In North America the evidence is that hunter-gatherers bounced back quite successfully within less than a millennium of the onset of the Younger Drya… - Graham Hancock

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In North America the evidence is that hunter-gatherers bounced back quite successfully within less than a millennium of the onset of the Younger Dryas, and thereafter there is a thin but fairly continuous archaeological record. What is mysterious is not so much the early appearance of mound-building in this new age — perhaps as early as 8,000 years ago, as we've seen — or the sophistication of sites such as Watson Brake 5,500 years ago, nor even their obvious astronomical and geometrical connections to later vast earthworks such as Moundville and Cahokia, but that in this early monumental architecture of the New World memes of geometry, astronomy, and solar alignments consistently appear that are also found in the early monumental architecture of the Old World at iconic sites such as Stonehenge and the Great Pyramid of Giza. A tremendous leap forward in agricultural know-how, coupled with the sudden uptake of eerily distinctive spiritual ideas concerning the afterlife journey of the soul, also often accompanies the architectural memes. It's therefore hard to avoid the impression that some kind of 'package' is involved here.