About 2,000 years ago, a new prehistoric people known as the Late Hunters appeared in Montana, thriving on a bison (buffalo) population living in open grassy areas on the plains and in river valleys. The earliest tribes are thought to have been the Kootenai, who stayed west of the Continental Divide, and the Flathead (Salish), and Pend d'Oreilles, who ventured east of the mountains into and east of the Three Forks country, southeast of Basin. In the 17th century, the Crow entered Montana from the east and the Shoshone from the south. Pressed by other tribes retreating west from white European settlers, the Blackfeet moved into Montana around 1730. Acquiring horses and firearms, and numbering about 15,000, they formed alliances with other incoming tribes, the Assiniboine and the Gros Ventres, and by the mid-18th century dominated the state. When the white explorers Lewis and Clark traveled up the Missouri River to Three Forks, they found only Blackfeet and Blackfeet allies. Heavily dependent on bison, the nomadic life of the Blackfeet "came to an abrupt end in the early 1880s when the buffalo became almost extinct."