El Nino is bashful about revealing its age.

Thoroughly sated and ready for sleep, a resident of coastal Peru hoisted himself from his campsite and dumped a pile of empty scallop shells onto a nearby trash heap. Six thousand years later, archaeologists digging through the leftovers of that meal report that they have discovered the origin of the climate feature called El Nino.

Known for its feverish warming of the central and eastern equatorial Pacific Ocean, El Nino strikes once every 4 to 7 years, disrupting weather around the world. Geoscientists have tried tracking its history using coral and other evidence, but their studies have failed to pinpoint when the pattern of warmings begins.

Archaeologist Daniel H. Sandweiss of the University of Maine in Orono and his colleagues began studying the problem when they noticed the wrong kind of mollusk shells in prehistoric garbage deposits, or middens. Modern species living along the shores of Peru are adapted to the cool waters of the Humboldt Current, which skirts the coastline. Middens dating to 5,000 years ago contain these same cool-water species, but middens going back 5,000 to 8,000 years have warm-water species, the researchers report in the Sept. 13 Science.

These unusual mollusks help date the onset of the modern El Nino pattern, says Sandweiss, because they reveal that warm waters continually bathed coastal Peru prior to 5,000 years ago. "El Nino is an occasional incursion of warm water along the coast of Peru, which is normally cool. If we see conditions in which there were permanently warm waters along the coast, then by definition there couldn't have been the El Nino pattern we know today," Sandweiss says.

The archaeologists first uncovered hints that the climate had shifted from warm to cool in the 1980s, while studying a site called Ostra. Last year, they documented a similar mollusk transition 400 kilometers north of the site. In support of their interpretation, Sandweiss notes that recent pollen studies from northern Australia also point to the initiation of El Nino warmings 5,000 years ago.

"The archaeological evidence is very compelling," comments Richard L. Burger of Yale University.

The shift toward cooler and more variable conditions may have played some role in fostering the development of more complex Peruvian societies, which appeared at roughly the same time. "Just after 5,000 years ago, the people began to build monumental structures in coastal Peru. How these [events] are linked needs to be looked at more closely," says Sandweiss.

The idea of a climate shift at that time gets a chilly reception from some other researchers who study coastal Peru. Geologist Lisa Wells of Vanderbilt University in Nashville traces the climate history of the region by examining deposits of giant floods, which occur when particularly intense El Ninos deluge the coastal desert with rains. Wells sees evidence of the El Nino-style floods going back to 7,000 to 8,000 years ago.

To explain the presence of warm-water mollusks off Peru more than 5,000 years ago, Wells suggests that they lived in shallow lagoons protected from the cooler ocean. She has documented a relict lagoon from this period at the Ostra site.