The aboriginal story began thousands of years ago. The ancestors' homeland of nearly six and a half million acres encompassed the Columbia River Plateau in today's southeastern Washington and northeastern Oregon. Their religion has been variously called the Dream, Seven Drums, or Washat, which professes belief in one creator and the resurrection of the spirit after death, as well as the organic unity between people and the earth. The original bands subsisted by fishing, hunting, gathering other foods and concocting medicines. In addition, they took part in trade with other bands that extended from the Pacific coast to Great Plains. The advent of the horse, which Europeans introduced into the Americas at the end of the 15th century, extended the tribes' mobility and range, and improved trade by increasing contact with the region’s other tribes. The size of the herds they husbanded have been estimated at 15,000 to 20,000 animals. The Cayuse bred a small horse called the cayuse. The name eventually became the standard term for Native American horses. At the beginning of the 19th century, the encroachment of such non-Indian outsiders as trappers, missionaries, settlers and U.S. soldiers, changed the land and significantly impacted the tribes' lifeways. Before the beginning of European hegemony and diseases, the Cayuse, Umatilla, and Walla Walla population was believed to be 8,000. By the 21st century, descendants would constitute about one third of that number. In 1855, the tribes and the U.S. Government negotiated a treaty that allowed the latter to legally claim the land and smooth the way for pioneers to settle on it. The tribes relinquished most of their 6.4 million acres in exchange for a reserved territory of 250,000 acres. The three tribes also reserved rights in the treaty that included the prerogative to fish at their customary places and to hunt and gather traditional foods and medicines on ceded lands. The tribes also reserved for ever their rights to pasture livestock and maintain self-government. As an outcome of congressional legislation in the latter 19th century, the 250,000-acre reservation was reduced to its current 172,000 acres.