From The Comanche Empire, by Pekka Hämäläinen (Yale U. Press, 2008), pp. 22-23:
Shoshones had created a flourishing and eclectic culture that belies the traditional image of the impoverished existence of Basin peoples; and yet over the course of the sixteenth century, they abandoned the Basin for the Great Plains. This migration was apparently triggered by a climate change, the beginning of the Little Ice Age, which ended the long dry spell and brought colder temperatures and higher rainfall. As steady rains once again nourished the grasslands, allowing the ailing bison herds to recover, humans began to move back, first in trickles, then in masses. What followed was one of the greatest migrations in the history of North America. As if pulled into a vacuum, people flooded in from the Rocky Mountains, northern woodlands, and the Mississippi valley, turning the plains into an agglomeration of migration trails. This human tide consisted mainly of groups that had lived on the plains before the great drought, but some of the immigrants were newcomers. Among these newcomers were the Shoshones.
Building on their century-old tradition of seasonal transmontane migration, more and more Shoshones filtered through the South Pass onto grasslands in the early seventeenth century, elbowing the Kiowas and other nations eastward to the Black Hills region. By midcentury a distinct branch of Plains Shoshones had emerged. Occupying the northwestern plains between the South Platt and upper Yellowstone rivers, these eastern Shoshones morphed into typical plains hunters who shaped their diet, economy, and culture around the habits of bison. They lived as nomads, following their migrant prey on foot, moving their belongings on small dog travois, and sheltering themselves with light, easily transportable skin tipis. In hunting bison, they alternatively surrounded the animals, ran them onto soft ice or deep snow, or drove them off steep precipices. These communal hunts absorbed a lot of time and energy and required careful planning, but astounding returns rewarded the efforts. The Vore site, a precontact buffalo jump near the Black Hills, contains partial remains of ten thousand bison, even though people used the site only once every twenty-five years or so. Hundreds of similar, if smaller, sites in the Shoshone range testify to a burgeoning economy and a flourishing way of life.
But prosperity did not translate into stability. Sometime in the late seventeenth century, the Shoshones suddenly splintered into two factions and left the central plains. Possibly seduced by larger and denser bison populations above the Yellowstone valley, the bulk of the people migrated onto the northern plains, where they were dragged into prolonged wars with the southward moving Blackfeet and Gros Ventre—wars that were still raging on when the first Canadian fur traders entered the northern plains in the 1730s. A smaller faction headed south and disappeared from archaeological record for several years. They reemerged in the early eighteenth century in Spanish records as Comanches, one of the many Native groups living along New Mexico’s borderlands.