We sometimes think of the Amazon rainforest as unmodified by humans, a glimpse into the planet’s past. In recent years, scientists have learned that many parts of the Amazon are not spared at all – they have been cultivated by indigenous peoples for thousands of years, and just centuries ago there were towns. and agricultural land. But this is not the case everywhere. In a new study in PNAS, the researchers determined that a rainforest in the Putumayo region of Peru is home to a relatively unchanged forest for 5,000 years, which means that the people who lived there found a long-term way to coexist with nature – and the proof is in microscopic pieces of silica and charcoal in the soil.
“It’s very difficult, even for experienced conservationists, to tell the difference between a 2,000-year-old forest and a 200-year-old forest,” says Nigel Pitman, ecologist at the Field Museum in Chicago and co-author of PNAS paper. “A growing body of research shows that many Amazonian forests that we think of as wilderness are actually only 500 years old, as that is when the people who lived there died from the pandemics brought by the Europeans and that the forest has pushed back. “
“Far from implying that the complex and permanent human settlements in the Amazon had no influence on the landscape of certain regions, our study adds much more evidence indicating that most of the severe impact of the indigenous population on the forest environment was concentrated in nutrient-rich soils near rivers, and that their use of the surrounding rainforest was sustainable, resulting in no detectable loss or disturbance of species, over millennia, ”says Dolores Piperno, researcher at the Smithsonian Tropical Research Institute and first author of the study.
Many plants take up silica from the soil and use it to produce microscopic mineral particles called phytoliths that provide structural support. After a plant dies, these phytoliths persist in the soil for thousands of years. Different types of plants produce phytoliths in different shapes, which means that phytoliths in the soil can be used to determine what types of plants lived there in the past.
For this study, Piperno and his colleague Crystal McMichael at the University of Amsterdam needed soil samples from the Putumayo region, in the Amazon rainforest of northeastern Peru. This is where Pitman comes in. In his work with the Field’s Keller Science Action Center, Pitman participates in “quick inventories” of the Amazon, intensive information-gathering trips to document a region’s plants and animals and to build relationships with people. who live there, in order to help build a case for the protection of the area. Piperno and McMichael contacted Pitman, a botanist, and asked if he would be able to collect soil samples while he inventoried the trees in the Putumayo area.
“The three or four days that we spend at one of these sites makes us feel like we are running a marathon. We have to do a lot of things in a very short time, and so we get up really early, we get up really. late, and somehow these soil cores had to be taken at the same time, ”says Pitman.“ Sometimes we collected the soil at midnight, or during torrential rains, when we couldn’t. examine the trees. “
To collect the soil, Pitman and his colleagues, including Field Museum associates Juan Ernesto Guevara Andino, Marcos Ríos Paredes, and Luis A. Torres Montenegro, used a tool called an auger. “It’s a long metal pole with blades at the bottom, and when you plant it in the ground and rotate it, it carves a column of soil about 2 to 3 feet long.” The team took soil samples at different heights on the column, placed them in plastic bags, and brought them back to the United States for analysis.
The age of the soil roughly correlates with its depth, with newer soil at the top and older soil deeper in the earth. Back in the lab, the researchers used carbon dating to determine the age of the soil, then carefully sorted samples under a microscope, looking for phytoliths that would tell them what types of plants were living in the area at any given time. .
They found that the types of trees growing in the area today have grown there over the past 5,000 years – an indicator that unlike other parts of the Amazon, the Putumayo was not home to cities and of agricultural land before European colonization.
In addition to the phytoliths, the researchers also looked for microscopic lumps of charcoal. “In the western Amazon, where it’s wet all year round, finding charcoal tells you people were there,” says Pitman. “There are no natural forest fires from lightning, so if something is burning it’s because someone set it on fire.”
The low levels of charcoal in the soil show that although the forest remained unchanged by humans for 5,000 years, people lived in the area – they simply coexisted with the forest in a way that did not change it.
“One of the scary things for environmentalists about research showing that much of the Amazon was once cities and cultivated land is that people who are not environmentalists are allowed to say, “If that was the case, then you environmentalists get angry for no reason – 500 years ago half of the Amazon was cut down and everything grew back, it doesn’t matter . We don’t have to worry so much about the Amazon cull, we’ve done it before and it turned out to be good, ”says Pitman. This study suggests that while people are able to coexist with the wilderness without modifying it, the Amazon is not simply a resource that can be destroyed and pushed back from scratch in a matter of centuries.
“To me, these findings don’t say that the indigenous people were not using the forest, but simply that they were using it in a sustainable way and did not significantly change its species composition,” explains Piperno. “We did not see any decrease in plant diversity during the study period. It is a place where humans seem to have been a positive force on this landscape and its biodiversity for thousands of years.”
“This is an important and hopeful find, because it shows that people have lived in the Amazon for thousands of years, in a way that allows them to thrive and the forest to thrive,” Pitman said. “And since this particular forest is still protected by indigenous peoples, I hope this study reminds us all of how important it is to support their work. “