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The Cutting Edge of Cognition

Acheulean handaxe

Four views of an Acheulean handaxe created 300,000–500,000 years ago in France. Credit: Didier Descouens/Wikimedia Commons

By Natalie Rogers

Modern brain scans are revealing whether Stone Age hominins planned to make specific tools or whether their craftsmanship determined the outcome of their endeavours.

For many decades, people have been fascinated by the archaeology of ancient Stone Age societies – the ancestors to our own species, Homo sapiens. What were these pre-human like? Did they think and behave like us?

Archaeologists, paleoanthropologists and philosophers have long believed that if we can understand their behaviour, particularly their tool-making behaviour, it might open a portal into the minds of these extinct hominins. After all, tool-making – also known as knapping – is a direct product of the mind of the hominin, so it would make sense that we could track the evolution of the human mind by studying how Paleolithic stone tools became increasingly more complex over time.

The Physics of Chipped Windscreens and Knapping

We now know that many stones used in knapping have mechanical properties similar to glass, so they fracture in much the same way. To make a stone tool, knappers cannot simply hit one rock (a hammerstone) against another rock (a core) to produce usable flakes; they must exploit the natural fracture mechanics of the stone. Essentially, when a hard round stone forcefully strikes a hard flat stone it creates shock waves that radiate from the point of the impact outwards in a cone shape to form what’s known as a Hertzian cone (Fig. 1). This principle of physics is superbly demonstrated when your car windscreen gets hit by a stone – leaving a cone-shaped hole – and appears to have been skillfully exploited by Stone Age knappers. Because cone-shaped rocks do not work well as cutting tools, knappers must adjust the angle of their striking so that only part of the Hertzian cone cuts through the rock (Fig. 2).

Stone Tools through Time

There are many different types of stone tools, and how they appear and were used is dependent upon many factors such as the species that made them, the type of rock used, where in the world they were made and what they were designed to be used for.

Oldowan

Named after Olduvai Gorge in Tanzania in the 1930s by Louis Leakey, the earliest stone tools actively produced by knappers are known as Oldowan tools. Although hominins such as Australopithecus afarensis used sharp stones as cutting tools as early as 3.4 million years ago, it appears they did not actively produce these sharp flakes until around 2.6 million years ago.

The Oldowan industry consists of simple sharp flakes and the cores from which they were struck. The flakes appear to have been used to scavenge flesh from carcasses left behind after large predators have had their fill, whereas the cores appear to have been used to break open the limb bones to extract the protein and fat-rich bone marrow – a valuable resource unavailable to the predators that killed the animal.

Acheulean Handaxe

Spanning an incredible 1.4 million years from 1.6–0.2 million years ago, the tear-drop shaped Acheulean handaxe is one of the most prolific and persistent tool types found in the archaeological record. Bifaces (or two-sided cutting tools) were made by Homo erectus and later by Homo heidelbergensis, and have been found in Africa, Europe, the Middle East and India. Curiously, although Homo erectus was present throughout Asia, handaxes are largely absent with a few notable exceptions in parts of China and South Korea. It is thought that bamboo provided an abundant and easily worked alternative resource.

The handaxe has been nicknamed the Swiss army knife of the Paleolithic. Microwear and residue analysis has indicated that these tools were used for everything from digging to animal butchery and wood working, and some have even suggested that they were thrown like a discus used as a weapon.

Levallois

Named after the Parisian suburb Levallois-Perret, the Levallois technique is a complex procedure where the core must first be shaped like a tortoise-shell before preparing a platform at the butt end and, finally, knocking of the final flake. These flakes were then used as cutting tools, as scrapers to remove fur from hides, and hafted as spearheads to be used as weapons. Although the specific Levallois technique is endemic to France, similar “prepared-core” techniques are found throughout Europe and in many parts of Africa but were only used by Homo neanderthalensis and very early Homo sapiens.

Blades

A blade is a thin flake tool that is at least twice as long as it is wide. The process required to make them is time-consuming and complex. First, the knapper must make a “blade core” by shaping their core roughly like a loaf of bread (Fig. 3a). Unlike other forms of knapping, where fakes are removed with “direct percussion” by hitting a hammerstone directly against the core, blades are removed with “indirect percussion” (Fig. 3b). Using this technique, the knapper positions a long, thin piece of wood or antler against the ridge where he or she would like to remove the blade. The knapper then hits the piece of wood or antler with a billet to “punch strike” the blade off the blade core.

Did Knappers Plan?

A question that has repeatedly been asked with respect to stone tool-making is whether or not the knapper had a plan in mind when making the tool. Although all mammals, birds and even cephalopods are capable of deliberately altering their behaviour to exploit a situation they predict will occur in the future, only Homo sapiens can develop complex plans. Cognitive planning utilises the prefrontal cortex, the area of the brain that houses our “higher functions”. It has been proposed that this cognitive ability to plan was only possessed by humans and that it was crucial for making the most complex stone tools, such as blades.

But how can we actually test this idea experimentally? This question has become the focus of my PhD – do people plan when they make complex, but not simple, stone tools?

Generally, if you want to find out what parts of the brain control a certain behaviour, such as mental multiplication, you pop experimental subjects in a brain scanner, present them with the task and scan their brain while they lie very still and solve the problem in their mind.

But therein lies the problem. Participants must lie very still during a brain scan. To answer my question, we need our participants to move while making tools.

The Secret Powers of Radioactive Glucose

Recently I ran an experiment that replicated a technique that was devised by Dietrich Stout of Emory University and colleagues from the Stone Age Institute using Positron Emission Tomography (PET) to see what brain regions are used during knapping.

First, an expert knapper was injected with a small amount of radioactive glucose and then made stone tools for the next half hour. While the knapper was working, the radioactive glucose was being used by all the parts of the brain that were required to make the tool. When the knapper had finished, we used the PET scanner to read the radioactive signature that was now saved inside the brain. This process was repeated over the next 3 days so we look at the brain signature when making simple tools compared with the brain signature when making complex tools.

Because movement produces quite a bit of “noise” in the images, as a control the knappers forcefully hit their hammerstones against a core without attempting to produce any sort of tool. This reproduced the visuomotor aspects of knapping, so we could subtract these images from the tool-making images to isolate the pure “tool-making” brain signal from the “hitting rocks together” brain signal.

So What Did We Find?

Although our data processing is still in its early stages, we have found no evidence that the prefrontal cortex was activated during any of the tool-making tasks. What this means is that it seems our knappers did not cognitively plan when they made either simple or complex tools. Contrary to many descriptions of stone tool-making, our participants did not construct a mental “to do list” and consistently update it as they went through the different stages of the tasks.

Instead, what knappers appear to be doing when they are making stone tools is essentially the same process that we do when we mindlessly drive to work. This brain circuit is a motor circuit that controls our day-to-day habits.

Although this circuit appears to have changed little throughout mammalian evolution, it can help us perform very complex tasks without the need for higher functions such as planning. This is because, after lots and lots of practice, even highly complex tasks, like steering a car or controlling the path of a Hertzian cone, can become automatic and second-nature.

We hope to publish the final results of this study in the coming months and would like to thank all of the hospital staff and experimental participants for supporting this exciting project.

Natalie Rogers is a PhD student at the University of NSW who is combining approaches from both archaeology and psychology to understand the evolution of the human brain and behaviour.