Wet climatic spells didn’t quench wildfires in East Africa’s Turkana basin 1.75 million years ago. Instead, they fuelled them. That’s what chemical traces from ancient plants in the region seem to suggest, and this pattern could have shaped our hominin ancestors’ evolution, including the use of fire and tools.
Rich in hominin fossils and archaeological finds, the Turkana basin – sometimes dubbed the Cradle of Humankind – has long been considered important for understanding the lives of our early ancestors. These include Homo erectus, a species that appeared about 2 million years ago and that is regarded as one of our earliest recognisably human ancestors. The first truly complex stone tools were found here, too, in layers dating back to around 1.76 million years ago.
Compared with earlier tools, these “Acheulean” tools were more advanced, and included refined multi-purpose hand axes, useful for both butchery and plant processing. The Acheulean technology then persisted for the next 1.5 million years. The 85-million-year timeline of human evolution One idea is that ancient hominins evolved the greater cognitive flexibility required to produce Acheulean tools – and to begin controlling fire – in response to dramatic environmental swings and rapidly changing ecosystems.
But although previous studies have found evidence in favour of this “variability selection hypothesis”, establishing exactly what kinds of shifting environmental conditions may have driven these cognitive and technological advances has proved challenging. Now, Rachel Lupien at Aarhus University, Denmark, and her team think they may have an answer. They analysed chemical signatures in sediments that formed in the Turkana basin over a 21,000-year period, roughly 1.75 million years ago.
With the information, they could reconstruct rainfall, vegetation and wildfire patterns when the earliest Acheulean tools were in use. An incredible Denisovan skull is upending the story of human evolution The chemical signatures came from ancient leaf waxes and plant combustion, revealing how the nature of the vegetation shifted from woody to grassy conditions, and how wildfire intensity changed across time. They found that the landscape was predominantly forested with limited wildfires for the first 5000 years of the study interval.
It then became far more variable for the next 5000 years, fluctuating between very dry grasslands with occasional wildfires, perhaps caused by lightning strikes, and wetter forested conditions with far fewer fires. But during the final, longer period of the study interval, something surprising happened: rainfall increased significantly to produce monsoon conditions – and the number of wildfires rose, too. Meanwhile, more fires limited vegetation expansion, which created a feedback loop.
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