Shrewd water use helped South America’s first empire thrive. So why did a drought destroy it?

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پنجشنبه 28 فروردین 1399 ساعت 18:12
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science mag
When Wari colonists arrived in the Moquegua Valley of southern Peru some 1400 years ago, people already living there were likely nervous. The Wari state, with its capital city of Huari high in the Andes near what is now Ayacucho, Peru, had been expanding its reach. The Wari takeover was violent in places; the invaders sacrificed local people and displayed their heads as trophies.
But this time the Wari colonists did something unexpected. Rather than trying to seize the fertile valley floor, where people already lived, the newcomers occupied high, dry land that no one else had figured out how to use. They constructed their government and religious buildings on top of a high mesa, now called Cerro Baúl, and erected canals and aqueducts that carried water much farther than any previously attempted in the valley. They carved mountain slopes into agricultural terraces, which efficiently trapped and distributed water from rain and snowmelt to plots of maize, quinoa, and peppery berries called molle. People from several other regions moved to the new farms and towns, forming a powerful labor force that helped maintain the sprawling water infrastructure.
Remote Cerro Baúl is home to some of the best preserved Wari canals and terraces, but the remains of their sophisticated water infrastructure have been found in both the Wari heartland and in several of the state’s many colonies, including around the Wari center of Pikillacta near present-day Cuzco and in the Huamachuco region, more than 700 kilometers to the north of Huari. Such innovative hydraulic engineering enabled Wari—which some scholars argue was South America’s first empire—to expand and thrive for some 400 years despite an often dry, drought-prone climate, recent studies suggest. (Archaeologists refer to this state as “Wari,” not “the Wari,” similar to the names of modern nations like Peru or France.) Wari colonists and those who joined their community were able to “settle empty zones and make them productive,” says Donna Nash, an archaeologist at the University of North Carolina, Greensboro. Archaeologist Patrick Ryan Williams of the Field Museum calls the Wari strategy “conquest by hydraulic superiority.”
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Those studying the Wari state’s rise and fall, however, confront a puzzle. Its end, about 1000 years ago, appears to have coincided with a severe drought. Across history, the pattern might seem familiar; other ancient civilizations, including the Classic Maya and the Old Kingdom of Egypt, appear to have collapsed in a time of drought. But how could drought have doomed Wari, a society that had been built on learning to take maximum advantage of limited water, and had seemingly even expanded through previous dry spells? To find an answer, researchers are trying to reconstruct two intricate, fragmented narratives—the human and the environmental—and weave them together. The history of climate “in the Andes is extremely complicated,” says Benjamin Vining, an environmental archaeologist at the University of Arkansas, Fayetteville. “And the only thing more complicated is human behavior.”
The Wari homeland around today’s city of Ayacucho is dry, like much of Peru. It sits just 200 kilometers from the Pacific Ocean but nearly 3000 meters above sea level, nestled in the Andes, and the vast majority of precipitation in South America falls far to the east, over the Amazon rainforest. As a result, Peru’s mountains and coast depend on rivers fed by mountaintop glaciers, plus what little precipitation falls. “That means water is one of the most valuable commodities,” Williams says.
Conditions in the Andes were at least as harsh around 600 C.E., when Wari was expanding beyond the Ayacucho region, recent and ongoing research suggests. Broxton Bird, a paleoclimatologist at Indiana University–Purdue University Indianapolis, is now analyzing a sediment core—drilled from Lake Pumacocha about 250 kilometers north of Ayacucho—that shows cool and relatively dry conditions between 475 and 725 C.E. Those data support evidence of aridity from another high-resolution record preserved in a mineral deposit (called a speleothem) from Huagapo Cave, also in the central Peruvian Andes, he says.
Such dry periods certainly stressed prehistoric communities—sometimes intensely, if they tipped into regional droughts. But for Wari, they also appear to have led to innovation, including new and better ways of storing, moving, and using precious water. Their canals were far longer and sturdier than any that came before, and although some other cultures had used agricultural terraces, Wari massively scaled up the technology and brought it to new regions. “In a moment of crisis, they came with the solution,” says Francesca Fernandini, an archaeologist at the Pontifical Catholic University of Peru (PUCP). Those technologies likely gave Wari colonists a strategic advantage—and a way to expand into new territories.
Wari’s expansion was not always peaceful, however. Wari art often depicts warriors, notes archaeologist Tiffiny Tung of Vanderbilt University. And strontium isotopes preserved in trophy heads uncovered in the Wari heartland site of Conchopata show the victims grew up elsewhere, evidence that Wari warriors captured and sacrificed people from faraway lands, she says. People buried at Conchopata also show more head trauma than people buried in communities controlled by other Andean cultures that existed before and at the same time as Wari.
At first, the Moquegua valley seemed likely to yield signs of a violent Wari takeover. Cerro Baúl was essentially a border outpost, butting up against territory occupied by colonists from Tiwanaku, another expansive Andean state that had its capital near Lake Titicaca in what is now northern Bolivia. But when researchers examined bodies from Tiwanaku cemeteries in the Moquegua area, they found that those buried after Wari colonists arrived showed no signs of increased violence. Instead, the valley’s peoples—Wari, Tiwanaku, and local Moquegua communities—appear to have coexisted for 400 years, from about 600 to 1000 C.E., each preserving its own style of pottery, architecture, temples, and burials.
