This paper proposes that parts of South America experienced a post-glacial climatic and ecological window especially favorable to agriculture and dense human populations. Seasonal moisture, megafauna-maintained landscape openness, and enhanced soil fertility created conditions more productive than those observed today. As megafauna declined, human societies inherited and maintained these landscapes through terracing, raised fields, controlled burning, and canal systems. Once established, such engineered environments could persist despite gradual climatic change, but collapsed rapidly when populations declined, allowing forests to reclaim and obscure prior land use.