Stay-At-Home Modern Humans Stranded in South Africa for 50,000 Years. What's Up With That?

Pinnacle Point Caves (PPC) in relation to Klasies River Mouth (KRM)  cave and Blombos Cave (BBC), three of the best known southern African localities with modern human accoutrements as early as 70 ka to 100+ ka. 

In the midst of the recent science-media frenzy about bows and arrows at Pinnacle Point, I thought I'd best expand on my reasons for doubting, en masse, the 70 ka to 90 ka [and beyond] dates for what appear to be unequivocally modern behaviours in a growing number of southern African archaeological sites. I've been working on this for several days, and the more I look, the more I see evidence of a real problem with optically stimulated luminescence (OSL), the technique used at all of these sites to estimate the age of the deposits. As I've mentioned previously [e.g. here], the primary assumption underpinning OSL age estimates is this: each individual quartz grain removed from the archaeological deposits MUST have been exposed to sunlight for long enough to have released any free electrons trapped in its crystal lattice. Given that all of these early dates derive from cave deposits, the primary issue for the OSL specialist is eliminating any quartz grains that have not been exposed in the manner just described. It's one thing to recognize this pesky problem; it's quite another to overcome it. And that, I think, is where OSL falls down. Whether or not a given age estimate is accurate becomes a real guessing game, as we'll see in a moment.  

Three representations of the stratification at Pinnacle Point PP5-6. Source: Brown et al. 2012

As you can see above, in the recently published profile from Pinnacle Point site PP5-6, the OSL picture is evidently not straightforward. The date of 53 ka highest up in the profile is superposed on a date of 60 ka. That is consistent with the stratigraphic relationship between the sediments that yielded the dates. However, the next lowest in the column is, again, 53 ka. Then come 56 ka, 52 ka, 50 ka, followed by 52 ka, 60 ka, 65 ka, 65 ka, 69 ka, then 66 ka, 66 ka, 64 ka, 72 ka, 71 ka,74 ka, 69 ka, 77 ka, 73 ka, 71 ka, 69 ka, 69 ka, and lastly 78 ka. Clearly this is not an 'internally consistent' temporal sequence. These age estimates are roughly sorted youngest to oldest, but given the lack of coherence, how is one to assess, much less find acceptable, either the youngest or the oldest dates?
     For example, does one conclude that the real age of this stratigraphic column ranges from 53 ka to 78 ka? How about 53 ka to 69 ka? Or 60 ka to 69 ka? Moreover, with such variance from what one should expect of the age succession in a sedimentary column that has maintained temporal integrity, how do we know that any of these estimates are accurate? All this points to, in my barely savvy opinion, serious difficulty with the technique, most likely due to the intractable problem inherent in the primary assumption of the technique. And, if the very latest results of the best of the best practitioners yields such shaky outcomes, what does it say about all of the other South African site chronologies? Here, then, lies the very real possibility that the OSL technique is single-handedly creating a false picture of modern human origins throughout the southern region.

Stratigraphic profile of La Quina, the type site of the Quina Mousterian. All of the objects illustrated are éboulis, clasts of all sizes and shapes, which have spalled off the bedrock and been incorporated along with the archaeological materials.

     In this I'm reminded of what's now a discredited relative dating technique that European archaeologists depended on for a good long time. I refer to the technique of éboulis analysis that underpinned [especially] the French Palaeolithic chronology for much of the late twentieth century. Briefly, the analyst qualitatively assessed the angularity of small pieces of bedrock breakdown and assigned an age estimate that coincided with what was then considered to be a four-stage European glacial cycle--Gunz, Mindel, Riss, Würm. The most angular pieces represented cold or dry or both, and the most rounded pieces warm and wet or both. It seems a bit naïve by today's standards. However, it was gospel for decades. One day it all came crashing down. After that, everyone picked up the pieces and moved on. Thus far archaeologists of the Middle Pleistocene haven't resorted to OSL for dating. However, they do use thermoluminescence, which depends not on individual quartz grains, but rather on pieces of flint that have been subjected to high heat. There's certainly less doubt about whether or not a piece of flint has been heated than that a tiny grain of quartz was ever exposed to sunlight. 
     In making this comparison between éboulis analysis and the OSL dates from southern Africa, I don't mean to suggest that the two techniques are equal. Rather, I make the comparison because, in the past, an error-prone technique was used to provide the chronological framework for an entire sub-continent. That's the direct comparison to be made. The southern African cave sites are universally dated using OSL or other equally shaky [to my mind] techniques.

