Shopping for rocks in the Olduvai Basin

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.

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.

What is more, the quartz-rich rocks in the Olduvai Basin:

• 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.  

Finally, it's not like volcanic rocks were not utilized at all. In fact, early humans appear in some cases to have selected them over metamorphic rocks when they wanted to create handaxes rather than simple flakes. This makes sense given that the more complex production sequence of a handaxe probably requires a more predictably fractured rock. Unfortunately, you can't subject fragile artifacts to the impact of a rebound hammer. However, if you can tell where the artifact originally came from, we can correlate the hardness of our geological specimens with their archaeological counterparts without subjecting the latter to any damage. Well, in addition to hammering rocks, we also subjected them to X-ray fluorescence, which can help identify their chemical composition. The volcanic rocks are easily distinguished from the metamorphic rocks just by looking at them, but the metamorphic rocks themselves, even those from different hills, can look very similar to each other. Luckily, the chemical signatures of each metamorphic hill are distinct enough for statistical algorithms to correctly match chunks of rock to the correct hill with 75-80% accuracy.

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

Paleolithic research in the Armenian Highlands and Anatolia

I returned safe and sound from Anakara a couple of weeks ago (an unexpected overnight stay in Terminal 5 at JFK not withstanding) and am happy to report that the trip was well worth it. It was an intimate event, with 14 papers and perhaps 20-30 audience members. Some highlights:

• The first paper was by Amilié Vialet and her colleague M. Cihat Alçiçek, who discussed the Kocabaş skullcap. I was particularly interested to hear this talk, as I knew next to nothing about this important hominin fossil. The remains consist of the frontal and parietal of a hominin that were recovered in 2002 during quarrying activities in the travertines of the famous Pammukale (Turkish: "Cotton Castle") hot springs (and UNESCO World Heritage Site) in western Turkey. (I actually visited this area, which also includes impressive ruins from Greek and Roman times, on a vacation way back in 2001.) The top of the skullcap was literally shaved off by heavy equipment (unfortunately, the rest of the skull, nor any other part of the skeleton, have ever been recovered) but, thankfully, the recovered bones were brought to the attention of Alçiçek, who recognized their importance. John Kappelman and his colleagues had previously published on the remains and suggested that (1) they dated to ~500,000 years ago, (2) provisionally, they represented H. erectus; and (3) the individual suffered from the earliest known case of tuberculosis. Vialet (who, apart from being a world-class paleoanthropologist, is a genuinely nice person) summarized more recent work on the fossil. Perhaps most importantly, they push back the age of the remains using a variety of dating techniques to ~1.4 million years ago, which makes them among the oldest in all of Eurasia. An audience member asked Vialet about tuberculosis, and she vacillated, saying that a confident diagnosis from their team would need to await further analyses. Their 3D reconstruction of the cranium, however, suggested a closer affinity of Kocabaş to Homo ergaster fossils from Africa rather than early Homo (including the crania from Dmanisi) fossils or later, classic Homo erectus populations from Asia. Vialet suggested, then, that Kocabaş represents an "expansion [from Africa that is] different from that represented by the Dmanisi fossils". It is becoming increasingly clear that the movement out of Africa by hominins ca. 2 million years ago was a complex series of events (emphasis on the plural) that involved many populations, some of which were able to establish long-term occupations and others that quickly went extinct. The new information from Kocabaş certainly reinforces this idea.
• What followed was a series of papers by our Turkish colleagues that summarized the Lower and Middle Paleolithic of Anatolia. What struck me is that apart from a few notable exceptions (Kaletepe Deresi 3, Karaïn Cave, Yarımburgaz Cave, and Üçazğlı Cave), much of what is known is based almost exclusively on surface assemblages that lack key contextual information (chronometric dates, paleoenvironmental reconstructions, etc.). For example, Kadriye Özçelik reported on surface surveys that documented for the first time the presence of Paleolithic material in the Denizli Basin of southwestern Anatolia (this is the same region in which the Kocabaş fossil was uncovered). It is quite amazing, if you think about it, that it took until 2014 to officially recognize the presence of Paleolithic tools in this region, especially since, as I later learned, handaxes have been known in Turkey since 1896. While those of us working in Armenia faced a similar situation−that is, a dependence largely on surface finds−a great deal of progress has been made on this front in the last 10 years, due largely, I think, to the support of the Armenian Academy of Sciences, which has done much to nurture Paleolithic research in Armenia. My conversations with Turkish researchers revealed that it is extremely difficult to acquire permits to even examine and collect surface assemblages in Turkey, much less excavate stratified sites. Until that changes, I fear that our knowledge of Paleolithic settlement in modern Turkey will continue to lag behind that of other nearby countries.
• Our paper summarized work on Bagratashen 1, and we officially announced OSL dates from the Middle Paleolithic horizon of around 35,000 years ago. While Middle Paleolithic sites of this age are not unusual, the types of stone tools uncovered from the deposit look like artifacts from other parts of the southern Caucasus and the Levant that are much older, somewhere around 150,000 to 200,000 years ago (see my previous post on Bagratashen 1 here). So, either our dates are way out of whack (which is possible), or we have an example of technological convergence where culturally and temporally unrelated hominin groups independently converged on similar tool types. There is a long history in archaeology of viewing stone tools to track human populations. That is, particular cultures made particular types of tools, so if you find similar tools, you have the same people. However, folks have long pointed out that, as complex a technology that lithic manufacture can be, there are only so many ways that one can reduce a chunk of rock into different shapes. Dan Adler and his colleagues made precisely this point in their paper about their excavations at Nor Geghi 1 in Armenia. What they found is that at about 330,000 years ago, folks in Armenia were creating both bifacial artifacts typically associated with the Acheulean and Middle Paleolithic Levallois products at the same time. This shows definitively that, at least at Nor Geghi 1, making Acheulean bifaces has little to do with being part of an Acheulean "culture" that represents a particular group or species of hominins. Likewise, it seems that the appearance of Levallois technology, again at least in Armenia, need not necessarily represent a new group or species of hominin. Perhaps it is time, as Dan simulated with a humorous slide, to flush many of these stone tool "cultures" down the toilet.

