Clamshells Reveal Secrets of Pre-Columbian Society's Decline

The history of that social catastrophe is told in clamshells, which reveal the local climate much like tree rings can.

The Moche Empire didn't suddenly collapse, said Fred Andrus, a geologist at the University of Alabama, in Tuscaloosa.

"The people adapted but did it in a way that was uncomfortable," he said. "They faced a series of challenges and dealt with them in ways that must have been difficult, and unpleasant."

The stable societies they built could not cope with the difficulties. Civil wars may have broken out, along with civil unrest. After a couple of centuries of upheaval new social organizations replaced the old, and the Moche simply became a different people, a culture known as the Chimu. 

The Moche lived along the northern coast of Peru between 100 and 800 A.D., with a capital at Huacas del Sol y de la Luna near the present-day towns of Moche and Trujillo. They were not an empire like the Incas or Aztecs.

"The Moche were not a unified group," said Ryan Williams, curator of South American Anthropology at the Field Museum in Chicago. "They were several small kingdoms." Each of those kingdoms had a different "trajectory," he said, so the collapse was not uniform.

"Most people now believe it was most likely a single religion adopted by different people but with political power still local," said Jeff Quilter, of the Peabody Museum of Archeology and Ethnology at Harvard, in Cambridge, Mass., who was involved in the clamshell research.

The Moche believed in gory human sacrifice and produced famously beautiful pottery, built huge, bizarre brick pyramids and had a complex and efficient irrigation system. Some of the aqueducts are still in use today.

Their society was sophisticated and agriculture-based. Ruins of their cities remain tourist destinations in Peru.

But they disappeared by 800, and many scientists think changes in the environment were largely responsible. Ice cores of glaciers in the area hint at the same cause.

El Niño has been a suspect since the 1970s, with dramatic shifts in the ocean pattern in the late 500s, Quilter said. The Moche seemed to have recovered for a while before being "transformed" into another people, he said.

Clamshells can provide more proof of these substantial climate changes. Researchers from Alabama, and the Universities of Arizona and Maine reported in the journal Geology that the Moche buried clamshells with their dead as tributes. Because of the climate and burial practices, Moche mummies are not uncommon.

According to Andrus, that combination makes it possible to produce a snapshot of the environment at the time of burial. The science of determining climate change through the study of shells is called sclerochronology.

Clams draw carbon from the ocean as they grow, Andrus said. The amount of carbon increases with upwelling. Cold ocean water is generally richer in nutrients and animals than warmer water, and the colder the water the more carbon there likely will be absorbed in the shells. These can be found in growth bands and dated through radiocarbon measurements.

By taking multiple samples of the shells, from the time a clam was born until it died, it is possible to trace the upwellings, he said. 

The clamshells support the theory that the pattern of El Niño changed. El Niño is a wind-driven equatorial phenomenon in which warm, nutrient-poor water replaces the cold, rich water off the South American coast. The clams indicate the cycles of El Niño between the sixth and the sixteenth centuries were more frequent and more intense than they are now, Andrus said.

The climate changed with it. Fields flooded in some cycles, and were replaced with sand dunes driven by extensive droughts in others, evidence suggests. The floods may have destroyed some of the irrigation canals, further altering the food supply, Andrus said. 

Other factors likely played a role in the collapse of the Moche society, Williams said, such as the eventual movement of highland groups, including the Wari and the Cajamarca, into the Moche's territory.

The effects on society would not have been sudden or dramatic, but nevertheless, the Moche apparently had difficulties contending with the changes, Andrus said. Eventually, they disappeared as a people.

"I think we can say climate change posed a severe challenge," Willams said. "It has as much to do with the social and political configuration of their society as it does with the actual climate impacts." 

Joel Shurkin is a freelance writer based in Baltimore. He is the author of nine books on science and the history of science, and has taught science journalism at Stanford University, UC Santa Cruz and the University of Alaska Fairbanks.

Inside Science News Service is supported by the American Institute of Physics.