The first Americans arrived thousands of years ago. Scientists believe they crossed a land bridge from Asia to Alaska during the last Ice Age. These nomads were moving from place to place searching for food. Several groups developed great civilizations. In some parts of Mexico and Central America, the Maya created great cities and studied astronomy and mathematics. The Aztec founded a permanent home in Central Mexico. Their city, Tenochtitlán, was the largest in the Americas, and one of the largest in the world. The empire of the Inca developed in the western highlands of South America. They built a large system of roads and bridges to maintain communication throughout their kingdom.
In North America, the Mound Builders, who lived in the area from present-day Pennsylvania to the Mississippi River valley, may have been influenced by the cultures of Mexico. The Hohokam and the Anasazi built complex structures in the desert regions of the Southwest. In the far north, the Inuit people adapted to a cold and frozen environment. Native Americans in the West developed a way of life that used the resources of the forest and the sea. On the Great Plains, the Dakota, Apache, and other tribes continued a nomadic way of life. Many of the groups who lived in the East formed complex political systems to govern their nations. Wherever they lived in North America, the first Americans developed ways of life that were well suited to their environments.