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Prehistoric humans harnessed fire to make sharp stone blades, say archaeologists who have recreated ancient fire-hardened tools dug up in South Africa. At 47,000 to 164,000 years old, the blades may date from the dawn of modern human behaviour, involving not just complex tool use but also language and art.
"These people were extremely smart," says Kyle Brown, an experimental archaeologist at the University of Cape Town, South Africa. "I don't think you could have passed down these skills from generation to generation without language."
Such adaptations may have given the first modern humans to leave Africa the tools and know-how to conquer the world.
Heat treatment occurred to Brown – who sculpts stone tools in his field laboratory – after he had trouble transforming the local crumbly silcrete stones into anything resembling the sharp, thin blades his team recovered at sites around Still Bay, several hundred kilometres east of Cape Town. However, Brown noticed that many of the ancient blades bore the same glossy sheen as North American tools created from heat-treated stone.
"It seemed like the most logical thing to do was take some of this poor quality material that we've been collecting and put it under a fire and see what happens," he says.
The rock was transformed after spending 5 to 10 hours buried beneath a fire and at temperatures between 250 and 300 °C, Brown says. "Heat treatment makes the stone harder, stiffer and more brittle," he says, "and it makes it easier to flake." Flaking is the process of chipping sharp, small pieces off a large rock.
But these experiments alone don't prove that ancient humans cooked stones before making blades. So, Brown's team looked for signs of heat treatment in artefacts 47,000 to 164,000 years old that were recovered from two Still Bay sites.
Firing leaves its mark in iron-containing rocks because heating realigns the geomagnetic orientation that was recorded when they formed. Brown's team found evidence for magnetic reorientation in all 12 of the ancient blades they tested with this method.
High heat can also eject electrons trapped in certain minerals. Electrons trickle back into the mineral once it cools, but so slowly that they can show if a rock has been heated recently. Every one of the 26 stone blades that Brown's team analysed in this way had been heated.
These tests still don't prove that the blades were burned intentionally: they could have been made from unbaked stone and then caught in a bush fire. The "greasy" sheen Brown noticed on the blades gives away rocks heated before flaking, however, he says. Most of 153 stone tools his team studied showed this signature, including 164,000-year-old blades.
This pushes the earliest evidence for fire-treated tools back about 140,000 years and from Europe to Africa, not long after the emergence of Homo sapiens in eastern Africa.
Climatic shifts may have forced many of those first humans into fertile refuges in South Africa, where conditions were right for modern human behaviour to emerge, Brown says. Other evidence suggests these humans fished, painted and wore jewellery.
Ancient humans probably used heat-treated tools to hunt or butcher animals, though many of the blades are too small to wield by hand. Brown imagines weapons constructed with multiple blades that could swapped out if one broke.
These adaptations, he argues, were part of a "tool kit" that modern humans took to other parts of the world. In Europe and west Asia, for instance, hunting and butchering with the fired blades could have given humans a competitive edge over Neanderthals, who might have ignored poor-quality rock to maketheir own stone weapons.
However Shea questions whether heat-treated stone would have given modern humans the edge over Neanderthals. Pre-cooking stones would eat up time and energy better spent on other ways of making a living. "It is tempting to see everything early humans did as adaptively advantageous, but this is not necessarily the case," he says.
"South Africa is about as far as one can get from Europe and the Near East and still be in Africa. It is an unlikely place to look for behavioural innovations that led to humans dispersing successfully into Eurasia," he adds.