There's a mythology about the native Americans, that they were all peaceful and in harmony with nature - it's easy to create narratives when there is no written record.

But archeology keeps its own history and a new paper finds that the 20th century, with its hundreds of millions dead in wars and, in the case of Germany, China, Russia and other dictatorships, genocide, was not the most violent - on a per-capita basis that honor may belong to the central Mesa Verde of southwest Colorado and the Pueblo Indians.

Writing in the journal American Antiquity, Washington State University archaeologist Tim Kohler and colleagues document how nearly 90 percent of human remains from that period had trauma from blows to either their heads or parts of their arms.

"If we're identifying that much trauma, many were dying a violent death," said Kohler. The study also offers new clues to the mysterious depopulation of the northern Southwest, from a population of about 40,000 people in the mid-1200s to 0 in 30 years.

From the days they first arrived in the Southwest in the 1800s, most anthropologists and archaeologists have downplayed evidence of violent conflict among native Americans.

"Archaeologists with one or two exceptions have not tried to develop an objective metric of levels of violence through time," said Kohler. "They've looked at a mix of various things like burned structures, defensive site locations and so forth, but it's very difficult to distill an estimate of levels of violence from such things. We've concentrated on one thing, and that is trauma, especially to the head and portions of the arms. That's allowed us to look at levels of violence through time in a comparative way."

It wasn't just violent deaths that poke holes in the harmony with the land and each other myth. A paper in June in the Proceedings of the National Academy of Sciences found that the Southwest also had a baby boom between 500 and 1300 that likely exceeded any population spurt on earth today. The northern Rio Grande also experienced population booms but the central Mesa Verde got more violent while the northern Rio Grande was less so.

Kohler has conjectures on why. Social structures among people in the northern Rio Grande changed so that they identified less with their kin and more with the larger pueblo and specific organizations that span many pueblos, such as medicine societies. The Rio Grande also had more commercial exchanges where craft specialists provided people both in the pueblo, and outsiders, specific things they needed, such as obsidian arrow points.

But in the central Mesa Verde, there was less specialization.

"When you don't have specialization in societies, there's a sense in which everybody is a competitor because everybody is doing the same thing," said Kohler. But with specialization, people are more dependent on each other and more reluctant to do harm.

If that sounds like rationalization based on Harvard psychologist Steven Pinker's The Better Angels of Our Nature: Why Violence Has Declined, it is. 

"Pinker thought that what he called 'gentle commerce' was very important in the pacification of the world over the last 5,000 years," said Kohler. "That seems to work pretty well in our record as well."

The episode of conflict in Southwest Colorado seems to have begun when people in the Chaco culture, halfway between central Mesa Verde and northern Rio Grande, attempted to spread into Southwest Colorado.

From 1080 to 1130, the Chaco-influenced people in Southwest Colorado did well. In the mid-1100s, there was a severe drought and the core of Chaco culture fell apart. Much of the area around Chaco lost population, and in 1160, violence in the central Mesa Verde peaked. Slightly more than a century later, everyone left that area, too.

"In the Mesa Verde there could be a haves-versus-have-nots dynamic towards the very end," said Kohler. "The people who stayed the longest were probably the people who were located in the very best spots. But those pueblos too were likely losing population. And it might have been the older folks who stuck around, who weren't so anxious to move as the young folks who thought, 'We could make a better living elsewhere.'" Older, or with too few people to marshal a good defense, the remaining people in the Mesa Verde pueblos were particularly vulnerable to raids.

At least two of the last-surviving large pueblos in the central Mesa Verde were attacked as the region was being abandoned. Some of their inhabitants probably made it out alive, but, says Kohler, "Many did not."