The hypothesis that the great serpent mound may relate to halleys comet is bolstered by

People depended on the Tigris and the Euphrates rivers. Irrigation transformed the original hunter-gatherers into small farming communities. - Metallurgy and began to replace stone and bone tools and weapons with metal, thus marking the end of the Stone Age and the beginning of the Bronze Age. It required the mining of ores, specialized technical training, and skilled artisans.