Seven Centuries Ago, Tens of Thousands of People Mysteriously
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important question. It’s not an easy place to reach,” says Donna Glowacki, an archaeologist now at the University of Notre Dame in Indi- AND THEN THERE ana, as she walks among the ruins. Even more perplexing is what happened after they settled GRITZ/GETTY JAMES there. The villagers occupied their cliffside houses for just a short time before everyone suddenly picked up and left. So did all the other farmers living in the Four Corners region WERE NONE of the American Southwest, where the mod- ern states of Colorado, New Mexico, Utah and Seven centuries ago, tens of thousands of people Arizona meet (see ‘Turbulent times’). All together, nearly 30,000 people disap- mysteriously fled their homes in the American Southwest. peared from this area between the mid-1200s Archaeologists are trying to work out why. and 1285, making it one of the greatest vanish- ing acts documented in human history. What had been one of the most populous parts of BY RICHARD MONASTERSKY North America became almost instantly a ghost land. Archaeologists have long puzzled over what drove these farmers, the ancestors of the Pueblo ultures carve lazy circles in the sky as When ancient farmers built these structures people, from their homes and fields. “That is a stream of tourists marches down around the year 1200, they had nothing like the one of the iconic problems of southwestern a walkway into Colorado’s Spruce modern machinery that constructed the tour- — and world — prehistory,” says archaeolo- Canyon. Watching their steps, the ist walkway. Instead, the residents had to haul gist Mark Varien, who Vvisitors file along a series of switchbacks leading thousands of tonnes of sandstone blocks, cut is executive vice-presi- Cliff Palace, a Pueblo to one of the more improbable villages in North timber and other materials down precarious dent of the Crow Can- dwelling in Mesa America — a warren of living quarters, storage paths to build the settlement, known as Spruce yon Research Institute Verde National Park, rooms, defensive towers and ceremonial spaces Tree House, in Mesa Verde National Park. in Cortez, Colorado. was a thriving village all tucked into a large cleft in the face of a cliff. “Why would people live here? That’s an Early scholars blamed in the 1200s. 26 | NATURE | VOL 527 | 5 NOVEMBER 2015 © 2015 Macmillan Publishers Limited. All rights reserved FEATURE NEWS nomads, the ancestors of the Apache and Navajo, for violently displacing the farmers. TURBULENT TIMES 100 km Over the past couple of decades, the main In the late 1200s, ancestral Pueblo farmers abruptly left explanation has shifted to climate — a profound their villages in the northern San Juan region of the US Southwest. Many of them resettled in the northern Rio drought and cold snap that hit in the 1270s. Grande area of New Mexico. COLORADO But a series of studies by Glowacki, Varian NORTHERN SAN Before they migrated, the ancestral and other researchers reveals a much more JUAN REGION Pueblo built cli dwellings that o complex answer. The scientists have used d provided added security. Some of ra lo the most spectacular are found in detailed archaeological analysis, fine-grained o C CENTRAL Mesa Verde National Park. climatic reconstructions and computer mod- UTAH MESA VERDE REGION els to simulate how ancestral Pueblo families Durango would have responded to their environment. MESA VERDE The interdisciplinary strategy has enabled the HIGHLANDS researchers to examine prehistoric societal Aztec NORTHERN S RIO GRANDE changes at a level unattainable in most other an J Ruins ARIZONA uan REGION regions. “We have enormous detail on this NEW archaeologically. Unparalleled detail,” says Chaco Canyon and the Aztec MEXICO Steve Lekson, an archaeologist at the Univer- Ruins National Monument area sity of Colorado Boulder. were power centres of the Pueblo Los Alamos world during the 1000s and Chaco The emerging picture is one of a society 1100s. When their inuence Canyon rocked by troubles until it eventually toppled. waned, it triggered political and More than a century before the Mesa Verde UNITED cultural changes across the region. villages emptied out, political disruptions STATES and a monster drought destabilized the entire Albuquerque ancestral Pueblo world. Thousands of peo- ple moved into the Mesa Verde region from nearby areas, straining the agricultural capac- Population data for the central Mesa Verde region show ALL GONE massive migration away from the area in the late 1200s. People ocked ity of the landscape and eroding established to the region in cultural traditions. This led to violent conflicts 30 the early 1200s. that further undermined the society, spurring some people to leave. When another drought 25 hit in the late 1200s, the remaining population 20 HTTP://DOI.ORG/8T6 (2014). departed en masse. Political instability, cultural conflict, violence, 15 overcrowding and drought. Many of the chal- lenges