The invention and proliferation of stone tool technology was one of the most significant events in human evolution--the ability to use stones as tools and, eventually, the wherewithal to modify them into sharp-edged knives and other implements enabled our early ancestors to access foods that would have been difficult or impossible to obtain and consume with their relatively small, unspecialized teeth. If you spend some time working with stones, it eventually becomes apparent that not all of them are created equal: some break easily, others are tough to fracture; some produce razor-sharp edges, others generate dull ones; some are close by and/or easy to get a hold of, others are far away and/or difficult to access; some are durable and last a long time, others are brittle and must be discarded after a single use.
Now, we know that a modern human can learn to recognize these attributes and, what is more, they can (not to say that they necessarily do) plan their days with them in mind ("well, let's see...there are two ways to get to the pond for fresh water, Path #1 and Path #2, but only Path #1 has an outcrop of durable rocks on the way, so I'll kill two birds with one stone and take Path #1"). The question, then, is this: to what degree did our early human ancestors appreciate the sometimes subtle differences among rocks, and what can this tell us about their cognitive capacities?
Before we can even answer this very interesting question, however, we need to figure out a way to (1) rank rocks in terms of their usefulness, and (2) determine where on the landscape early humans were getting their rocks in the first place. There is a long history of research on these topics in Paleolithic archaeology, and my colleagues and I added some data to the debate in a recently published paper in Quaternary International. My interest in the topic goes back to the late 2000s, when David Braun wrote a couple of really interesting papers on the stone tools from Kanjera South, a two-million-year-old site in Kenya. Most studies on rock "usefulness" are based on rather subjective and imprecise categories. These categories, and the studies that utilize them, have provided key insights, including the fact that rock selection by early humans was not random. Braun, however, explored the possibility that the material sciences might provide some useful tools to help archaeologists objectively describe the characteristics of rocks.
As I mentioned above, there are a host of features that one might consider when selecting a rock. We chose to concentrate on fracture predictability, largely because the creation of many types of stone tools involves breaking a rock into smaller (and hopefully useful) pieces. If a rock breaks differently every time you hit it, there is no way to predict what you're going to end up with. Sure, it might be useful, but, then again, it might not. With a rock that fractures predictably, though, you can be reasonably sure that the time and energy you've expended will pay off with the production of a useful tool. Flint knappers have known for a long time that homogenous rocks break more predictably than do heterogeneous rocks because they are stronger (they can resist strain) and more elastic (they can resist deformation) when impacted by an outside force. Thankfully, a rock's strength and elasticity are highly correlated with its hardness, something that can be quickly assessed with a rebound hammer. These nifty handheld devices, which were originally designed for use on concrete, fire a spring-loaded plunger onto the surface of a stone. The plunger then bounces back, or "rebounds," after impact. The distance of that rebound reflects the hardness of the stone. Braun and others have used this technique to estimate fracture predictability for the rocks available to early humans at several Pleistocene archaeological sites in Kenya.
In 2014, we set out to produce comparable data in our neck of the woods, the Olduvai Basin of northern Tanzania. What is today a deep gorge surrounded by open grasslands was, about two million years ago, a stream-fed soda lake surrounded by lush vegetation. Largely unchanged, however, are the volcanic highlands that border the basin to the south and east and the numerous hills--remnants of Archean-aged metamorphosed bedrock--that rise above the plains. Importantly, both the volcanos and the hills are made up of rocks from which stone tools can (and, in the past, could) be made.
With the help of students from the UNCG Olduvai Gorge Paleoanthropology Field School and Earlham College's Summer Collaborative Research Program, I and my good friends and colleagues Cynthia Fadem and Ryan Byerly have been traipsing around the Olduvai Basin hammering as many rocks as we can get a hold of. Since 2014, we've accumulated a database of 110 specimens, and some interesting patterns have emerged. It turns out that the volcanic rocks that occur as rounded cobbles within the seasonal streams that drain the volcanic highlands have high rebound values and, thus, high fracture predictability, while the metamorphic rocks from the hills show either intermediate or low rebound values. Now, if early humans were selecting their rocks based on fracture predictability, we might expect that most of the artifacts from the archaeological sites would be made from volcanic sources. It turns out, however, that among Olduvai's artifact assemblages, volcanic rocks tend to be very rare, while metamorphic rocks, especially those made largely of quartz, are very common, which implies that fracture predictability was not a major concern. But why not? We might interpret this pattern to mean that early humans in the Olduvai Basin were not clever enough to recognize the value of predictably fractured volcanic rocks. We're skeptical of this hypothesis, though, because experimental work indicates that there are good reasons not to select volcanic rocks, since they:
In the future, we should be able to match Olduvai's metamorphic artifacts to the hills from which they were being collected, which in turn will give us an idea of how far early humans travelled when shopping for their rocks.
