As Europeans spread across the New World, native Americans were overmatched. People who had never even learned how to write were up against soldiers with muskets - and new diseases they had no immunity against.

But lost in all of the anthropological speculation is any real evidence; did Europeans wipe out native populations with disease and war shortly after their first contact, and did it happen so fast it left tell-tale fingerprints on the global climate?

For obvious reasons, other social scientists dispute such climate change and infectious disease appropriation of the 15th and 16th centuries. 

Matt Liebmann from the Department of Anthropology at Harvard, and colleagues instead say that evidence in northern New Mexico shows that disease didn't break out until nearly a century after the first European contact with Native Americans,  which overturns the impact of disease as a driver of native American decline.

Then somehow, generations later, disease did strike and 60 years after that, native populations dropped from approximately 6,500 to fewer than 900 among the 18 villages they investigated.  

"In the Southwest, first contact between native people and Europeans occurred in 1539," Liebmann said. "We found that disease didn't really start to take effect until after 1620, but we then see a very rapid depopulation from 1620 to 1680. (The death rate) was staggeringly high -- about 87 percent of the Native population died in that short period.

"Think about what that would mean if you have a room full of people and nine out of 10 die. Think of what that means for their social structure, if they're losing the people who know the traditional medicine, their social and religious leaders, think of the huge impact it would have on their culture and history." 

What else occurred due to fewer humans? Contrary to what environmentalists claim, less population was not a good thing. Forest fires raged out of control precisely because humans were not heating and cooking and building. 

This flies in the face of claims that a new geological era -- dubbed the Anthropocene -- began around 1610, when, according to ice core records show, global CO2 levels dropped dramatically due to Europeans killing off native Americans.

"The argument hinges on the notion that the depopulation of the Americas was so extreme that it left its mark on the atmosphere and climate at global scales," Liebmann said.

Looking at the patterns of fires in the tree rings, they could see that up until about 1620, fires were small and sporadic. Human development was acting as literal fire breaks. But as the forest started re-growing, much more widespread fires occurred. That continued until almost exactly 1900, when a combination of increased livestock grazing and a change in federal forest management policies began to suppress all fires.

Californians recently learned that lesson as well, as environmental lawsuits that prevented any real forest management have led to runaway wildfires on an annual basis, which ultimately hurts the ecology more than sensible logging.

How and when depopulation happened, and the ecological fallout from it, is far more complex than scholars have previously claimed.

Published in the Proceedings of the National Academy of Sciences.