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Scientists adjust their picture of the Amazon in the age before Columbus

Rhett A. Butler / mongabay.com

New evidence challenges the idea that the Amazon Basin was densely inhabited before the arrival of Europeans.

The historical portrayal of the Amazon Basin's residents before 1492 has swung from the stereotype of backward savages to a vision of sophisticated stewards of the land — but a newly reported survey suggests that wide swaths of the Amazon's forests, particularly in the western and central regions, were relatively untouched by humans.

The findings could play into the debate over the Amazon's future as well as its past.

""You can't use an idea of past transformed landscapes to justify modern deforestation," Crystal McMichael, a paleoecologist who analyzed Amazonian soil as part of her research at the Florida Institute of Technology, told me. McMichael is the lead author of a study published in today's issue of the journal Science.

She and her colleagues collected 247 core samples of soil from 55 sites throughout the central and western Amazon, in Brazil and Peru, to check for signs of human disturbance. Their objective was to provide a reality check for what some researchers have called the "1491 hypothesis": the idea that areas of the Amazon Basin were intensely managed centuries ago, but reverted to a more natural state after the arrival of explorer Christopher Columbus and his European brethren, due to the decline of indigenous culture.

One of the foremost critics of that view is Dolores Piperno, a senior scientist at the Smithsonian Institution's National Museum of Natural History and the Smithsonian Tropical Research Institute. Piperno is a co-author of the Science paper.

"Drawing on questionable assumptions, some scholars argue that modern Amazonian biodiversity is more a result of widespread, intensive prehistoric human occupation of the forests than of natural evolutionary and ecological processes," she said in a Smithsonian news release. "Climatologists who accept the manufactured-landscapes idea may incorporate wholesale prehistoric Amazonian deforestation, widespread fires and carbon emissions into their models of what caused past shifts in atmospheric carbon dioxide and methane levels. But we need much more evidence from Amazonia before anything like that can be assumed."

The evidence from the soil samples, including samples taken from sites with previously known human impacts, runs counter to those assumptions. Most of the samples showed little sign of charcoal, which would have been left behind by land-clearing fires. There were few signs of silica deposits known as phytoliths, which are indicators of ancient agriculture. The researchers did pick up the signature of "terra preta" — that is, earth enriched by human waste — but mostly around riverbanks rather than far into the forest.

Crystal McMichael

Researchers Crystal McMichael and Monica Zimmerman collect soil samples in the tropical rainforests of Peru.

"Together, the data suggest that human population densities in the sampled regions were low and highly localized, and were not consistent with major population centers with associated areas of widespread, extensive agriculture," the researchers wrote.

The findings came as no surprise to Michael Heckenberger, an anthropologist at the University of Florida. Heckenberger is perhaps best-known for his study of ancient urban communities in the Upper Xingu region of the Brazilian Amazon, east of the areas surveyed by McMichael and her colleagues. His work was discussed in "The Lost City of Z," a best-selling book by David Grann.

"I was delighted to see the paper, because it does act as a cautionary note," Heckenberger told me. 

Heckenberger said the research fits in with the view that the pre-Columbian Amazon Basin had wide areas of forest land that showed relatively little human alteration, as well as areas that supported substantial concentrations of human population.

"This clearly has moved the debate forward," he said. "I hope we don't digress back to [a debate over whether] the Amazon was the setting par excellence for primordial forests and primitive tribes vs. an area that was dominated by large, complex societies. It's neither one nor the other. ... There were patches of dense, complex societies, and then there were other areas that were, if not completely untouched, then something very like untouched forest."

Heckenberger said he was "still of the opinion that as time progresses, we're going to find more and more of the Amazon that did support large populations." But he praised the work published in Science and said he hoped to see more sampling of sites from broader stretches of the Amazon Basin.

"I'd love to grab that team and bring them to my research site, to use that to some degree as a control against what you might expect," he told me. "The flip side of that is to jump into the pickup truck with that team and look for archaeological signatures in the area that they've been studying."

McMichael thought that was a fine idea. "He's done some excellent work," she said of Heckenberger.

She speculated that pre-Columbian tribes preferred to live near rivers rather than in the forest interior "so they could connect with other communities" more easily. She also suspected that the eastern side of the Amazon Basin was settled more intensely than the western side because it was drier and more amenable to forest-clearing. However, even if large settlements existed in some parts of the Amazon before Columbus, that shouldn't be used as a defense for 21st-century deforestation, McMichael said.

"The amazing biodiversity of the Amazon is not a byproduct of past human disturbance," she said in a news release. "We also can't assume that these forests will be resilient to disturbance, because many have never been disturbed, or have only been lightly disturbed in the past. Certainly there is no parallel in western Amazonia for the scale of modern disturbance that accompanies industrial agriculture, road construction, and the synergies of those disturbances with climate change."

More about Amazonian culture:

In addition to McMichael and Piperno, authors of "Sparse Pre-Columbian Human Habitation in Western Amazonia" include M.B. Bush, M.R. Silman, A.R. Zimmerman, M.F. Raczka and L.C. Lobato.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.