It is possible now to discern three periods in this early phase of Shoshone history. The first, the footgoing period, is unknown, and little can be inferred of Shoshone location and movements. The second period is characterized by the acquisition of the horse, and we would conjecture that a good deal of territorial expansion took place after that time. The Comanche differentiated from the main Shoshone group at the beginning of the eighteenth century. Although the Comanche maintained communication with their northern colinguists, the territories of the two groups were not contiguous by the end of the century, and their histories followed separate courses. The extension of the Shoshone into the northern Plains may possibly have predated the acquisition of the horse, for it seems quite likely that they occupied fairly extensive areas east of the Rockies in the footgoing period. But, on the other hand, it seems unlikely that in the period immediately preceding 1700, the most probable date for the acquisition of the horse, they extended from the Arkansas River on the south to the Bow River in Saskatchewan. This would be especially unlikely if later distribution of Shoshone-speakers in the Basin-Plateau was substantially the same in the earlier period also. We would conjecture, then, that equestrian life gave the Shoshone the mobility to extend into the Canadian buffalo grounds but that they were pushed back beyond the divide well before 1800. As Shimkin has noted, the ravages of smallpox and the resulting decline of population probably contributed to their territorial shrinkage (Shimkin, 1939, p. 22). By 1810, the early explorations of Thompson, Lewis and Clark, and others bore fruit in the commercial exploitation of the far Northwest. The fur trade had already reached the Plains and the Rockies in Canada, and a fierce competition was being waged by the Hudson's Bay Fur Company and the North West Company for the patronage of the Indians there. The British interests pushed southward from Canada, and in 1809 David Thompson established North West Company trading posts on Pend Oreille Lake at the mouth of the Clark Fork River and another farther up that stream within the borders of present-day Montana. At the same time, American traders were pushing westward, and Andrew Henry crossed the Continental Divide to locate his trading post on Henry's Fork of the Snake River in 1810. Also, John Jacob Astor's Pacific Fur Company established the ill-fated Astoria post at the mouth of the Columbia River in 1811. Although this particular American enterprise foundered, Manuel Lisa, the guiding spirit of the Missouri Fur Company, penetrated the Missouri River country and founded a post on the Yellowstone River at the mouth of the Big Horn in 1807. From this point his trappers spread into the near-by mountain country. The most famous of these mountain men was John Colter, who trapped the country of the Blackfoot and Crow and discovered Yellowstone Park.