Meanwhile, the Wari community, which Williams estimates numbered about 3000 people, pursued far more ambitious water projects than its neighbors. Whereas people living close to the valley floor typically dug 1- to 3-kilometer-long irrigation canals from the river to their low-lying fields, the Wari community built a 20-kilometer-long canal that snaked high up the mountain slopes and brought water to several settlements built along its path. The earth and gravel terraces on which the Wari farmed—an agricultural innovation independently developed in many hilly, water-stressed ecosystems—retained moisture around crop roots while allowing excess water to drain to the terraces below. Wari leaders living on and ruling from Cerro Baúl were “able to sculpt the landscape and put water where they wanted it,” Nash says.
That hydraulic infrastructure required an incredible amount of labor to build and maintain, Nash says. She has excavated in the agricultural settlement of Cerro Mejía, just 2 kilometers away from Cerro Baúl’s elite civic and religious center. Based on the variety of domestic pottery and other material, she thinks people from four separate cultures came together under the Wari umbrella in Cerro Mejía, including some Moquegua locals and people from the coast. “I envision this as a multiethnic, pioneering frontier colony,” she says. Wari leaders appeared to be able to marshal them all “to perform huge amounts of labor,” Williams says.
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Formal diplomacy, likely with Tiwanaku representatives, was probably conducted in the form of elite feasts in the palace atop Cerro Baúl, which also housed a brewery. The site is now littered with thousands of molle berries, used for making the typical Wari beer called chicha de molle . Tiwanaku-style jewelry found in the Wari palace, as well as a small Tiwanaku shrine tucked into Cerro Baúl’s Wari palace, suggest other ties, perhaps formed as elite Tiwanaku women married into the Wari power structure, Nash says.
For many centuries, the system appeared to sustain peace and social cohesion, even during hard times. Between 850 and 950 C.E., for example, excavations show that part of the Wari colony at Cerro Baúl suffered a devastating landslide that buried people, houses, and large swaths of farmland. “It was a massive disaster,” Nash says. “But when the Wari government was going strong, they could [cope with] things like this.” The terraces were soon rebuilt.
Still, the reconstruction was flimsy, perhaps reflecting a rush to prevent starvation immediately after the landslide. And in what may be an early sign that support for the Wari government had begun to crack, no one mobilized additional efforts to improve the new terraces. The community appears to have fractured even further by the time a new natural disaster hit: an extreme drought. Paleoclimate records like the core from Lake Pumacocha and the speleothem from Huagapo Cave show this drought was much more severe than the dry period at the beginning of Wari’s expansion, Bird says.
By the time the drought reached the Moquegua valley in the 11th century, archaeological evidence suggests the Wari colony at Cerro Baúl was already weakened, Williams says. Around 900 C.E., after centuries of relatively separate coexistence, more Tiwanaku villagers started to move into Wari territory, as shown by the remains of Tiwanaku-style houses, ceramics, and cemeteries. Small Tiwanaku temples appeared on top of abandoned Wari agricultural fields, suggesting parts of the canal system were no longer functioning, which would have weakened the community’s ability to cope when the drought arrived. Increasing factionalism and decreasing cooperation to maintain infrastructure “might mean this society is more vulnerable to even the beginnings of a changing climate,” Williams says.
Around 1050 C.E., the administrative and religious Wari buildings on top of Cerro Baúl were abandoned, after what Williams calls an “end of times party.” The revelers intentionally burned particular rooms, including parts of the chicha de molle brewery, then scattered smashed drinking vessels on top, a common offering in Andean societies. Archaeologists can’t be sure whether the elite Wari leaders of Cerro Baúl left the region entirely or blended into the new, smaller communities that sprang up in Wari’s wake. The more middle-class residents of Cerro Mejía likely retreated into smaller towns located higher on mountain slopes, Nash says; these were easier to defend against attacks and nearer to short segments of the Wari canal that were still functioning.
Cerro Baúl wasn’t the only Wari community in trouble around 1050 C.E. Across the Wari state, people appear to have abruptly abandoned settlements they had worked hard to build and maintain. Even the capital of Huari emptied out, with its residents likely moving closer to the coast, Tung says. It remains a mystery, however, whether the collapse of the Wari capital rippled out to weaken colonies like Cerro Baúl, or the colonies gave up first and ceased to send tribute and supplies back to the heartland, eroding the state from the outside in. In any case, Williams says, “The Cerro Baúl colony couldn’t sustain itself without being part of the larger whole.”
Cerro Baúl’s story is adding to the increasingly nuanced view that scholars have of drought’s role in the collapse of ancient societies around the world. Once, it was conventional wisdom that drought had toppled ancient civilizations such as the Maya. Now, scholars rarely see a lack of water as the sole cause. Rather, they say, there is often a complex interplay between the social and natural environments. Sometimes, droughts simply drive wedges deep into existing cracks in political and economic systems.
In Cerro Baúl, for instance, it appears that Wari hydraulic expertise should have enabled the colony to cope with that final drought, Nash says. “But if the politics were bad, if their institutions were unraveling,” then the community was vulnerable. “So, you don’t blame the drought,” she says. “You blame the government.”
“The critical moment,” says Luis Jaime Castillo, an archaeologist at PUCP, is not necessarily when canals run dry. It “is the moment when people lose confidence in the system.”
Ironically, Wari engineering long outlasted the state itself. Beginning in the 1300s, the expanding Inca Empire repurposed Wari canals, roads, and agricultural terraces to feed and connect their far-flung territories. Some of the ancient terraces, with their Incan and Wari roots, are still in use today, Williams says. Indeed, he notes, terracing is being revived as a sustainable and hydraulically efficient way to farm in the Andes, as today’s communities confront the ancient problem of drought, but now with a new face: human-caused climate change.
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