Fire engineers of the Stone Age

Stone Age toolmakers didn’t need motivational speakers to get fired up at work. People living on the southern tip of Africa 72,000 years ago decided on their own to heat stones with carefully controlled fires in order to make the rock more suitable for tool manufacturing, a new study finds.

Coastal residents of southern Africa may even have heated stones as a first step in toolmaking as early as 164,000 years ago, say Kyle Brown, an anthropology graduate student at the University of Cape Town, South Africa, and his colleagues.

Until now, evidence of heat treatment extended back no further than about 25,000 years ago in Europe, Brown’s team notes in the Aug. 14 Science.

“We’ve shown that, at least 72,000 years ago and perhaps much earlier, modern humans had an understanding of how fire and heat could transform the materials around them to suit their needs,” Brown says. This technological development bridged the taming of fire for cooking, warmth and protection at least 750,000 years ago (SN: 5/1/04, p. 276) to the appearance of pottery around 30,000 years ago and metal working 5,500 years ago, in his view.

Brown’s results “push the antiquity of heat treatment back much earlier than previously supposed,” remarks archaeologist John Shea of Stony Brook University in New York. It’s not surprising that ancient Homo sapiens used fire in this way, he adds. In 2003, another team reported that modern humans living in the Middle East 90,000 years ago heated chunks of a yellow mineral to give it a reddish hue before grinding the mineral into pigment.

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. Aborigines in northwestern Australia engaged in much the same practice until a few decades ago.

At South Africa’s Pinnacle Point cave, the researchers first identified magnetic and molecular signatures of intense heating on 26 tools made of a type of stone called silcrete. The tools came from an excavation of sediment layers dating to between 72,000 and 47,000 years ago. The researchers estimate that the artifacts were heated at maximum temperatures of 300° to 400° Celsius.

High levels of surface gloss, based on a measure of the relative amount of light reflected by stone, further indicated that a majority of 129 stone artifacts from the same layers had been heated, probably before being made into tools.

Gloss levels on 24 Pinnacle Point stone tools with an estimated age of 164,000 years suggested that they too had been heated at high temperatures.

Brown’s group also experimented with modern samples of silcrete from the Pinnacle Point area. They found that intense heating makes silcrete less brittle and allows sharpened edges to be created more precisely, the scientists say.

In contrast, Shea argues that heat treatment yields only marginal improvements in stone quality for the amount of time and energy it demands. He speculates that Stone Age heat treatment in southern Africa represented a kind of conspicuous consumption. Its practitioners may have wanted to gain the upper hand in rivalries for mates and social status that intensified within rapidly growing populations.

South African sites have also provided remnants of symbolic behavior by humans at around the same time as the earliest possible appearance of heat treatment. People ground pigments—apparently for decoration—around 164,000 years ago (SN: 10/20/07, p. 243) and carved geometric designs into pigment chunks 100,000 years ago (SN Online: 6/12/09).

Technological and symbolic behaviors typical of modern humans did not first appear 50,000 years ago, as some researchers have argued, contends anthropologist and study coauthor Curtis Marean of Arizona State University in Tempe. Marean suspects that such behaviors spread rapidly after the evolutionary launch of modern humans around 200,000 years ago.

Anthropologist Sally McBrearty of the University of Connecticut in Storrs considers it more likely that modern human behavior developed slowly over several hundred thousand years. Ancient people gradually developed a series of practical applications for controlled fire beyond cooking, she proposes, including making wooden tools in a charring-and-scraping process and melting tree sap for adhesives. “I don’t think the use of heat treatment required a big cognitive leap,” McBrearty says.

Marean also hypothesizes that heat-treatment expertise gave modern humans a leg up on Neandertals, who apparently lacked advanced toolmaking skills. But unlike Pinnacle Point people, Neandertals had access to good-quality flint for tools, comments archaeologist Nira Alperson-Afil of Jerusalem’s Hebrew University, so they may not have needed heat treatment.