About 70,000 years ago, people in South Africa were shoving rocks into fires to heat them and make it easier to break off flakes that could be used as tools. The first detailed study looking at this process has revealed how it was done.
Heat treatment to make tool edges sharper and straighter was used widely around the world in the Late Stone Age, and probably invented independently by many different peoples. Stones modified in this way 70,000 years ago at several sites in South Africa are the oldest evidence of the consistent use of the process, although much older sporadic examples exist, and nearby sites are of similar age. It took 50,000 years before similar heat treatment of tools became widespread in Europe and Asia.
Dr Anne Delagnes, of the University of Bordeaux, found that 90 percent of the silcrete tools at the Klipdrift Shelter in the southern Cape had been through fire. Silcrete is a type of stone common in Africa and Australia. In its raw form it is difficult to work with, but high temperatures change the stone structure, making it more brittle and less porous, after which it is easier to break off flakes suitable for use as tools.
In PLOS ONE, Delagnes reports that heating occurred early in the process of turning rocks into tools, with what she and her co-authors call “post-heating scars”, where tools were broken off the rock. Many tools showed signs of also having been exposed to fire after completion, but this may have been accidental rather than part of the tool-making process; the floors of living areas were extensively and repeatedly burnt for unknown reasons.
Occasional heating fractures suggest the stones were inserted directly into embers, rather than slow-heated in warm sand. Non-silcrete tools at the site were not similarly heated.
A: The contrast between rough and smooth surfaces identifies a pre-heating scar, B: A heat-induced fracture, C: Post-heating scars, D: Potlid fractures caused by sudden heating. Delagnes et al/PLOS ONE
Silcrete was so prized in the Middle Stone Age that implements made from it have been found 200 kilometers (130 miles) from where they were quarried, although the people living at Klipdrift had a much shorter journey to obtain their materials.
Delanges and her co-authors describe the deliberate heat treatment of silica rocks as a “major technological milestone in prehistory... It provides the first evidence of a transformative technology, and marks the emergence of fire engineering.”
The more we learn about the way some animals use tools, the more sophisticated they appear to be. Certain birds even appear to have mastered spreading fire, long thought to be an exclusively human activity. Some stone tools previously thought to have been made by our ancestors might be the products of monkeys instead.
Still, the combination of fire and stone flaking is distinctively human, marking a point where our technology took a step no other species has replicated.