Excavations at two sites on the Island of the Sun in Lake Titicaca, called Ch'uxuqullu and Titinhuayani, discovered significant preceramic (ca. 2000 B. C.) through Formative period (up to ca. A. D. 400) occupations. Excavation data indicate that there was a continuous occupation from the first known settlements on the island up to the present day. The early occupations on the island were culturally linked to the populations on the mainland, as indicated by ceramic analysis and the presence of nonlocal obsidian from the Colca Valley source, 275 km to the northwest. Our excavation data also reveal that there was a brisk exchange network between the island and mainland beginning at least during the Formative period, and probably earlier during the end of the Late Archaic period. Furthermore, paleoenvironmental data indicate that the island has been isolated from the mainland during most or all of the human occupation. The Lake Titicaca region of Peru and Bolivia was characterized by a system of exchange that relied, in part, on watercraft beginning at least by 1600 B. C.