The Guaraní expansion in the Upper Uruguay River. Chronology, colonization strategies, social impacts and environmental changes

The Guaraní archaeological record reflects the material culture and associated behaviors of ancient Amazonian forager-horticulturalists who expanded from southwestern Amazonia to the Río de la Plata River, leading one of the most significant pre-Columbian migrations in the Americas. This study focuses on the Guaraní colonization of the Upper Uruguay River, located within the tropical rainforests of southeastern South America. Utilizing a geographic and chronological database specifically developed for this research, along with statistical tools, pollen, and charcoal records from a sediment core recovered in the region, we present the first comprehensive analysis of the Guaraní colonization process in this valley, including its environmental impact and effects on pre-existing non-Guaraní populations.

The first evidence of Guaraní occupation in the Upper Uruguay River is dated between 773 and 880 CE, within a well-developed deciduous forest. This initial phase, extending between ∼ 770 and 1000 CE, indicates the presence of small founder populations that left a weak archaeological signal characterized by a few scattered settlements along the valley. Subsequently, between 1000 and 1400 CE, a gradual expansion of these groups is observed along the valley, leading to the first environmental impacts, such as an emerging deforestation process, an increase in species typical of secondary forests, and the earliest detection of Zea mays. Between ∼1400 and 1600 CE, there was a sudden increase in the number of Guaraní sites, which were distributed almost continuously along the 240 km of the valley, leading to the displacement of non-Guaraní indigenous societies. During this phase, the Guaraní population occupied the entire deciduous forest area up to the headwaters of the Uruguay River, at the confluence of the Canoas and Pelotas rivers, where they reached the limit of their ecological niche. During this period, significant environmental changes occurred, including widespread deforestation due to the intensification of slash-and-burn practices, an increase in maize cultivation and other edible species, and a greater frequency of species typical of open areas growing within disturbed forests. By 1600 CE, the valley experienced significant depopulation, coinciding with Portuguese and Spanish colonization of the region, leading to the full regeneration of the forest as a secondary forest.