Ancient Canals in Andes Reveal Early Agriculture

Nicholas Bakalar
for National Geographic News
December 5, 2005
Despite a lack of solid evidence, archeologists have thought for some time that farmers used irrigation canals in agricultural villages in Peru as long as 4,000 years ago.

New discoveries in the Peruvian Andes may push that date back another 2,700 years.

Scientists say they have unearthed evidence of the oldest canals ever found in South America.

The ancient canals were almost certainly designed for irrigation and were built in the Andean foothills in northern Peru's Zaña Valley, about 40 miles (60 kilometers) from the Pacific coast.

Researchers describe their find in the November 22 issue of the journal Proceedings of the National Academy of Sciences.

"This is an important paper with terrific results," said Jonathan Haas, an anthropologist at the Field Museum in Chicago, who was not involved in the research.

"Every day we have a better understanding of the beginnings of agriculture and domesticated economy. But the whole question of irrigation has so far been indirectly inferred based on the existence of the plants," he said.

The new study "has nailed that down very solidly. Irrigation starts remarkably early in the Andes. You're getting agriculture based on irrigation in the Andes as early as anything seen in the rest of the New World," he added. "It is just stunning work."

Early Agriculture

Researchers found three canals that date to at least 5,400 years ago buried by sediment layering. A fourth possible canal was also found, which special radiocarbon dating techniques revealed to be 6,700 years old.

"Some colleagues in the Andes surmise early canal irrigation based on the presence of crops at 9,000 to 10,000 years ago," said Tom D. Dillehay, an anthropologist at Vanderbilt University in Nashville, Tennessee, who lead the study.

"Yet the actual canal evidence is probably later," he added.

The new find suggests that agricultural settlements may have appeared in South America at about the same time they did in the Middle East. Dillehay notes that the earliest evidence of irrigation canals in Sumeria is between 7,000 and 8,000 years old.

The Peruvian canals were dug along the Ñanchoc River and drew water from small streams. The irrigation scheme suggests an agricultural system built on deliberate manipulation of the environment, rather than one dependent on planting and harvesting in naturally wet areas.

The canals appear to have been designed to draw water by gravitation toward planted fields at lower levels.

Social Reorganization

There is no evidence that a centralized bureaucracy built and maintained the structures, researchers say. But the canals' existence suggests they were built by an increasingly complex society with a mixed economy.

Such a community would have included not only hunters and gatherers but also people who survived partly or exclusively by producing their own food.

This meant not just a shift from foraging to food production, but a reorganization of society and significant changes in social roles.

Donald T. Rodbell, a professor of geology at Union College in Schenectady, New York, says the find raises another key question. "This is an interesting article, as it points to further evidence for a major [post-Ice Age] transition in the way societies lived in Peru," he said.

"This occurred at roughly the same time as a climatic reorganization that featured the onset of the modern frequency of the El Nino Southern Oscillation"—atmospheric weather changes tied to shifting Pacific Ocean currents—"which leads to the question of whether or not climatic change was the impetus for this important societal transition."

Researchers say population size increased with the building of the canals and that housing design shifted from circular to rectangular structures. Public rituals also increased, indicating communal cooperation.

Construction Details

The topmost canal, designated Canal 1 by the study authors, was made of rough stones and burned clay. Rocks line Canals 2 and 3 and the possible fourth canal. The stones appear to have been brought to the site from nearby hillsides.

Water deposited gravel and sand in Canals 2 and 3, which also contain charcoal that researchers dated.

Canal 1 is about 2.5 miles (4 kilometers) long from its point of intake to its termination. Canals 2 and 3 are about half that length.

The canals' design, the study authors report, suggests that their builders were capable of measuring small slopes to control the flow and deliver constant amounts of water to their fields.

The Oldest Canals

Environmental data, the researchers say, suggests that the area was semitropical and that it underwent a period of increasing aridity between 8,000 and 5,000 years ago.

Accumulated sediments filled and eventually buried the older canals, and new ones were periodically built on top of them.

Canal 1 was apparently built during a period of increased rainfall, when surrounding streams carried water year round.

Because Canals 2 and 3 were abandoned at some point, researchers believe there must have been lengthy periods during which the site was uninhabited.

The Zaña Valley canals are the oldest ever found in South America and provide evidence that agricultural production was as important as marine foods in the early civilizations of the Peruvian coast.

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