Mysteries in the Indiana Jones saga tend to follow the kind of logical progression of a detective story.

The audience learns that thousands of years in the past, for example, the Ark of the Covenant was lost, literally, to the sands of time in a city swallowed up by the desert. During the course of the movie, the search begins, the clues are followed, the prodigious obstacles overcome and, at the end of the day, the Ark is found.

While the problems Jones faces may not be inconsequential, the structure of good storytelling tends to point the hero toward the light at the end of the tunnel. And even if the tunnel is filled with Nazis and other villains who need to be overcome, the goal, usually a single relic or artifact, almost always is in sight, which is a big help to the audience that has to follow the tale.

For real-life archaeologist Jeffery Clark, one of his biggest professional challenges was approached from the opposite direction.

To Clark, an NSF-funded researcher at the Center for Desert Archaeology in Tucson, Ariz., the "what" was obvious: sometime between 1300 and 1450—long before Europeans and accompanying diseases arrived—at least 50,000 people "disappeared" from the desert Southwest of the United States.

The "how" and the "why" were what fascinated him and his colleagues, whose work focuses on the causes and consequences of human migration.

Obviously, as a scientist, some sort of supernatural or paranormal explanation would not suffice.

While some might be tempted to say these people simply "vanished," that would not do either, for obviously something big happened to them that can be accounted for by environmental, economic or social processes.

After years of selective digging at sites and rummaging through old museum collections, Clark and his colleagues have found important clues that shed light on the fate of these people. Their research tells a story about environmental change and migration that not only affected people directly, but also how they interacted with each other. Their story is directly relevant to contemporary society, which is trying to cope with mass migrations related to climate change, social stress and economic inequality.

"We don't like to use the word 'vanished,' but some of these 50,000 people died, others left and still others remained, but became archaeologically invisible," he said. "Any way you look at it, before Coronado and the Spanish arrived, there was a major population collapse and dramatic change in the way people lived in the southern Southwest."

Using evidence gleaned from the ruins of ancient dwellings and the chemical analyses of pottery and stone tools, Clark and his team discovered a complex but strong correlation between the arrival of migrants from the drought-stricken Kayenta region of northeast Arizona in southern Arizona in the late A.D. 1200s, and heightened social tensions within the local Hohokam population.

Some Kayenta groups migrated more than 300 kilometers (almost 200 miles) to the river valleys of southern Arizona, where they encountered long-established Hohokam communities that used irrigation to farm the land.

What happened to the Kayenta groups varied widely, depending on their destination.

"You have to study the impact of this migration valley by valley," Clark says. "Each has a different story to tell. The whole spectrum of human relations is represented across the southern Southwest, from ethnic conflict and segregation to coexistence and close integration. Sometimes relations went back and forth within the same valley"

Based on the archaeological evidence, Clark says, a strong Kayenta migrant identity persisted for at least a century, despite the fact that they were in the minority status, had lost their homeland and were dispersed. The scattered migrant communities continued to maintain connections, even in exile, to their social and economic benefit, similar to Jewish, Armenian, Cuban and other communities in diaspora today.

These strong migrant ties presented a serious challenge to local Hohokam communities. The Hohokam initially responded by moving closer together, marking their territories and actively asserting their own identities.

In some valleys, cliff dwellings and other fortified settlements were built near the boundaries of migrant and local communities, and groups remained segregated and engaged in "ethnic" conflict.

But in other areas, migrant and local groups coexisted and closely integrated. They established trade relations and ultimately developed inclusive religions and hybrid identities that combined elements of both cultural traditions.

Regardless of the relationship between migrants and locals, people continued to live in close proximity, which caused local environmental degradation, poor health and population declines.

After several decades of population loss, boundaries between social groups slowly dissolved as they continued to coalesce into increasingly smaller territories to maintain irrigation systems and social networks that required a minimum population level. When this minimum level could no longer be met, groups either moved elsewhere or radically changed their lifestyle.

These problems were exacerbated by another interval of climatic change in the late 1300s and early 1400s.

Population loss accelerated as families with connections to more successful settlements outside the southern Southwest, immigrated or became invisible to archaeologists. Very few pre-contact archaeological sites in the region can be identified after A.D. 1450.

By "archaeologically invisible," Clark means that groups either adopted a highly mobile lifestyle, where they lived lightly on the land and left little behind, or they moved into areas like river floodplains, where settlements would have eventually been buried or eroded by subsequent floods.

Given the high population density in the southern Southwest today and its increasing dependency on "imported" water, Clark notes a similar collapse could happen.

"If some dramatic event disrupted our infrastructure or the climate became substantially drier, you'd have a drop in population from hundreds of thousands of people down to tens of thousands of people," he says. "Families would be scattered to the winds, so to speak, going to wherever they have friends and kin. To an archaeologist, it would look like a mass disappearance."

With a new NSF grant, Clark and colleagues will investigate what happened when the population of southern Arizona plummeted in the late 14th century. Limited evidence suggests some of the mixed Kayenta-Hohokam groups migrated into southwestern New Mexico, where they joined small Kayenta populations living in a relatively isolated frontier.

Based on the Kayenta and Hohokom experience, Clark believes that despite enormous differences in population size and technological level, the experience of several hundred years ago has lessons for today.

As large population movements become more common across the globe, because of economic and social stress, migration research will be increasingly important. Several variables that were vital to understanding Kayenta migration and subsequent relations with Hohokam groups can predict the outcomes of other migrations that bring societies and cultures into close contact.