Humanity’s environmental impact did not start with the bang of agriculture or industrialization but a whisper initiated long ago—one that scientists are finally learning to hear.
New archaeological and paleoenvironmental findings now date human activity that transformed our natural surroundings to more than 80,000 years ago,after early modern humans settled on the northern shores of Lake Malawi at the lower tip of eastern Africa’s Great Rift Valley. These humans dramatically modified the landscape and ecosystem by burning forests to yield a sprawling bushland that remains today, according to a report published on Wednesday in Science Advances.
The finding marks the oldest evidence yet of humans profoundly changing their environment with fire. And it could represent the earliest known case of people deliberately doing so, the researchers hypothesize. “It represents a really powerful cultural capacity to transform the landscape in a way ... that will enhance the survival of the people,” says archaeologist Amanuel Beyin of the University of Louisville, who was not involved in the new study.
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Lake Malawi is one of the world’s largest lakes today, but it has dramatically fluctuated in size across the ages. In a 2018 study, paleoecologist Sarah Ivory of Pennsylvania State University and her colleagues examined fossils, pollen and minerals in two sediment cores drilled from the lake bed. Their analysis revealed that the lake’s water level and vegetation exhibited a consistent climatic pattern over the past 636,000 years. Dense forests along the lake’s shores typically disappeared during drought periods when the lake ran dry and then returned when it filled up again.
But the pollen records showed an abrupt break from this cycle when the wet period returned about 86,000 years ago. Although the lake level was high again, the shoreline forests just briefly recovered before collapsing. Only some fire-tolerant and hardy species persisted, while grasses became more widespread in the landscape.
When Ivory discussed these data with Yale University paleoanthropologist Jessica Thompson and her colleagues, who were excavating nearby archaeological sites along the northern shores of the lake, an explanation came into focus: human activity. The first known settlements in the area pop up roughly 92,000 years ago, as evidenced by tens of thousands of stone artifacts found by Thompson and others with help from their colleagues in Malawi. Many were tools likely used in hunting and cutting. The researchers observed that the humans’ appearance was followed by a spike in charcoal deposits in the lake cores, suggesting that people started intensively burning the forest just as it was growing back, thereby preventing a full recovery.
Alternative explanations are possible. The charcoal deposits could instead have stemmed from a few fires that spiraled out of control or perhaps from people at that time burning timber for cooking or warmth. But Thompson proposes that the population deliberately burned the forests, as some hunter-gatherers do today. Cleared forest areas allow a patchwork of new grasses and shrubs to emerge, enabling a mosaic habitat with a variety of food sources that attract different animal species—and hence new prey for humans. Thompson thinks the scale of burning is more consistent with this kind of continuous habitat transformation than accidental fires or wood harvesting. Doing the latter efficiently would have required tools that were not available then, she adds.
The use of fire by human ancestors dates back at least a million years, scientists have found. But during the Middle Stone Age—between 315,000 and 30,000 years ago—humans began to wield fire in new ways. For instance, around 164,000 years ago in southern Africa, people likely used fire to heat stone to render it more malleable for toolmaking. “This realization that you could use fire ... as a tool to modify the productivity of your immediate environment” would be one of many inventions that took place in this broader period, Thompson says.
Previously, some of the oldest possible evidence of humans using fire to manage their environment stemmed from the Great Cave of Niah in Malaysian Borneo. Scientists hypothesize that humans 50,000 years ago used fire in a dense tropical forest near that location to foster the growth of specific plant species. Other studies propose similar activities about 45,000 years ago in New Guinea and 40,000 years ago in Australia.
It is not easy to prove that humans rather than climatic factors ignited such fires, notes Patrick Roberts, an archaeological scientist at the Max Planck Institute for the Science of Human History in Jena, Germany, who was not involved in the new study. But he thinks the evidence it unearthed around Lake Malawi makes a fairly convincing case that humans were the culprit—given the paleoenvironmental record in the lake cores that spans more than 600,000 years and the fact that those cores were extracted close to the archaeological site.
Although human intent is also hard to prove, Roberts says he sees no reason to assume that people were not cognitively capable of taking such action to make their environment more productive. “Why else would you go and set fire to the landscape?” he asks.
Beyin suggests that the early modern humans living around Lake Malawi may have been part of populations migrating from drier environments to the north or south. When they encountered unfamiliar forests there, he says, it is possible that they “may have resorted to using fire to create ... this familiar woodland environment.” The study also underscores the value of integrating ancient environmental records such as those documented in the lake cores with classic archeological data to detect clues to human cultural innovations, Beyin adds.
The ancient people appear to have left another impression on the landscape near Lake Malawi. After the forests disappeared, rain fell on deforested highlands, gradually eroding sediments to form large triangle-shaped deposits called alluvial fans. Over time, the process of erosion buried and preserved artifacts in the fan deposits. Thompson says she wouldn’t be surprised if more evidence of early modern humans’ environmental impact emerges over the coming years. “If we actually just think of this as something we associate with the human ‘condition...,’ if you shift your perspective that way,” she adds, “suddenly, I think, you’re going to see this stuff all over the place.”