreat is largely accomplished and the sea levels have risen to near current levels.
The idea that the last glaciation was simply too cold or dry or whatever does not fly with me; for everyplace that we consider hospitable today that was under ice or tundra or taiga too cold to be an early civilization center I figure there would be someplace else that was more optimal for it then than today. But if those reasonable cradles were constantly being shifted around I can see why that would discourage the necessary chain of developments that could lead to proper civilizations, I guess. So that does explain a lot.
Put me down for the camp that says that if a proper, full civilization could have developed during the last glaciation somewhere, it would leave clear traces today--we'd inherit probably not one but several agricultural plants from that era; the civ would have spread out so that even if extinguished in its heartland some colony somewhere would expand on newly-hospitable land; they'd have done some earthworks that would leave traces; bottom line, even if the descendants of this high age fell on hard times, humanity would have hit the early interglacial era running and we'd have the first great cities on lands they could exist in today some five thousands of years before OTL early civilization dates.
We don't find traces; my belief is that that is because the hypothetical pre-interglacial civ never existed.
Also the way I understand things, our species is not so old we could have been around during the last interglacial; the relevant time scale is therefore under 100,000 years before present.
I do think language formation was a "trigger" event but that it happened about 100,000 years ago in Africa, and what it triggered was the spread of ours species through that continent and then past the Red Sea north and south of it (via Yemen) into Eurasia and beyond. I'm on the fence as to how early the very first colonization of the Americas took place after that, but it would seem clear enough that the vast majority of Native American ancestry came in after the ice barriers began to die back, just in the last 12,000 years.
Neanderthals, I've been given to understand, had brains just as big as ours, but with a different layout--one that in particular does not make room for development of key brain centers our brains use to process speech, understanding it and uttering it.
It might be possible that instead their brains enabled language by another path, using different deep grammar.
But I'm partial to S. Mithin's "Singing Neanderthal" hypothesis, that language in our species ultimately derives from song which derives from dance, whereas Neanderthals would be quite good with music--perhaps commincating far more than we can imagine.
Nevertheless despite their quite-large brains I don't think Neanderthals or other pre-H.sapiens sapiens hominid cousins could found civilizations, though I do think they might participate in one founded by members of our species (and presumably contribute unique skills to it.