encountered by the ancestral Pueblo 10 seem all too familiar in 2015, as hundreds of Population (thousands) 5 thousands of migrants flee from the Middle East J. ARCHAEOL. METH. THEOR. METH. J. ARCHAEOL. and Africa towards Europe. When Glowacki 0 600 700 800 900 1000 1100 1200 1300 looks at the events of more than seven centu- Year ries ago at Spruce Tree House, she sees many similarities. “There was a splintering that went on and an implosion of this political system. It 1200, and added more right up until the last to a major interdisciplinary effort called the SOURCE: S. ORTMAN was a rejection, them saying, ‘We can’t live that residents abandoned the site around 85 years Village Ecodynamics Project (VEP), which way anymore. There has to be a better way’.” later. The researchers can narrow construction launched in 2002. Funded by the US National dates to within a year or two by analysing tree- Science Foundation, the nearly US$2.5-million STONE WORK ring patterns in the wooden support beams initiative is assessing how social and environ- It was chance that first carried Glowacki into in the ceilings and then matching them to an mental factors influenced the populations the world of the ancestral Pueblo. Before established tree-ring chronology for the region. of prehistoric Pueblo farmers from about starting graduate school, she ended up in a Despite the tedious nature of the work, 600 to 1300, says Tim Kohler, the VEP’s prin- summer job as a ranger at Mesa Verde National Glowacki says that it never loses its appeal. cipal investigator and an archaeologist at Park, where she fell for the landscape and its “There are rooms that are fully intact, and you Washington State University in Pullman. archaeology. She has spent the past 23 years, can stand in them — and they were built in the In one strand of research, the team drew on on and off, researching the region’s ancient 1240s. In this country, being able to stand in the rich history of archaeology in the region to populations. something that was built at that time is really compile a database of 18,000 prehistoric sites, At Spruce Tree House, Glowacki pulls out pretty magical.” which allowed them to measure the popula- a map showing the latest results of an archi- The cliff dwellings were a last resort for the tion and how it shifted over time1. With such tectural analysis that she is helping the park to park’s prehistoric Pueblo residents. When a massive database, the researchers could look carry out. The work is laborious — researchers farmers first arrived in the region around at population changes in narrow time bands sometimes sit in front of a wall of sandstone ad 600, they settled on the fertile highlands averaging about 40 years (see ‘All gone’). blocks for days, studying the mortar and rocks above the canyons, which gave them easier “There are not many places in the world to work out how the structure was first built access to their fields. But by 1200, some- where archaeologists can look at changes in and then altered over time. thing began to force them over the edge into such discrete slices of time,” says Varien, who Gradually, a history of the village has taken the giant alcoves that naturally form in the is a co-principal investigator of the VEP. The shape, showing that people assembled the sandstone cliffs. analysis1 suggested that people started leaving first set of rooms in the alcove around the year Insights into that shift are emerging thanks the Mesa Verde region at least 15 years before 5 NOVEMBER 2015 | VOL 527 | NATURE | 27 © 2015 Macmillan Publishers Limited. All rights reserved NEWS FEATURE the drought hit. “It looks as though the final 1100s and 1200s. In the model, the farmers processing, so the team will graduate to a depopulation began with a trickle and ended spread out farther across the landscape than supercomputer for future simulations, which with a flood,” says Scott Ortman, an archae- they actually did in reality. So something seems are planned for later this year or early next year. ologist at the University of Colorado Boulder to have caused the real ancestral Pueblo to Nothing of this scale has been done before in who developed the model for the project’s huddle together more tightly than expected. the field, says Kohler. “Archaeologists do not population analysis. Kohler and his colleagues wondered have the reputation of being users of high-per- Another part of the VEP looked at how the whether fear might have been a factor. To find formance computing environments,” he says. farmers fed themselves. The researchers used out, they surveyed the archaeological litera- “But I don’t think we’ll be the end of the road temperature and precipitation estimates from ture and tracked levels of violence in the area for this kind of work.” tree-ring data to create a model of where the through time by tallying how many skeletons Among the ruins at Spruce Tree House, communities could have grown maize (corn) had broken arm bones, fractured skulls or Glowacki takes a different approach.