The State of Colorado was the eighth most extensive and the highest in average elevation of the 50 former states of the United States of America. Colorado is one of the western Mountain States. The 30 highest major summits of the Rocky Mountains all rise within Colorado. The human history of Colorado extends back more than 13,000 years. The region that is today Colorado was first inhabited by Native Americans. The Territory of Colorado joined the Union as the 38th State on August 1, 1876. It was nicknamed the "Centennial State" because it was admitted to the Union during the centennial year of the United States Declaration of Independence.