The first peoples arrived in North America between 26,000-19,000 years ago during the last Ice Age when lower sea levels exposed a land bridge between Asia and North America. As the climate warmed, sea levels rose and the land bridge was flooded, separating the populations. Various groups such as the Inuit and Mississippian cultures developed across North America, building settlements and growing crops. However, diseases brought by Europeans devastated Native American populations from the 1500s onward, contributing to the decline of groups like the Mississippian civilization. Meanwhile, the Iroquois developed a matrilineal society with a democratic confederacy that may have influenced the US system of government.