People were assumed to have moved into high environments only after the lower-elevation, more productive environments had been fully settled, and only during specific seasonal windows when the alpine zone offered resources not available elsewhere.

The evidence from high-altitude sites in the Rockies has challenged this assumption repeatedly.

Sites at elevations above 10,000 feet have produced evidence of human occupation extending back several thousand years — earlier than the assumption of marginal and late occupation would predict.

The resources available in the alpine and subalpine zones — particularly the large game animals that use high-altitude terrain, including bison, bighorn sheep, deer, and elk — were real and substantial.

People who knew how to operate at high altitude and who were physically adapted to the physiological demands of low-oxygen environments could access resources that were not available at lower elevations.