The oceanographic phenomenon known as El Niño is the subject of intensive recent study. Any hypotheses regarding physical causes and predictability of El Niño should consider its geological history. New geoarchaeological evidence suggests that the El Niño phenomenon did not exist along the northern and central coasts of Peru before about 5000 years BP. Molluscan faunas from archaeological sites at Pampa las Salinas and Salinas de Chao permit temporal bracketing of a major structural change in the East Pacific water mass. The boundary between the warm Panamic Province and the cold Peruvian Province, which today occurs at about 5 degrees south latitude, was some 500 km further south from at least 11,000 years BP to about 5000 years BP. This conclusion is corroborated by many other lines of evidence including phosphorite distribution, timing of glacial retreat, sea level change, radiolarian, diatom and fish scale distributions, and beach ridge patterns. The present day arid coastal climate of north central Peru is probably a post-5000 year BP development. Hunter-gatherer populations of the area would most likely have exploited more landmbased seasonal resources from grasslands and forests before 5000 years B.P, and relied less upon the diminished productivity of warm water maritime resources.