Stone Age humans found new use for fire: making tools

It appears that everything early humans needed to know to be considered "modern" they learned in Africa.

Perhaps as far back as 164,000 years ago, the residents of caves along what is now South Africa's southern coast used fire to harden stone for crafting into knives, spear points, and other cutting tools, according to a study by an international team of scientists published in the current issue of Science.

Until now, the earliest evidence for the process appeared in Europe and was dated to about 25,000 years ago.

Heat-treated stone flakes from the caves along South Africa's Mossel Bay suggest that "these early modern humans commanded fire in a nuanced and sophisticated manner," notes Kyle Brown, a paleoanthropology PhD candidate at the University of Cape Town and the field director for the site where the evidence emerged. The new study says the strongest evidence for the emergence of heat-treating points to 72,000 years ago.

Genetics and anatomy have shown that modern humans emerged in Africa between 100,000 and 200,000 years ago, researchers note. But it's been tougher to pin down when these people began to develop intellectually, as seen in advanced toolmaking or the use of symbols for expression, for instance.

Two years ago, researchers working the Mossel Bay sites found evidence for the use of the pigment ocher roughly 164,000 years ago, suggesting that the residents had developed some early form of symbolic expression – perhaps using the pigment as body decoration.

Test of fire

The stone flakes recently discovered in these caves were made of silcrete, a rock that forms from thin layers of silica-rich soils. Silcrete is hard, but like concrete, it's not easy to shape in its natural form into flakes useful for knives, spear tips, or other cutting tools.

Besides, the color of the flakes was unusual compared with silcrete found in rock outcrops, says Mr. Brown. The team also found a large silcrete flake in an ash deposit in one of the caves, but someone could have simply dropped it there by accident. Or, the ash could have resulted from the burning of organic material in the cave after the flake was formed.

But the find did pose the question of whether heat would turn such a poor raw material for sharp tools into something more suitable. So one evening, Brown placed some raw silcrete into the expedition's fire pit, covered it with hot coals, and let it bake. The next morning, it had taken on the color of the flake found in the cave. And it was easily worked into sharp-edge flakes.

Dawn of engineering

That still left the problem of figuring out whether the inhabitants heat-treated the rocks to make into tools, or the artifacts had been exposed to wildfires or to burning organic matter in the caves afterward.

The team answered that by selecting 26 samples from their dig that did not come from soil layers affected by fire. Then they exposed the samples to three independent lab tests for heat treatment. The tests determined that the flakes didn't come from stones "pretreated" by wildfires. The verdict: All 26 samples had been intentionally heat-treated.

Other, later cultures, including Australia's aborigines and native Americans, have used a similar process to turn mediocre raw materials into something they could readily use.

"Heat treatment technology begins with a genius moment," and then the approach gets passed down through succeeding generations, with each adding its own improvements to the process, explains Curtis Marean, an Arizona State University paleoanthropologist who is directing the project in South Africa.

It's a development process that is unique to humans, he adds.

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