Supplementary Methods Phytolith analysis. A total of 26 samples were processed from profiled walls and features from Mound Gamma (Fig. 3) and the central part of the TBN trench transect and residue from three plant processing tools from Mound Gamma. Phytoliths were isolated from modern plants and archaeological sediments using standard methods of analysis (ref. 28). To maximize the recovery of phytoliths of different size classes, such as those that derive from the rinds of Cucurbita spp. fruits and leaves and cobs of maize, archaeological sediments were separated by wet-sieving into silt (2-50 µm) and sand (50-2000 µm) fractions. Phytolith slides were scanned at 400X using an Olympus B2 light microscope. In selected archaeological sediment samples, a minimum of 200 short cell phytoliths were counted for each sample. Entire slides were scanned for diagnostic Zea mays and Cucurbita spp. phytoliths. Given the dominance of Poaceae morphotypes in most of the samples, and in order to emphasize the variability of short-cell phytoliths, percentages of phytoliths from the Panicoideae, Chloridoideae, Pooideae, and Oryzoideae sub-families of grasses were calculated on the sum of the short-cell phytolith types alone. Percentages of non-grass phytoliths were calculated on the basis of the total sum of phytoliths. Phytolith identifications were made by comparison to a large modern reference collection of plants from the study region consisting of nearly 300 species, including 60 Poaceae, 21 non-grass monocotyledonous, 21 of herbaceous dicotyledons, and 7 woody dicotyledonous. The modern reference collection of over 2000 specimens of plants from the Americas housed at the Smithsonian Tropical Research Institute (STRI) was also closely utilized. In addition, seven modern surface soil samples were analyzed from wetlands, wet prairies, upland prairies, gallery forest, and palm forest. All phytoliths were rotated to see all sides and identify the three-dimensional characteristics. Starch grain analysis. While the excavations were in progress, all potential plant processing tools (e.g., those which exhibit ground or polished surfaces) were wrapped in aluminum foil and set apart for phytolith and starch grain residue analysis (Supplementary Fig. 3). The unwashed tools were processed following standard procedures as described by Piperno and colleagues (ref. 23). The modern reference collection curated at STRI was used for comparison and identification of the starch granules. For this study we also added twelve species of Poaceae and four species of Canna native to Uruguay to STRI’s collection of over 300 species from more than 36 economically important different plant families. Maize starch grains (ref. 23) were recovered from three plant-processing tools as follows: a) a subspherical mano excavated at 205-210 cm deep, 5 cm above a date of c. 3460 14C yr BP (3980 to 3470 cal. yr BP) (Supplementary Fig. 3a) (starch grain size, x =17 µm; range 14-20 µm; n=2); b) the used surfaces of a rectangular fragment of a milling stone base of grainy rhyolite (9.1 x 7.2 x 4.1 cm) excavated from the level 175-180 cm depth, sector 4/C (Supplementary Fig. 3b) (starch grain size, x =16 µm; range 10-22; n=11), and c) the worked faces of a rectangular milling stone base (20 x 12 x 6.2 cm) at Mound Gamma 150-155 cm depth, sector 3/E (Supplementary Fig. 3c) (starch grain size, x =16 µm; range 10-18 µm; n=4). Further comparisons of starch grain size and morphology between wild and domestic species are required to determine if the Canna sp., Calathea sp, and Phaseolus sp. starch grains represent domesticated or wild plants. Paleoenvironmental analysis. A 1.70 m core was extracted from the wetlands of India Muerta in the vicinity of the site. Pollen samples were extracted every 5 cm and processed following standard methods of analysis (ref. 28). Additional samples were taken at 62.5, 77.5, 83.5, and 89 cm depth to obtain a better resolution for the midHolocene period. Pollen preparations were examined under magnifications of 1000X. Identifications were based on comparison with the pollen reference collection at the Department of Biological Sciences in the Florida Institute of Technology and with published pollen atlases and keys. A minimum of 200 grains were counted per slide. Percentages of all taxa were calculated from the total pollen sum. Phytolith samples were taken every 10 cm, and at 5 cm intervals in the early and mid-Holocene sections of the core. Samples were processed and analyzed at STRI following procedures described above.