The history of New Mexico is based on archaeological evidence, attesting to the varying cultures of humans occupying the area of New Mexico since approximately 9200 BCE, and written records. The earliest peoples had migrated from northern areas of North America after leaving Siberia via the Bering Land Bridge. Artifacts and architecture demonstrate ancient complex cultures in this region. The first written records of the region were made by the Spanish conquistadors, who encountered Native American Pueblos when they explored the area in the 16th century. Since that time, the Spanish Empire, Mexico, and the United States (since 1787) have claimed control of the area. The area was governed as New Mexico Territory until 1912, when it was admitted as a state. The relatively isolated state had an economy dependent on mining. Its residents and government suffered from a reputation for corruption and extreme traditionalism. New Mexico introduced the Atomic Age in 1945, as the first nuclear weapons were developed by the federal government in the research center it established at Los Alamos. Ethnically, New Mexico's population consists of a mixture of Native Americans, Hispanics, and Anglos. Human occupation of New Mexico stretches back at least 11,000 years to the hunter-gatherer Clovis culture. They left evidence of their campsites and stone tools. After the invention of agriculture, the land was inhabited by the Ancestral Puebloans, who built houses out of stone or adobe bricks. They experienced a Golden Age around AD 1000, but climate change led to migration and cultural evolution. From those people arose the historic Pueblo peoples who lived primarily along the few major rivers. The most important rivers are the Rio Grande, the Pecos, the Canadian, the San Juan, and the Gila. The Pueblo people built a flourishing sedentary culture in the 13th century CE, constructing small towns in the valley of the Rio Grande and pueblos nearby. By about 700 to 900 CE, the Pueblo people began to abandon ancient pit houses dug in cliffs and build apartment-like structures with rectangular rooms.