A Stone Age campsite on the coast of South Africa has revealed the earliest evidence of early humans who used fire to make better, sharper stone tools. Researchers had been surprised to find spear points and other stone implements made of silcrete, a crumbly rock that doesn’t respond well to the flaking, chipping process that early tool-makers employed. But lead researcher Kyle 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 [New Scientist].
Brown buried silcrete stones in a fire pit and kept a roaring fire going for up to 10 hours at a time. When the blaze eventually died down and the rocks had cooled, they looked different, with a new reddish sheen. They also had different physical properties. “The stone becomes harder and stiffer,” Brown says. “It basically becomes more brittle, which is great if you are breaking something [and] you want it to break more easily” [NPR News]. The flakes from the treated stones were also sharper than those created from untreated silcrete.
The study, published in Science, pushes back the onset of “pyro-engineering.” Previously, scientists had believed the first fire-hardened tools were created 25,000 years ago in Europe, but the treated blades found in South Africa date from about 70,000 years ago. Researchers say the technique illuminates the transition between using fire to cook food (which early humans probably figured out about 800,000 years ago), and its more sophisticated use in pottery making and metal working. Brown says that by “72,000 years ago, people are doing more than just using fires for cooking, heat, light or protection…. I think heating stones is the dawn of human engineering” [BBC News].
Heat treatment of stones for toolmaking occurred in several steps that required complex thinking abilities, the researchers assert. Toolmakers buried selected pieces of stone beneath a fire at a campsite or workshop, probably for a day or more, they suspect. Stones were then removed and worked into shape as cutting tools [Science News]. The complicated process shows that the toolmakers had mastered advance planning, and Brown goes so far as to suggest that they must have had language as well, so that instructions could be passed down from generation to generation.
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Image: Science / AAAS. A replicated tool with blades made from heated silcrete.