Evidence that early modern humans living on the coast of the far southern tip of Africa 72,000 years ago employed pyrotechnology – the controlled use of fire – to increase the quality and efficiency of their stone tool manufacturing process, is being reported in the Aug. 14 issue of the journal Science. An international team of researchers, including three from the Institute of Human Origins at Arizona State University, deduce that "this technology required a novel association between fire, its heat, and a structural change in stone with consequent flaking benefits." Further, their findings ignite the notion of complex cognition in these early engineers.
"Our illumination of the heat treatment process shows that these early modern humans commanded fire in a nuanced and sophisticated manner," says Kyle Brown, the lead author and a doctoral candidate at the University of Cape Town, and field and lab director in Mossel Bay, South Africa, for ASU's Institute of Human Origins.
"We show that early modern humans at 72,000 years ago, and perhaps as early as 164,000 years ago in coastal South Africa, were using carefully controlled hearths in a complex process to heat stone and change its properties, the process known as heat treatment," Brown says.
"Heat treatment technology begins with a genius moment – someone discovers that heating stone makes it easier to flake," says Curtis Marean, the project director and a co-author on the paper. Marean is a paleoanthropologist with the Institute of Human Origins and a professor in the School of Human Evolution and Social Change at Arizona State University's College of Liberal Arts and Sciences.
"This knowledge is then passed on, and in a way unique to humans, the technology is slowly ratcheted up in complexity as the control of the heating process, cooling and flaking grows in sophistication," Marean says.
This creates a long-chain technological process that the researchers explain requires a complex cognition, and probably language, to learn and teach.
The heating transformed a stone called silcrete, which was rather poor for tool making, into an outstanding raw material that allowed the modern humans to make highly advanced tools.
The eureka moment
The focus of Brown's research involves experimentally replicating the types of tools and production debris found at African archaeological sites to understand how and why people made and used these tools.
"In numerous field surveys with co-author David Roberts, who is a leading expert on silcrete formation, we were unable to locate stone outcrops with material that matched the fine-grained texture and often reddish color of the silcrete artifacts we excavated at Pinnacle Point," Brown says. "The silcrete we had collected was just not suitable for tool production."
Most of the silcrete they found was intensively flaked. It was unusual to find a piece larger than a few centimeters. However, one day in 2007, while Brown and Marean were at the Pinnacle Point Site 5-6 (PP5-6) they found a huge flake of silcrete embedded in ash – the largest piece of silcrete they had ever seen on an archaeological site, nearly 10 centimeters in diameter.
"It looked like it had been accidentally lost in a fire pit," Brown says. He recalls how many of the silcrete tools from the site had a sheen or gloss that reminded him of tools he had examined in North American collections that were heat-treated.
"That is when we developed the heat treatment idea," Marean says. "The co-association of the ash cemented to the silcrete, the red color of the silcrete, and its inexplicably large size was the genesis conditions of our eureka moment."
To test their theory, Brown placed some of the silcrete stone beneath their fire pit one evening, building a hot fire over the top.
"When I returned to dig the stone out the following day, the results were amazing. After heating, the silcrete became a deep red color and was easily flaked. Most importantly, it looked exactly like silcrete from site PP5-6. Using heated silcrete we were then able to produce realistic copies of the actual silcrete tools," Brown says.