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Imagine a science in which discoveries are made after a bulldozer digs up the ground for a new highway, oil pipeline, or strip mall. That’s the way most archaeological sites are unearthed today, particularly in the booming Southwest. “The vast majority of information about prehistoric people is learned this way,” says Rick Moore, an associate director with the Grand Canyon Trust, an Arizona-based conservation group that has incorporated archaeology into its mission. For example, in Phoenix, which is sprawling ever faster across the Sonoran Desert, 700-year-old remnants of the ancient Hohokam settlement—everything from farming tools to subterranean pit houses—are still routinely found.

Similar evidence of other lost cultures turns up frequently all over the Southwest. The ruins—sometimes whole villages—are remarkably well preserved by the region’s dry climate and often still largely intact, at least until the latest highway widening. Scientists have grown accustomed to working one step ahead of the bulldozer. “Where there is destruction of the landscape, you’ll find archaeologists,” says Steve Simms, an archaeologist at Utah State University. “We’re like morticians of the environment.”

Ecologists, who also are called to action when a species is at death’s door, have the Endangered Species Act (ESA) to help them ward off extinction. The federal law can be invoked to prevent the last remaining habitat of an imperiled animal or plant from being converted into a casino or condominium development. Archaeologists have the National Historic Preservation Act to at least help them slow the backhoes. To mitigate potential damages, they get a short reprieve to document or excavate the ruins before they’re lost forever.

For both wildlife and archaeology, one of the best refuges has long been public lands, as President Bill Clinton decreed when he began establishing national monuments in the mid-1990s specifically for their ecological and archaeological value. He distinguished these monuments as scientific havens and eventually grouped them into a special classification within the Bureau of Land Management (BLM) called the National Landscape Conservation System. (There are more than 100 national monuments, most of them managed by the National Park Service.) This novel breed of BLM monuments soon prompted a number of ecologists and archaeologists to step out of their separate universes and join forces.

Today that newfound familiarity has bloomed into a pioneering, collaborative venture at Agua Fria National Monument, a mosaic of canyons, mesa tops, and grasslands located in a sparsely populated, mountainous region of central Arizona. President Clinton created the monument in 2000, based largely on its 450 known prehistoric sites, which include an unusual assortment of multi-room houses made of mud and stone (known as pueblos) perched on a rocky mesa. There archaeologists and ecologists from Arizona State University (ASU) are studying the age-old ruins and the ecosystem surrounding them. The project, started in 2003 and called Legacies on the Landscape, is now yielding surprising insights about the people who lived there 700 years ago, as well as about the long-term ecological changes they wrought on the land.

Additionally, the research sheds new light on a volatile period in the Southwest, when prehistoric cultures were forced to adapt to unpredictable climate changes. Some scientists believe that the stage is being set for similar environmental conditions today, as the region contends with explosive development, cyclical droughts, and rapid water depletion. In some circles there’s even a school of thought, argued recently in Jared Diamond’s best-selling Collapse: How Societies Choose to Fail or Succeed, that the current rate of population growth and environmental exploitation in the Southwest is about to collide with harsher climatic conditions in ways that may soon mirror what occurred across the region in the 13th century, which archaeologists consider a period of tremendous societal upheaval. This also happens to be the time when people started moving into the forbidding, isolated moonscape that now comprises Agua Fria National Monument.

Perry Mesa, a rolling, high-desert landscape within the 71,000-acre monument, is only 40 miles north of Phoenix, but it is so remote and stretches so far toward the horizon that it might as well be the end of the earth. The monument’s entrance lies just off a lonely stretch of Interstate 17. There, a winding gravel road descends three miles to the Agua Fria River before climbing through narrow valleys and grassy hills to the top of Perry Mesa, 3,500 feet above sea level.

Steep, 1,000-foot canyons cut the mesa. Right here, along several edges, sit a number of hollowed-out pueblos, some with 50 to 70 rooms. The people of Perry Mesa started constructing this settlement around 1275, stayed for most of the 1300s, and then suddenly vanished into history. The only remaining tangible clues, it would seem, are crumbling masonry walls, pottery and other artifacts excavated from the ruins, and the many rock-art images pecked onto cliff faces, including those of bighorn sheep, deer, and bizarre geometric designs.

