Shea, an expert at making replicas of Stone Age tools, notes that pressure flaking can be taught in 30 minutes to a novice. “It is, literally, so easy a caveman can do it,” he says.
Stone-tool makers living in southern Africa 75,000 years ago pushed the cutting edge in more ways than one. These intrepid folk sharpened the thin tips of heated stone spearheads using a forceful technique previously dated to no more than 20,000 years ago, a new study finds.
This stone-tool making method, called pressure flaking, was invented and used sporadically in Africa before spreading to other continents, according to a team led by archaeologist Vincent Mourre of the University of Toulouse-Le Mirail in France. Having a flexible repertoire of tool-making methods aided the survival of modern humans who left Africa beginning around 60,000 years ago, the scientists propose in the Oct. 29 Science.
The finding fits with the idea that symbolic art, rituals and other forms of modern human behavior developed gradually over hundreds of thousands of years, not in a burst of cultural innovation marked by cave paintings and other creations that appeared after 50,000 years ago in Western Europe.
Excavations of sediment dated to 75,000 years ago in South Africa’s Blombos Cave produced stone artifacts displaying signs of pressure flaking, Mourre and his colleagues say. “The Blombos evidence for pressure flaking is the oldest we know,” says anthropologist and study coauthor Paola Villa of the University of Colorado Museum of Natural History in Boulder.
Pressure flaking doesn’t add much sharpness or strength to a cutting instrument, Shea adds. Blombos tool makers probably employed this technique to advertise their skill or to denote users’ social identity, he proposes.
Archaeologist Curtis Marean of Arizona State University in Tempe calls the evidence for pressure flaking at Blombos “suggestive but not completely convincing.” Further work needs to confirm that pressure flaking of replicated silcrete artifacts consistently produces marks like those on the Blombos finds, Marean asserts.
Knowledge of pressure flaking doesn’t imply any special mental or toolmaking abilities, he remarks. Like Shea, Marean regards pressure flaking as a simple way to finish shaping tools made from certain types of stone.
“If the authors are correct that pressure flaking occurred at Blombos Cave, the result is important in that that it extends the time range of the technique,” Marean says. “But it’s not game-changing in our understanding of the origins of complex cognition.”
In questionnaires after the typing test, subjects by and large took the blame for the introduced errors and took credit for the researchers’ corrections. No matter what he actually typed, when the typist saw that the word on the screen matched the word he had intended to type, he assessed his own performance as accurate.
But the speed of the typists’ keystrokes revealed something else. After hitting the wrong key, a typist’s fingers slowed down for the next keystroke, even if the researchers sneakily fixed the error so that the typist didn’t notice it. In these cases, a typist wasn’t explicitly aware of the mistake, but the brain’s motor signal changed nevertheless.
Logan says that this change in timing reflects a kind of automatic assessment of performance. “The body is doing one thing and the mind is doing another,” he says. “What we found was that the fingers knew the truth.”
Many psychologists thought that the mind was capable of detecting errors in several ways, but “nobody had pinned it down,” says cognitive neuroscientist Jonathan Cohen of Princeton University. “Here, they developed a very clever set of experiments to tease the types of system apart.”
The results may reveal a hierarchical method of error correction — with a “lower” system doing the actual work and a “higher” system assigning credit and blame, Logan suggests. These multiple layers of control may be evident in tasks such as playing music, speaking and walking to a destination, Logan says. As a man heads toward a new restaurant, his brain is noticing landmarks and keeping on the right course. Meanwhile, his feet steadily plod along, navigating the terrain automatically.
