Humans Burnt Down Forests For Farming 1000 Years Ago, Study Finds

Christine Daniloff/MIT

Deforestation is usually the calling card of modern industrial economies. However, according to a recent study, humans in Madagascar burnt down large areas of forest land for agriculture as early as 1,000 years ago. The study was recently published in the journal Quaternary Science Reviews.

Remarkably, they arrived at this conclusion through a pair of 2-meter (6.5-foot) stalagmites found in a Madagascan cave. Although they’re hidden deep in underground caves, stalagmites actually provide an extremely accurate record of the ecosystem above them. They are essentially preserved groundwater deposits that can document water composition through the ages as they form layer upon layer.

The team of scientists from the Massachusetts Institute of Technology (MIT) and the University of Massachusetts looked at the calcium carbonate composition of these two stalagmites. They discovered that the layered composition of the stalagmites underwent a relatively quick and dramatic change in carbon isotope ratios. This change suggests that the land above underwent some kind of transition from heavy tree and shrub coverage to grassland.

The data was obtained by looking at the ratio of two radioactive isotopes, uranium to thorium, in the different layers of these stalagmites. Through this technique, they revealed that the shift happened in less than a century, around 1,000 years ago.

This change alone could have been by caused some kind of massive weather event or climate change. However, while the carbon isotope ratio showed a big change, the analysis showed that the stalagmites' oxygen isotopes remained the same at the same time. This eliminated any suggestions that this could be caused by climate change or any drop in rainfall.

And among sediment deposits from ancient lakes in the same region of the stalagmites, they found high amounts of charcoal microparticles – a clear indication of fires.

Burning trees to clear land for agriculture, infrastructure and industry is still a common practise today in tropical rainforests. Image credit: akiyoko/Shutterstock.

But this fire was no act of mindless destruction. The shift neatly coincides with the time that humans started to practise agrarian agriculture and bring cattle over to Madagascar. The authors believe that humans scorched the land to make way for agriculture, in perhaps one of the earliest examples of “slash-and-burn” land clearing.

"The transition from ephemeral forager to dedicated agro-pastoralist occurred, probably across Madagascar, around 1,000 years ago," David McGee, from MIT, said in a statement.

"We know that a dramatic landscape transformation occurred in the northwest.  We know that this transformation was not triggered by climate change. But we don’t yet know whether similar shifts, also unrelated to natural aridification, occurred elsewhere on the island, and if so, when, exactly. We are currently seeking to answer these questions."

Around this time, there was an extinction of giant lemurs and drops in pygmy hippo and giant tortoise populations. With further research, the team hope to see if the forest burnings also had a destructive effect on Madagascar’s wildlife, along with the island's plant life.

Laurie Godfrey, professor of anthropology at University of Massachusetts Amherst, who worked on the study, concluded: "I think this is one more piece of evidence that human impacts on the environment don't just start with Europeans and the Industrial era."

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