The Hopewell Culture that flourished in parts of the American Midwest rose around 200 AD, but by 400 AD, most of the long-distance trade and interaction of the Hopewell people had ceased (it was not clear what they called themselves). This period of decline lasted from 400 to 500 AD according to the Biblical Timeline Chart with World History. With it came the decrease of mound building (of which they were known for) as well as rituals and other art forms.
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The decline of the Hopewell trade was attributed to several reasons including war, climate change, and a shift in hunting technology. The use of atlatl or spear-thrower, widely used before by the Hopewell people and other early Americans, disappeared in the area after the invention of the bow and arrow. The use of the bow and arrow improved hunting and increased their food supply, but its use also increased the instances of warfare. Perhaps the efficiency of the bow and arrow in warfare forced the people to move from the villages to more secluded and larger settlements in the highlands and interior valleys. They protected their new settlements with defensive fortifications (such as walls and ditches). The abrupt decline of the Hopewell culture and trade can also be attributed to climate change which resulted in a drastic decrease in their food sources.
http://www.cabrillo.edu/~crsmith/hopewell.html
Fiedel, Stuart J. Prehistory of the Americas. Cambridge: Cambridge University Press, 1987
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