In southern Colorado, the ruins of a home of the Anasazi people, who went south for reasons that are unclear.

By GEORGE JOHNSON

Perched on a lonesome bluff above the dusty San Pedro River, about 50 kilometers east of Tucson, Arizona, the ancient stone ruin archaeologists call the Davis Ranch Site doesn’t seem to fit in. Staring back from the opposite bank, the tumbled walls of Reeve Ruin are just as surprising.
Some 700 years ago, as part of a vast migration, a people called the Anasazi, driven by unknown motives, wandered from the north to form settlements like these, stamping the land with their unique style.
“Salado polychrome,” says a visiting archaeologist turning over a shard of broken pottery. Reddish on the outside and patterned black and white on the inside, it stands out from the plainer ware made by the Hohokam, whose territory the wanderers had come to occupy.
These Anasazi newcomers - archaeologists have traced them to the mesas and canyons around Kayenta, Arizona, not far from the Hopi reservation - were distinctive in other ways. They liked to build with stone (the Hohokam used sticks and mud), and their kivas, or underground ceremonial chambers, like those they left in their homeland, are unmistakable: rectangular instead of round, with a stone bench along the inside perimeter, a central hearth and a sipapu, or spirit hole, symbolizing the passage through which the first people emerged from mother earth.
“You could move this up to Hopi and not tell the difference,” said John A. Ware, the archaeologist leading the field trip, as he examined a Davis Ranch kiva. Finding it down here is a little like stumbling across a Asian pagoda on the African veldt.
For five days in late February, Dr. Ware, the director of the Amerind Foundation, an archaeological research center in Dragoon, Arizona, was host to 15 colleagues as they confronted the most vexing and persistent question in Southwestern archaeology: Why, in the late 13th century, did thousands of Anasazi abandon Kayenta, Mesa Verde and the other magnificent settlements of the Colorado Plateau and move south into Arizona and New Mexico?
Scientists once thought the answer lay in impersonal factors like the onset of a great drought or a little ice age. But as evidence accumulates, those explanations have come to seem too precise - and deterministic. The Anasazi (or Ancient Puebloans ) were presumably complex beings with the ability to make decisions, good and bad, about how to react to a changing environment.
Some archaeologists are studying the effects of warfare and the increasing complexity of Anasazi society. They are looking deeper into ancient artifacts and finding hints of an ideological struggle .
“The late 1200s was a time of substantial social, political and religious ferment and experimentation,” said William D. Lipe, an archaeologist at Washington State University.
“You can’t have a situation where it just happens that hundreds of local communities for their own individual, particularistic reasons decide to either die or get up and move,” Dr. Lipe said. “There had to be something general going on.”
Archaeologists have proposed that colder weather contributed to the downfall. Growing seasons were becoming shorter. But the combination may have been less than a decisive blow. Soon after the abandonment, the drought lifted. Though the rains returned, the people never did.
In the remains of Sand Canyon Pueblo, in the Mesa Verde region, Kristin A. Kuckelman of the Crow Canyon Archaeological Center in Cortez, Colorado, sees the story of a tragic rise and fall.
As crops withered, the inhabitants reverted from farming maize to hunting and gathering. Defensive fortifications were erected to resist raiders. The effort was futile. Villagers were scalped, dismembered, perhaps even eaten. The pueblo was burned and abandoned. Curiously, the victors didn’t stay to occupy the conquered lands.
Ultimately the motivation for the abandonments may lie in the realm of ideology. By studying changes in ceremonial architecture and pottery styles, Donna Glowacki, an archaeologist at the University of Notre Dame in Indiana, is charting the rise of what may have been a new puebloan religion. For more than a century, the established faith was distinguished by multistory “great houses,” with small interior kivas, and by much larger “great kivas” - round, mostly subterranean and covered with a sturdy roof. Originating at Chaco Canyon in northwest New Mexico, the temples seem designed to limit access to all but a priestly few.
Though Chaco declined as a regional religious center during the early 1100s, the same architecture spread to the Mesa Verde area. But by the mid 1200s, a different style was also taking hold, with plazas and kivas that were uncovered like amphitheaters - hints, perhaps, of a new openness.
Though the dogma may be irrecoverable, Dr. Glowacki argues that it rapidly attracted adherents. Then around 1260, before the drought, the residents began leaving the pueblo, perhaps spreading the new ideology.
Amid the swirl of competing explanations, one thing is clear: The pueblo people didn’t just dry up and blow away like so much parched corn. They restructured their societies, tried to adapt and when all else failed they moved on.