Heat Treatment Makes Better Stone Tools


August 17, 2009

The first evidence for the controlled use of fire appears about 790,000 years ago when fire was used for simple tasks like cooking, heat production, light, and protection from predators. Fire, heat, and their use to manipulate raw materials remain the foundation of modern technology. But how did people make the technological leap from the simple direct benefits of a flame for cooking and light, to understanding that fire and heat can alter the properties of raw materials so that people may command those materials in new ways?

Then sometime around 10,000 years ago people began to use fire to make ceramics, and around 5,000 years ago fire was used to make metals. In the August 14, 2009 issue of Science, researchers report the foundation of this technological leap. 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, a process called heat treatment. This heating transformed a stone called silcrete, rather poor for tool making, into an outstanding raw material that allowed them to make highly advanced tools. Here are the beginnings of fire and engineering, the origins of pyrotechnology, and the bridge to more recent ceramic and metal technology.

The genetic and anatomical evidence shows that biologically modern humans arose between 200 and 100 thousand years ago in Africa. There is no consensus as to when modern human behavior appears, but by 70,000 years ago there is good evidence for symbolic behavior. Many researchers are looking for technological proxies for complex cognition, and heat treatment is likely one such proxy. Heat treatment technology begins with a genius moment – someone discovers heating stone makes it easier to flake. This knowledge is then passed on, and in a way unique to humans, the technology is ratcheted up in complexity as the control of the heating process, cooling, and flaking grows in sophistication. This creates a long-chain technological process that requires a complex cognition, and possibly language, to learn and teach. Our discovery shows that these early modern humans had this complex cognition.

Heat treatment was widely regarded as first occurring in Europe at about 25,000 years ago. Last week’s announcement pushes this back at least 45,000 years and perhaps 139,000 years, and places it on the southern tip of Africa at Pinnacle Point, a promontory approximately 400 km east of Cape Town. Two years ago this same team of researchers documented in the pages of Nature the earliest evidence for exploitation of marine foods and modification of pigments (see “Early human use of marine resources and pigment in South Africa" in NEWS). These results sharply advance our knowledge of modern human origins, and show that something special in human cognition was happening on the coastline of South Africa during this crucial final phase in human origins.

Illumination of the heat treatment process shows these early modern humans commanded fire in a nuanced and sophisticated manner. It is widely believed these modern humans left the warm confines of Africa sometime around 60-50,000 years ago and penetrated into the colder glacial environments of Europe and Asia, where they encountered Neanderthals. By 35,000 years ago these Neanderthal populations were mostly extinct, and modern humans dominated the land from Spain to China to Australia. This command of fire provides, the authors of the Science paper assert, a potential explanation for the rapid migration of these Africans across glacial Eurasia – they were masters of fire and heat and stone, a crucial advantage as these tropical people penetrated the cold lands of the Neanderthal.

Here is a photograph of Pinnicle Point Site 5,6 on the coast of South Africa east of Capetown.  This and companion Site 13B have yielded evidence of heat treatment.

This discovery of heat treatment resulted from a transdisciplinary research program combining thermo-luminescence, magnetic analysis, optically stimulated luminescence dating, experimental stone tool production, and field archaeology. The program is called SACP4 (South African Coast Paleoclimate, Paleoenvironment, Paleoecology, Paleoanthropology).