Until recently, scientists familiar with the archaeological sites had speculated that the former inhabitants moved to Perry Mesa out of desperation. A drought lasting decades rocked much of the Southwest in the 1200s, setting off a fierce scramble for dwindling food resources. During this dark period, many populations moved away from valleys to upland areas, where larger groups clustered together in defensive-oriented settings. The most famous example—still widely debated—is that of the Anasazi, a sophisticated culture in the Four Corners area that had built an impressive network of farming villages over hundreds of years before abandoning them, beginning in the mid-1200s, for precarious cliff-top dwellings.

The case of Perry Mesa is no less befuddling, if not nearly as well known. When I visited there last spring, I had a hard time imagining how anyone could survive the harsh conditions for a month, much less decades. The 75-square-mile tract has a post-apocalyptic, Planet of the Apes aura. Wind whips constantly. Walking is arduous; the terrain, uplifted atop a bed of volcanic rock, is strewn with broken boulders. Thick masses of thorny mesquite and prickly pear cactus hug the brownish ground, which also sprouts patches of frail tobosa grass.

When Hoski Schaafsma starts describing the ancient gardens that once blanketed the landscape, I stare at the dirt in disbelief. All I can make out is a gray, rocky jumble. But Schaafsma, a pony-tailed ASU plant biologist, has in the past few years decoded an astonishing pattern of land use atop Perry Mesa. “This whole hillside is terraced,” he says, sweeping his arm across a lumpy area of the mesa. “What we’re standing on here, this sort of linear clump of rocks, is drainage, where they were catching the water and soils for agricultural purposes.” What appears, at first blush, to be a random assortment of large stones are actually the remnants of an intricate network of irrigation and cultivated farm fields.

Seven centuries ago rocks were moved and positioned in a way to create walls perpendicular to a given slope so that they caught water and soil after monsoonal rains. Some of the surface soil at Perry Mesa is sandy, which is not the best for farming. But the manipulation of rocks into a series of walls that form terraces engineered a loam for maximum agricultural productivity.

The people of Perry Mesa thus teased corn, beans, and agave from a marginal environment—relying only on rainwater. The technique, known as dry farming, was perfected by prehistoric Indians in the hot, arid Southwest. But the extent to which it was practiced seven centuries ago on this bleak terrain has proven revelatory.

Even seasoned archaeologists who have spent years studying Perry Mesa admit to being surprised by the vast agricultural footprint. “If you walk across the landscape and all you’re looking for is stone houses, then that’s all you’ll find,” says Schaafsma, who had an archaeology background before expanding his portfolio with academic training in ecology. To date, he and his colleagues have documented about 1,000 terraces, with many more expected. As Rem Hawes, manager of Agua Fria National Monument, puts it, Perry Mesa’s “entire landscape was intensively managed to control and collect water, grow crops, and support a thriving society.”

In doing so the indigenous inhabitants—whose descendants are thought by some to be the Hopi, a tribe that still practices dry farming in northern Arizona—totally changed the composition of the soil and altered the plant community, a legacy that continues today. “These people [on Perry Mesa] essentially created their own unique ecosystem,” marvels Connie Stone, a BLM archaeologist who advises on the management of the monument.

How a prehistoric culture transformed this inhospitable environment is just becoming clear, but the most obvious questions still mystify scientists. “Why did they move up there, and why did they leave?” asks Jeffery Clark, an archaeologist with the Tucson-based Center for Desert Archaeology.

An even bigger mystery ties archaeologists in knots. Seven hundred years ago long-established prehistoric cultures started collapsing across the Southwest. The region had been booming for centuries, abetted by the spread of agriculture and complex trading networks. Then, about 1350, the Anasazi disappeared from the Four Corners region. At the same time the rugged Fremont people, dispersed in semi-subterranean pit-house villages in Utah, also vanished from the archaeological record. The chaotic upheaval extended from what’s now Salt Lake City to southern Arizona and into Mexico for another 200 years. By the 1400s the highly organized Hohokam, which had built extensive irrigation canals in the Phoenix area, collapsed, punctuating the grim span.

