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Computer Models Help Unravel Mystery of Puebloans’ Disappearance

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Remember playing “The Oregon Trail” computer game in middle school? As a pioneer leading your family westward in a covered wagon, you hunted virtual deer, rabbits and bison—but not too many. You had to leave enough game animals alive to sustain your party until you reached Oregon. And along the way, you were subject to chance events such as snowstorms and snakebites, and the most dreaded fate: “You have died of dysentery.”

Mesa Verde via Wikimedia Commons

Yes, the game was delightfully unrealistic. But controlling the use of finite natural resources and adapting to changing conditions have been central to human survival in the American West for ages. In fact, Washington State University archaeologist Timothy Kohler and his partners in the Village Ecodynamics Project are using computer models to better understand the processes that affected prehistoric societies in the Southwest. Kohler gave an update on the project on February 16 at a symposium on long-term climate vulnerabilities, part of the annual meeting of the American Association for the Advancement of Science.

The Village Ecodynamics Project aims to solve a longstanding mystery in the prehistory of the American Southwest. Kohler refers to this puzzle as “the elephant in the room.” Ancient Puebloan peoples, also known as the Anasazi, occupied the present-day Four Corners region (where Utah, Colorado, Arizona and New Mexico meet) for hundreds of years. They cultivated corn and lived in large settlements, most famously at Mesa Verde, now a U.S. National Park and UNESCO World Heritage Site.

Curiously, nearly all the villages in the northern Southwest were abandoned in the 1200s. Patterns found in ceramics and architecture suggest the Puebloans migrated south, settling more arid regions of New Mexico and Arizona, where their descendants still live. Early tree ring analyses conducted in the 1920s showed evidence of a severe, long-term drought, which many assumed was responsible for the migration. But the dry spell is not the whole story, Kohler says. “It’s a classic problem in Southwestern archaeology, and it’s embarrassing that it hasn’t been completely solved by now. But I think we’re going to solve it. We’re very close,” he says.

Like many of today’s scientific quandaries, this knot will be unraveled with the help of computer models. These simulations are the best way to integrate information about past climate and resources, agricultural productivity and human populations. The latest work from the Village Ecodynamics Project models a larger swath of the populated region and utilizes more accurate methods for estimating maize productivity than previous analyses.

Study areas of the Village Ecodynamics Project in southern Colorado and northern New Mexico (in boxes). The VEPIIN northern study area includes Mesa Verde National Park (MVNP) and Canyon of the Ancients National Monument (CANM), while the southern study area incorporates contemporary tribal lands and parts of the Santa Fe National Forest, as well as Bandalier National Monument (BNM). CNHP, Chaco Culture National Historic Park. Archaeological data suggest that people from the Four Corners region migrated south during the 1200s. Image credit: Kyle Bocinsky.

Why maize productivity? Today’s Paleo diet proponents and slow foodies criticize government subsidies and food companies for promoting over-reliance on corn, which feeds our livestock and also wheedles its way into much of our processed food. But our love affair with the crop goes back much further. People in the American Southwest probably got more than 70 percent of their calories from maize beginning nearly 5,000 years ago. And in the Four Corners region, after hunting depleted deer populations, people turned to domesticated turkeys for protein. And they fed those turkeys corn.

With new reconstructions, archaeologists now understand that drought was not the only factor contributing to the maize shortfall in the 1200s. Temperature may also have played a role. At the time, the northern Southwest was at the upper range of maize production. People grew a maize variety called chapalote, which was originally brought to the area from Rio Balsas, Mexico—a warmer region with more reliable rainfall. So cool, dry conditions could have created a devastating combination. Rather than using productivity data for modern hybrid corn, archaeologists have now grown the chapalote variety and measured its yield across a range of rainfall and temperature levels.

Chapalote maize, a Mexican landrace. Photo credit: International Maize and Wheat Improvement Center (CIMMYT).

Temperature can be difficult to reconstruct, because tree ring width tends to be more dependent on changes in precipitation. But trees at higher elevations, such as the bristlecone pine (the world’s longest-lived organism), are less limited by moisture—the wider the growth ring, the warmer the year.

Scientists in the Village Ecodynamics Project are combining tree ring measurements with pollen records for more accurate temperature reconstruction. Palynologists take cores from bogs and lake bottoms and analyze the proportions of tree species in each time period, then find modern plant communities with similar compositions to extrapolate the temperatures during that time.

These new, more careful reconstructions of past climate and productivity can help archaeologists determine which areas were available for farming and how large a population they may have sustained, which could reveal some of the patterns that led up to the infamous depopulation of the 1200s. But past land use projections based solely on climate and resources don’t always jibe with the dwellings that archaeologists find on the landscape. That’s where the vagaries of human behavior come in. (See my upcoming Q&A with archaeologist Margaret Nelson, who believes we can learn from the food insecurities of prehistoric societies.)

The author in 1992—age 8—at Chaco Culture National Historic Park in northwestern New Mexico. Chaco Canyon was home to thousands of Puebloans from 850 to 1250. Photo credit: Brian Sharlach.

“My own feeling,” Kohler says, “is that the steep declines in both the crude birth rate and the life expectancy after 1300 have to do with people being compacted into these areas.” In other words, though we may look back in awe at the complex societies built by Puebloan peoples at sites such as Mesa Verde and Chaco Canyon, roots that go too deep can be restrictive. Deeply ingrained social structures may have led people to use up all available land and resources—including animals to hunt and fuel wood to heat and cook.

The new phase of the Village Ecodynamics Project takes human factors into account, including the exchange of maize among related groups and the possible escalation of hostilities in the region during this time.

“I think that a lot of the general public considers archaeologists more as explorers than as scientists,” Kohler says. “That’s really not so true anymore. We are trying to put numbers on things.”

Kohler hopes the modeling techniques developed by the Village Ecodynamics Project can be applied to other regions of North America, such as Cahokia. Known as America’s first city, Cahokia was a complex settlement near present-day St. Louis, and also declined around the 1200s—probably a coincidence, but possibly also due to drought. As in the Southwest, multiple forces likely led to the depopulation.

Kohler hopes this research will draw public attention that goes beyond curiosity about how people lived in the distant past: “We’re in the midst of an era of rapid climate change. What we’re showing is the relationship between climates and production. This research should be of interest to everyone who likes to eat.”

The views expressed are those of the author and are not necessarily those of Scientific American.

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