For the tropical west coast of South America, where El Nino/Southern Oscillation (ENSO) is most pronounced, archaeological and associated paleontological deposits in northern Peru revealed a major climate change at about 5000 years before the present (yr B.P.). The data implied the presence of stable, warm tropical water as far south as 10 degrees S during the early mid-Holocene (about 8000 to 5000 yr B.P.). These data suggest that ENSO did not occur for some millennia preceding 5000 yr B.P., when global and regional climate was warmer than today.