Wet climatic spells didn鈥檛 quench wildfires in East Africa鈥檚 Turkana basin 1.75 million years ago. Instead, they fuelled them. That鈥檚 what chemical traces from ancient plants in the region seem to suggest, and this pattern could have shaped our hominin ancestors鈥� evolution, including the use of fire and tools.
Rich in hominin fossils and archaeological finds, the Turkana basin 鈥� sometimes dubbed the 鈥� has long been considered important for understanding the lives of our early ancestors. These include听Homo erectus, a species that appeared about 2 million years ago and that is regarded as one of our earliest recognisably human ancestors.听
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 鈥�Acheulean鈥� 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.
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One idea is that ancient hominins evolved the greater cognitive flexibility required to produce Acheulean tools 鈥� and to begin controlling fire 鈥� in response to dramatic environmental swings and rapidly changing ecosystems. But although previous studies have found evidence in favour of this 鈥渧ariability selection hypothesis鈥�, establishing exactly what kinds of shifting environmental conditions may have driven these cognitive and technological advances has proved challenging.
狈辞飞,听听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听rainfall, vegetation and wildfire patterns when the earliest Acheulean tools were in use.听听
鈥淏ecause these three powerful climate and ecosystem indicators are all from the same sample, we could directly look at how their relationships vary through time with no guesswork,鈥� says Lupien.听
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 鈥� and the number of wildfires rose, too.听
鈥淲e find that higher precipitation leads to more vegetation growth, which allows fires to burn because of the additional biomass,鈥� explains Lupien. Meanwhile, more fires limited vegetation expansion, which created a feedback loop. 鈥淲hat we can say from our data is that if听humans were taking advantage of fire in Turkana, it would have been during these鈥� wet intervals when monsoons were magnified.鈥�
鈥淗ere we鈥檙e now beginning to see the definite evidence that bushfires were there in their world,鈥� says听听at the University of Liverpool, UK, who wasn鈥檛 involved in the analysis. It wouldn鈥檛 be surprising if ancient hominins then experimented with using those fires to improve their access to resources, he says.听
For instance, frequent natural fires would have increased hominin opportunities for 鈥榝ire foraging鈥�: gathering up animals or plants that had been killed and cooked in the flames. It would also have allowed hominins to experiment with taking burning branches from natural wildfires and feeding them to keep fires burning longer, so they could cook food for themselves.
鈥淚t is far easier to keep a fire going than it is to start one,鈥� says听, a study co-author at the University of Liverpool, UK. 鈥淚n this way, hominins could have started to reap the benefits of fire long before they were able to create it.鈥�
听presented evidence of this type of fire use听between 1.07 million and 1.79 million years ago at South Africa鈥檚 Wonderwerk Cave. Tentative evidence of fire use in Turkana dates back to around 1.5 million years ago.听
The appearance of Acheulean stone tools during this interval may be further evidence of behavioural changes linked to the extremely variable environments in which ancient hominins evolved. at the Smithsonian Institution in Washington DC, who pioneered the variability selection hypothesis in the 1990s, thinks a link is plausible.听
鈥淭his is the first paper I鈥檝e encountered to illustrate the emergence of the Acheulean as a response to ecological variability and the process of variability selection,鈥� he says. 鈥淓pisodic and intense natural fires offer a novel contribution.鈥�
Quaternary Science Reviews
