Several Native American groups originally inhabited the region now known as Wyoming. The Crow, Arapaho, Lakota, and Shoshone were but a few of the original inhabitants white explorers encountered when they first visited the region. What is now southwestern Wyoming became a part of the Spanish Empire and later Mexican territory of Alta California, until it was ceded to the United States in 1848 at the end of the Mexican-American War. French-Canadian trappers from Québec and Montréal went into the state in the late 18th century, leaving French toponyms such as Téton and La Ramie. John Colter, a member of the Lewis and Clark Expedition, itself guided by French Canadian Toussaint Charbonneau and his young Shoshone wife, Sacagawea, first described the region in 1807. At the time, his reports of the Yellowstone area were considered to be fictional. Robert Stuart and a party of five men returning from Astoria discovered South Pass in 1812. The Oregon Trail later followed that route. In 1850, Jim Bridger located what is now known as Bridger Pass, which the Union Pacific Railroad used in 1868—as did Interstate 80, 90 years later. Bridger also explored Yellowstone and filed reports on the region that, like those of Colter, were largely regarded as tall tales at the time.