This essay suggests that the evolution of human cooperation over the course of human history should be viewed as a two-layer process. A foundational layer, rooted in subsistence and ecological pressures, shaped cooperative dispositions unevenly, whereas an expansionary layer, rooted in conflict and stratification, generated large-scale cooperation in societies in which its seeds were formed. The first evolutionary layer unfolded over the grand arc of human evolution, reinforcing the capacity for small-scale cooperation in hunter-gatherer societies while favoring traits complementary to cooperation only in some sedentary societies. The second evolutionary layer emerged as rising population density heightened external threats, fostered coercive centralized authority, and raised the returns to public infrastructure. In environments where cooperative traits had already evolved, warfare, extraction, and infrastructure provision reinforced these predispositions, transforming them into durable collective institutions. Yet in settings where such cultural foundations were absent, large-scale collective action was more challenging, and conflict was often destabilizing, magnifying division and political fragility. Recognizing the profound global heterogeneity in this foundational layer of cooperative behavior is essential for identifying the origins of large-scale cooperation and the conditions under which conflict reinforced cooperative capacity rather than intensifying fragmentation.