The rapid spread of numic peoples into the Great Basin about 500-700 years ago is a major anomaly in the prehistory of that region because, according to current interpretations, it occurred in the absence of major adaptive change. A review of existing evidence suggests that this view is incorrect: we propose on alternative notion of important contrasts between prenumic and Numic adaptation in terms of the relative reliance on large game and small seeds. These contrasts explain why the numic speakers were consistently able to expand at the expense of prenumic groups.