This dissertation is a multidisciplinary study combining both archaeological and paleoecological data to examine the rise of early Formative societies in Uruguay, La Plata Basin.

It is contextualized within broader anthropological concerns related to the emergence of cultural complexity, the significance of ritual and public architecture in intermediate-level societies, and the role of human-environment interactions during the mid-Holocene. This investigation generated the first Late Quaternary paleoclimatic record, based on pollen and phytolith analyses, documenting that the mid-Holocene (ca. 6,620 to ca. 4,040 bp) was a period of environmental flux and increased aridity. It describes the occupational history of the Los Ajos site from the creation of a household-based community integrating a centralized communal space during the Preceramic Mound Component (ca. 4,190 – 3,000- 2,500 bp) to the Ceramic Mound Component (ca. 3,000 2,500 bp to the Contact Period), where Los Ajos acquired a strong public ritual character through the formatilization and spatial segregation of its mounded architecture. During the Ceramic Mound Period, the site exhibited both internal stratification (inner versus outer precincts) and dual asymmetrical architecture in its central sector, which suggest the emergence of incipient social differentiation. This study also marks the earliest occurrence of at least two domesticated crops in the region: corn (Zea mays) and squash (Cucurbita spp.), showing that the early Formative societies adopted a mixed economy shortly after 4,190 bp. Collectively, these results challenge the long-standing view that the La Plata Basin was a marginal area by evidencing an early and idiosyncratic emergence of social complexity never before registered in this region of South America.