I should also point out that Phil was able to bring together Turkish and Armenian researchers to discuss Paleolithic archaeology. This shows the power of science in general, and archaeology in particular, to transcend political and ethnic enmity and bring together people with common goals. I was honored to be part of such a momentous event.

Preliminary results from Bagratashen 1

I returned from Yerevan on Sunday night, having been forced to sprint to my last connection into Raleigh/Durham because Air France was unable to get the cargo hold open to release our baggage. This was my first experience with an Armenian spring, and there were some brisk days at ~3,000 feet (I'm usually there in the summer, when it can be broiling, particularly in the arid north where we work).

As I mentioned in my previous post, the goal of this trip was to conduct a preliminary analysis of the stone tools from Bagratashen 1 and, after having looked at them over the past two weeks, it appears as if they are going to be very interesting indeed. My colleague, Boris Gasparian, and I examined over 500 pieces from our excavations.

The Bagratashen 1 lithic material laid out for analysis.

While it's obviously going to take some time to sort out what's going on (not to mention additional excavations to recover a larger sample size−this stuff came from a small 6m² excavation), I can relate some observations:

• There's definitely (and unsurprisingly) Levallois technology represented. 
• We've got quite a few points and point fragments in the assemblage (over ten). What's interesting is that we have both unretouched and retouched Levallois pieces and non-Levallois pieces. So, MP folks used different techniques to produce stone tools with the same characteristic; i.e., pointy ends. I took some measurements on tip cross sectional area, which can, according to John Shea (2006) and others, help determine whether a point could have been used as an effective projectile (note that this attribute cannot say that a point definitively was used as such). A couple of these points look, at least in a general morphological sense, very similar to the elongated retouched points found in Levels 1 and 2 at Djruchula Cave (Georgian Republic) and Lower E at Hayonim (Israel). This is potentially significant for us, since we have yet to successfully date the Bagratashen 1 sediments and Djruchula and Hayonim have been dated to between about 300,000 and 150,000 years ago. We have to be careful here, though: morphological similarity does not necessarily mean temporal similarity, since technology, especially lithic technology, is subject to independent development. 
• Boris and I noticed that a number of the artifacts, most of which are dacite, display a "rotted" surface texture, and some even feel lighter, as if they've been leached somehow. Boris suspects that this may be related to thermal damage and, sure enough, another colleague of ours, Dmitri Arakelyan, told us that he has tossed dacite into fire before and it does indeed show this sort of modification. More systematic experiments are of course in order, but we may have some indirect evidence for fire at the site (whether its natural or anthropogenic is also another issue).
• Nearly all the pieces were covered by carbonate crust, likely imparted well after the materials had been buried. We recorded what face of each artifact was facing skyward before it was pulled out of the ground and, because carbonates tend to form on the "downward" faces of clasts, we should therefore get an idea of whether or not they moved around in the sediment post-depostionally.
• There also appears to be quite a bit of truncation going on. In essence, this means that after knocking off a flake, hominins chose to subsequently remove, either through a single, massive blow or finer retouch, one or both ends of the piece. Why one would bother to truncate a seemingly well-made flake is another question. One possibility is that this truncation provides a new platform for the removal of smaller flakes from the original piece. The knappers may have wanted to remove the old platform and/or the bulky bulb of percussion to artificially thin the piece, for instance. Whatever the reason, people were doing it at Bagratashen 1.

Alright, enough chatter, let's get to the analysis....

References:

Shea, JD (2006). The origins of lithic projectile technology: evidence from Africa, the Levant, and Europe. Journal of Archaeological Science 33, 823-846.

Off to Armenia

I just arrived in Yerevan yesterday evening. As part of my spring research leave, I will be here studying the lithic collections from sites that our team excavated between 2010 and 2011. I'll concentrate on Bagratashen 1, which is an open-air site discovered during our 2009 survey. Excavations in 2011 and 2012 recovered several hundred well-preserved Middle Paleolithic artifacts from a discrete find horizon. Unfortunately, no fauna has yet been uncovered; we are pretty excited about it the site nonetheless, since geological work suggests that the assemblage is largely undisturbed. So, we hope to extract some fine-grained behavioral information.

Look out for more updates as the analyses proceed....