Humans have used fire for millennia to lure out game when hunting and to convert woodland to agricultural land, leaving their mark on the landscape. New archaeological and paleoenvironmental evidence from Lake Malawi, Africa, shows that the effects on the landscape of humans' use of fire is tens of thousands of years older than previously thought, according to an international team of researchers.
"The vegetation in the area around Lake Malawi is a little mysterious," said Sarah Ivory, assistant professor of geosciences at Penn State. "It's right in the middle of the tropics, and we think of tropical forests as being these icons of biodiversity where all the world's species are housed. Yet here, in the middle of tropical Africa, is this extensive forest that is really species poor."
Fire geographers and modern ecologists have thought that humans may have played a role in shaping the landscape, but how long ago humans impacted the landscape and exactly how they did so remained unclear until now, Ivory said.
The researchers studied dense clusters of stone artifacts, some dating as far back as 92,000 years, collected from the northern shores of the lake, and fossil pollen and charcoal samples taken from a sediment core drilled from the lakebed. The evidence, when analyzed separately, raises more questions than answers, but combined it tells a story of early modern humans using fire in a way that prevented regrowth of the region's forests and created the sprawling bushland that exists today, according to the researchers. They reported their findings today, May 5, in Science Advances.
"This is the earliest evidence I have seen of humans fundamentally transforming their ecosystem with fire," said Jessica Thompson, assistant professor of anthropology at Yale University. "It suggests that by the Late Pleistocene, humans were learning to use fire in truly novel ways. In this case, their burning caused replacement of the region's forests with the open woodlands you see today."
The scientists identified the artifacts as being of Middle Stone Age type, which describes a widespread way of making stone tools across Africa for at least the last 315,000 years. During this time, the earliest modern humans made their appearance, with the African record showing the earliest significant advances in cognitive and social complexity.
"We got interested in the region because we wanted to investigate the start time of the Middle Stone Age," said David Wright, professor of archaeology at the University of Oslo, Norway. "But we couldn't understand why all of our sites were dating to the end of it."
Thompson and Wright had logged several field seasons of archaeological work before a conversation with Ivory made the patterns in their data make sense.
"Of the entire 600-thousand-year record captured in the sediment core, the last 85,000 years were the most ecologically interesting," said Ivory, who also holds an appointment in Penn State's Earth and Environmental Systems Institute. "I suspected that humans might be responsible for the unusual patterns I was seeing in the charcoal and vegetation records, and the archaeological data supported it."
A montane forest in south-central Africa. The researchers think the study area in Malawi used to look like this forest, high in biodiversity and dense with vegetation, before humans' use of fire tens of thousands of years ago permanently altered the landscape.
Lake Malawi is the world's fifth-largest freshwater lake, but its water levels have fluctuated over the ages. During the lake's driest periods, the last of which ended about 85,000 years ago, it shrunk into two small, saline bodies of water. The lake recovered from these arid stretches and its levels have remained high ever since.
Despite the consistently high lake levels, which implies greater climatic stability than the previous interval, the fossilized pollen samples showed that the number of distinct species inhabiting the region went flat following the last arid period.
"Over previous climate cycles in the lake, rainy environments had produced forests that provide rich habitat for an abundance of species," Ivory said. "The pollen that we see in this most recent period of stable climate is very different than before. Specifically, trees that indicate dense, structurally complex forest canopies are no longer common and are replaced by pollen from plants that deal well with frequent fire and disturbance."
A spike in charcoal accumulation occurred shortly before the flattening of the region's species richness, the scientists said. An alluvial fan - an accumulation of sediment eroded from the region's highlands - that developed during steady lake levels after the last arid period is dense with archaeological sites. The burning and climate-driven changes set the conditions that preserved so many artifacts in the region, the researchers said.
Previous transitions from dry to wet conditions did not result in a similar alluvial fan, nor were they preceded by the same charcoal spike.
The long-term scale of such impacts is typically associated with farmers and herders rather than hunter-gatherers. This suggests that hunter-gatherers had important impacts on the landscape on par with modern people and may also explain why the archaeological record formed in the first place.
The traditional view of when people started modifying ecosystems in a way that would be visible in the geologic record extends, in some places, back to the advent of farming 12,000 years ago, said Ivory.
"But this paper shows that people have had this long relationship with fire," she added. "Fire clearly has been an important component in our evolutionary story, and this research identifies an interesting turning point. Humans, through their use of fire and being able to ignite their own fires, are shaping the landscape to benefit themselves. It's a shift in the balance of power from an ecological perspective, and the timing is right as modern humans are moving out of southern Africa."
National Geographic-Waitt Foundation, the Australian Research Council, the Wenner-Gren Foundation, the University of Queensland Archaeological Field School, the Korean Research Foundation Global Research Network, Deutsche Forschungsgemeinschaft, Emory University, the Belmont Forum, Purdue University, the Research Council of Norway, the National Science Foundation and the International Continental Scientific Drilling Program supported this research.