References:
Egeland, CP, Fadem, CM, Byerly, RM, Henderson, C, Fitzgerald, C, Mabulla, AZP, Baquedano, E, Gidna, A (2019). Geochemical and physical characterization of lithic raw materials in the Olduvai Basin, Tanzania. Quaternary International. doi.org/10.1016/j.quaint.2019.09.036
Now, we know that a modern human can learn to recognize these attributes and, what is more, they can (not to say that they necessarily do) plan their days with them in mind ("well, let's see...there are two ways to get to the pond for fresh water, Path #1 and Path #2, but only Path #1 has an outcrop of durable rocks on the way, so I'll kill two birds with one stone and take Path #1"). The question, then, is this: to what degree did our early human ancestors appreciate the sometimes subtle differences among rocks, and what can this tell us about their cognitive capacities?
Before we can even answer this very interesting question, however, we need to figure out a way to (1) rank rocks in terms of their usefulness, and (2) determine where on the landscape early humans were getting their rocks in the first place. There is a long history of research on these topics in Paleolithic archaeology, and my colleagues and I added some data to the debate in a recently published paper in Quaternary International. My interest in the topic goes back to the late 2000s, when David Braun wrote a couple of really interesting papers on the stone tools from Kanjera South, a two-million-year-old site in Kenya. Most studies on rock "usefulness" are based on rather subjective and imprecise categories. These categories, and the studies that utilize them, have provided key insights, including the fact that rock selection by early humans was not random. Braun, however, explored the possibility that the material sciences might provide some useful tools to help archaeologists objectively describe the characteristics of rocks.
As I mentioned above, there are a host of features that one might consider when selecting a rock. We chose to concentrate on fracture predictability, largely because the creation of many types of stone tools involves breaking a rock into smaller (and hopefully useful) pieces. If a rock breaks differently every time you hit it, there is no way to predict what you're going to end up with. Sure, it might be useful, but, then again, it might not. With a rock that fractures predictably, though, you can be reasonably sure that the time and energy you've expended will pay off with the production of a useful tool. Flint knappers have known for a long time that homogenous rocks break more predictably than do heterogeneous rocks because they are stronger (they can resist strain) and more elastic (they can resist deformation) when impacted by an outside force. Thankfully, a rock's strength and elasticity are highly correlated with its hardness, something that can be quickly assessed with a rebound hammer. These nifty handheld devices, which were originally designed for use on concrete, fire a spring-loaded plunger onto the surface of a stone. The plunger then bounces back, or "rebounds," after impact. The distance of that rebound reflects the hardness of the stone. Braun and others have used this technique to estimate fracture predictability for the rocks available to early humans at several Pleistocene archaeological sites in Kenya.
In 2014, we set out to produce comparable data in our neck of the woods, the Olduvai Basin of northern Tanzania. What is today a deep gorge surrounded by open grasslands was, about two million years ago, a stream-fed soda lake surrounded by lush vegetation. Largely unchanged, however, are the volcanic highlands that border the basin to the south and east and the numerous hills--remnants of Archean-aged metamorphosed bedrock--that rise above the plains. Importantly, both the volcanos and the hills are made up of rocks from which stone tools can (and, in the past, could) be made.
 |
| A view of Olduvai Gorge in the foreground and, in the background, Naibor Soit, a granulite outcrop from which quartz could be procured (photo: Amy Schnell). |
With the help of students from the UNCG Olduvai Gorge Paleoanthropology Field School and Earlham College's Summer Collaborative Research Program, I and my good friends and colleagues Cynthia Fadem and Ryan Byerly have been traipsing around the Olduvai Basin hammering as many rocks as we can get a hold of. Since 2014, we've accumulated a database of 110 specimens, and some interesting patterns have emerged. It turns out that the volcanic rocks that occur as rounded cobbles within the seasonal streams that drain the volcanic highlands have high rebound values and, thus, high fracture predictability, while the metamorphic rocks from the hills show either intermediate or low rebound values. Now, if early humans were selecting their rocks based on fracture predictability, we might expect that most of the artifacts from the archaeological sites would be made from volcanic sources. It turns out, however, that among Olduvai's artifact assemblages, volcanic rocks tend to be very rare, while metamorphic rocks, especially those made largely of quartz, are very common, which implies that fracture predictability was not a major concern. But why not? We might interpret this pattern to mean that early humans in the Olduvai Basin were not clever enough to recognize the value of predictably fractured volcanic rocks. We're skeptical of this hypothesis, though, because experimental work indicates that there are good reasons not to select volcanic rocks, since they:
- usually occur as rounded cobbles, which are tough to flake because they don't have very many of the acute angles that make flake removal possible;
- require more raw muscle power to flake; and
- may not be as durable as other rock types.
- are readily available from conspicuous landscape features that are very close to most of the archaeological sites; and
- are very friable, which, although reducing their fracture predictability, makes them relatively easy to smash into lots of small chunks, among which are typically a handful of useable tools.
In the future, we should be able to match Olduvai's metamorphic artifacts to the hills from which they were being collected, which in turn will give us an idea of how far early humans travelled when shopping for their rocks.
References:
Egeland, CP, Fadem, CM, Byerly, RM, Henderson, C, Fitzgerald, C, Mabulla, AZP, Baquedano, E, Gidna, A (2019). Geochemical and physical characterization of lithic raw materials in the Olduvai Basin, Tanzania. Quaternary International. doi.org/10.1016/j.quaint.2019.09.036
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