Native Americans long lived along the Mississippi and its tributaries. Most were hunter-gatherers or herders, but some, such as the Mound builders, formed prolific agricultural societies. The arrival of Europeans in the 1500s changed the native way of life as first explorers, then settlers, ventured into the basin in increasing numbers. The river served first as barrier – forming borders for New Spain, New France, and the early United States – then as vital transportation artery and communications link. In the 19th century, during the height of Manifest Destiny, the Mississippi and several western tributaries, most notably the Missouri, formed pathways for the western expansion of the United States.