14 AUGUST 2009 VOL 325 SCIENCE www.sciencemag.org 820 PERSPECTIVES F aced with the difficulties of surviving in often hostile surroundings, early humans manufactured various flaked stone tools to increase the success of food gathering and the efficiency of food process- ing. Special-purpose tools increased the like- lihood of obtaining food and were particu- larly important when it was hard to secure adequate food resources ( 1, 2). For exam- ple, projectile points are more likely to inflict a lethal wound if they have needle- sharp tips and symmetri- cal, straight, razor-sharp edges that reduce drag on penetration. Such sophisti- cated tools require not only special skills but also high- quality stone materials. When such materials were unavailable, early humans developed the ability to improve the quality of the available materials through controlled heat treatment by burying selected pieces of stone beneath a fire at a campsite or a specialized workshop, usually for a day or more. On page 859 of this issue, Brown et al. show that in coastal South Africa, the deliberate use of this technique dates back at least 72,000 years (and perhaps as long as 164,000 years), predating its use outside Africa ( 3). This use of fire as an engineering tool is an early step in the evolution of means by which humans could more effectively con- trol their environment. Heat treatment in Africa appears at roughly the same time as widespread evidence for symbolic behav- ior, signaling the development of increas- ingly complex cognitive ability. By enabling the manufacture of more efficient tools, heat treatment may have played a key role in allowing early modern humans to spread rapidly from the relatively benign environ- ments of southern Africa into the colder, more hostile environments of Europe. The Neandertals in Europe apparently lacked this technique, perhaps giving the early modern humans an evolutionary advantage as they moved into Eurasia. Heat treatment was practiced until recently in the Kimberleys in northwestern Austra- lia ( 4). Kidja aborigines built a large fire in a ~0.5-m-deep pit in sandy soil. When the fire had burnt down, the coals were removed and the bottom of the pit was covered with sand; roughly flaked nodules (blanks) of white chalcedony were placed on the sand and covered with more sand, so that the stone was not exposed to intense heat. Coals and hot sand were shoveled back into the pit and left for 3 to 4 days. When the pit was cold, the blanks were removed and worked by pressure flaking [which involves removal of flakes using pressure rather than percussion (striking)] into long, thin, bifacial “Kimberley points” by a limited number of master crafts- men (see the figure). Heat treatment was cru- cial in enhancing the flakeability and aes- thetic qualities of the stone, and Kimberley points were prized as beautiful objects, given as prestigious gifts, and exported along indig- enous trade routes. Most flaked stone artifacts are made from microcrystalline siliceous rock types (such as chert, flint, chalcedony, and jasper). Fine pressure-flaking of these materials is extremely difficult in their natural state; their flakeability is substantially improved by con- trolled heat treatment. As Crabtree and Butler showed in their pioneering replicative studies of stone tool manufacture ( 5), heat treatment increases the length of detached flakes, for example, from a half-inch to 2 inches. Lon- ger, thinner, and sharper flakes can be pressed because heat-treated material can be flaked at angles nearly parallel to the surface; this is not possible on unheated material. Heat treatment also increases homogeneity, allowing greater control, ease, and precision of fracture and resulting in fewer production failures. Heat treatment markedly reduces a mate- rial’s fracture toughness ( 6), so that the flak- ing properties become more like those of the best quality stones, such as obsidian. The reduction in fracture toughness of microcrys- talline siliceous materials during heat treat- ment is due to recrystallization and, to a lesser extent, healing of microcracks ( 7). During heat treatment, strongly interlocking, variably sized quartz crystals recrystallize to become more equidimensional and more similar in size, allowing intergranular fractures (which require less energy than intragranular cracks) to propagate more readily. However, although the heated material is much easier to flake and retouch, it is also less durable and more prone to edge fracturing. Thus, intentional thermal alteration was not desirable in the manufac- ture of heavy-duty tools. Heat-treated artifacts are most commonly recognized by a vitreous luster (see the figure) on post-treatment fractures, contrasting with matte flake scars predating heat treatment ( 3). Color changes caused by heat treatment are variable, but generally are toward more reddish hues, and heat treatment was some- times deliberately used to enhance the beauty of stone materials. Carnelian bead manufac- turers in Cambay, western India, have used thermal treatment for at least 4500 years to change the gray to cream colors of agates into vivid red to orange shades ( 8). Archaeological and ethnographic records document that most stone knappers practiced heat treatment to produce retouched tools, especially projectile points. Heat treatment is particularly beneficial in the manufacture of blades, microblades, and bifacial points by percussion flaking with a soft hammer of wood, bone, or antler, as used by Paleo-Indian cultures of North America to produce the dis- tinctive, leaf-shaped Folsom points, which have wide, shallow grooves running almost the entire length of the point ( 9). Heat treatment was used to improve the flaking properties of local but poorer quality stone materials and to conserve raw mate- rial. Heat-treated stone can be worked by pressure blade techniques, which produce up to four or five times as many blanks from the same volume of material compared with percussion flaking ( 10). This was particu- larly important in the harsh tundra environ- ments of the Northern Hemisphere, where deep snow cover, frozen ground, and often frost-shattered stone reduced access to suit- able raw materials and caused seasonal shortages. Pressure blade techniques may have been invented by reindeer hunters in Fire and Stone PALEONTOLOGY John Webb and Marian Domanski For at least 72,000 years, humans have used heat treatment of stone materials to make more efficient tools. Benefits of heat treatment. This Kim- berley point from northwestern Austra- lia shows the vitreous luster and finely controlled working typical of heat- treated stone artifacts. CREDIT: WEI MING/LA TROBE UNIVERSITY Environmental Geoscience, La Trobe University, Victoria 3086, Australia. John.Webb@latrobe.edu.au Published by AAAS on October 27, 2009 www.sciencemag.org Downloaded from