I'm talking the last million years, not the last 10,000.

The claim of an unknown, relatively advanced (in comparison to the other, known, stone age tribes) is an extraordinary one. It requires extraordinary proof, not armchair archeology.

The fact is that, as far as we can tell, it took the better part of a million years for mankind to go from the most primitive stone tools to learning how to work the first metals. As hard as it is to imagine our supposedly intelligent ancestors needing those 10s of thousands of generations to make what seems like an incredibly minor leap in technology, those are the facts as we know them.

Postulating that repeated, lost civilizations explain the apparent gap in advancement isn't useful unless you have evidence that supports the idea. Instead we're faced with the evidence that it did take that long to make the leap, so it's more useful to postulate on what the causes for the lag are, rather than wishing them otherwise.

The facts are that we can trace the distinct evolution of stone and bone tools over the last million years. Globally. In populations distinct and entirely separated from each other. No one known tribe gets very far ahead of any of the others in tool types and general forms (though the specifics differ quite radically) at any given point of time. There is, however, rough convergence of parallel evolution in these forms. This suggests that the same issues and problems faced by any one specific tribe are cut from the same general cloth as those faced in other, disparate regions.

This in turn suggests that the rate of human advancement is somehow predictable, given certain base variables. Eg when a people/tribe/culture achieves A, B, and C, then discovery D is all but bound to happen.

We can look at our hominid brothers and sisters on the tree of life for corresponding evidence to this hypothesis. They had cultures and tribes for much more time than we humans have graced the planet and you can see that their tools changed far less than ours did over their time as the dominant intelligent species on the planet. This suggests that they failed to achieve A, B, or perhaps C, which kept them locked out from ever discovering D at all.