The debate over what happened to all of these people has been notoriously contentious for decades; researchers have pointed variously to drought, warfare, or, lately, a mass exodus to areas where the landscape could better support farming. Perry Mesa appears to have been among the best of the worst places. Recent tree-ring analysis by one of the ASU researchers, Scott Ingram, points to a wetter period in central Arizona during the 1300s, when the pueblos atop Perry Mesa expanded, supporting as many as 1,000 people. It’s possible they came out of desperation in the late 1200s but then used their ingenuity and an improving climate in the 1300s to set down roots. “I can understand why people would live there at this time, but what I don’t understand is the configuration of the houses,” he says, referring to their placement along canyon rims.

David Wilcox, an archaeologist at the Museum of Northern Arizona, in Flagstaff, hypothesizes that the settlement was “organized for war, ” with the canyon walls “being utilized like the walls of a castle.” Until ASU researchers uncovered evidence of the massive agricultural fields, Wilcox theorized that the Perry Mesa people raided farming villages in the Phoenix basin where the Hohokam were located. Next, perhaps, they got caught in a “raiding and retribution” cycle, which then required an elaborate system of defense. Now he wonders if they simply sought refuge there from the escalating warfare that seems to have spread throughout the region.

For her part, Katherine Spielmann, an archaeologist at Arizona State University, is reluctant to theorize about Perry Mesa’s architectural layout or about what caused people to first arrive and then, a hundred or so years later, pick up and leave. She contends it’s simplistic to reduce their decision making to warfare or environmental stresses when religious factors may also have been involved.

Still, one part of ASU’s focus is examining the constraints imposed by climate and determining how the Perry Mesa people overcame them—or perhaps eventually succumbed. One theory being considered is that a growing population in the 1300s atop the mesa may have exceeded the land’s carrying capacity through excessive agriculture, which could have depleted soils of their productive nutrients. There is also evidence the region was hit with another extended dry spell in the early 1400s that could have dried up their crops. What’s the point of sticking around if you can’t feed yourself and your family?

Some scientists believe that this climatic and social instability offers an instructive modern-day parallel. The Southwest is booming: Major urban hubs like Phoenix, Las Vegas, and Salt Lake City are sprawling across the desert, with strip malls and subdivisions replacing biologically rich habitat; finite water resources are being exhausted at alarming, unsustainable rates. All this explosive growth comes amid new projections from climatologists, published last year in the journal Science, that the Southwest is entering a natural cycle of high aridity and that brutally long periods of drought will be the norm all through the 21st century. Rising temperatures from global warming are expected to make things even hotter and drier.

“The drought that nearly brought this country to its knees in the 1930s wasn’t all that long,” notes Kevin Jones, Utah’s state archaeologist. Could we survive a 30-year drought the likes of which hit the Southwest around the time numerous cultures disintegrated into thin air and people started building houses and forts atop a harsh mesa top in central Arizona? “Absolutely not,” says Duncan Metcalfe, a professor of anthropology at the University of Utah. “We don’t have that kind of buffering capability built in,” he explains, referring to reservoirs and other water control technology.

What we do have going for us, though, is the scientific knowledge that enables us to be aware of such historic drought cycles and the kinds of changes we’re making to the climate today, says Julio Betancourt, a U.S. Geological Survey paleoecologist based in Tucson. “It is almost certain that pre-modern societies lacked sufficient understanding of their physical and biological environment to escape natural disasters or avoid many land-use impacts,” he points out.

Today, for example, scientists can pinpoint wet and dry periods going back thousands of years just by examining tree rings. This detailed reconstruction of the historical environment would seem to be an invaluable resource to planners and policy-makers alike. “Yet tree-ring science is not utilized by water planners,” says Betancourt, who adds: “We invest considerable sums of money and effort in investigating nature and how it works, but all this information is underutilized.”

Whatever happened to the Anasazi at Mesa Verde or the people of Perry Mesa may never be solved, but the landscapes they left behind stand as haunting testaments to the turbulent time in which they lived. “It’s almost cliché to say that if you don’t understand history, you’re going to repeat some real mistakes you don’t want to,” says Metcalfe. “The fact is, when we include prehistory in North America, it gives us that much more to be mindful of.”