The crux of the southern African OSL dates. In relation to the earliest evidence of modern human behaviour everywhere outside of Africa, and the speed at which Eurasia was 'colonized', the southern African dates stand out like a sore thumb. 

     Having made the comparison between éboulis and OSL, I can now introduce what I believe to be several data sets suggesting that the southern African cave age estimates are anomalous when compared to the rest of the world. For, when it comes to the early dates for modern human behaviour in southern Africa there is an absence of parsimony. Parsimony, if you'll recall, is the central point of Occam's Razor, the idea that, when deciding which competing hypothesis is the more correct, the one that makes the fewest assumptions should be selected. Occam's Razor is by no means a law of nature. It merely crystalizes what empirical scientists have know for some time--the more auxiliary hypotheses you need to support your main conclusion, the less likely it is to be the right one. In the case of the early dates for modern human behaviour in South Africa, there is much that flies in the face of Occam's Razor.
     Take, for example, the overwhelming evidence for the almost instantaneous spread of behaviorally modern humans to the furthest reaches of Asia, which happened about 45 ka. Australia was colonized at this time. So were some of the the furthest reaches of Europe--England, for example, at 44 ka. The best explanation for the absence of modern human archaeological assemblages prior to this time is that there simply were no modern humans outside of Africa prior to that time. The question thus becomes, if humans could expand the length and breadth of Asia in an eye-blink of geological time, why didn't it happen earlier, when modern human behaviour is claimed to have had its start 70 ka to 90 ka ago in southern Africa?
     To explain the apparent lag between the emergence of modern human behaviour in southern Africa and that which inhered everywhere else in the world post 45 ka, the various archaeological teams must invoke untested, and indeed untestable hypotheses as to the reason for those moderns staying put for anywhere between 164 ka and [taking the youngest date from PP5-6] 53 ka. One hypothesis has that population bottlenecks occurred due to environmental circumstances [as yet unrecorded], after which cultural achievements of the modern populations were stultified, only to reappear some 30,000 years later, at which time the archaeology records the rapid spread of modern humans throughout what's been called the Old World.
     In July of this year we saw a report from Border Cave of a modern human archaeological assemblage that dates to about 44 ka, a result in accord with the beginning of modern human behaviour in Eurasia. But, because they found no earlier deposits, the authors find it necessary to replay the tape of the 'early' teams, that the Border Cave moderns must represent the hypothesized re-fluorescence of modern humans preparatory to world domination. Once again. I think such intellectual reaching is unnecessary, at best, and obfuscatory at worst. I do not mean to suggest that the obfuscation is deliberate. The archaeologists who rely on OSL age estimates have nothing else to go on. Or, at least, they have no reason to think that their information is in error. After all, the best of the best are working at Pinnacle Point. Who could doubt their veracity? Who would, much less?

Modern human artifacts from Border Cave at 44 ka. Touted to be the earliest evidence for the same culture that existed ethnographically in the vicinity of Border Cave. [Take that with an anthropological grain of sand.] Source: D'Errico et al. 2012.

     I know. I know! We're dealing with Culture. And culture theory says that the early moderns in southern Africa might just have decided that they wanted to stay put and not colonize the known universe. If you think about it, how would they have known they were choosing not to colonize the rest of the world, unless they knew there was a 'rest of the world' to colonize. And, if they did know that there was a 'rest of the world,' why is there no evidence for those modern humans until about 45 ka everywhere but southern Africa. If, and I have no doubt, the early moderns in South Africa were anything like modern moderns, or a great many ethnographic moderns, they couldn't have resisted seeing what was beyond the next ridge. Yet it would seem that they 'chose' to stay put, enjoying the fruits of their modernity all by themselves, thus endangering their persistence by allowing themselves to be buffeted by the whims of the environment. That doesn't sound like modern me to me. [N.B.: I'm only being half facetious.]
     I supposes one could suggest that there were natural barriers to their spread out of southern Africa. But, there are no natural barriers to movement in coastal Africa, nor would there have been when the sea levels were depressed during the glacial stages of the Pleistocene. If they could process pigments and decorate ochre, make beads, and arrows, they were equally capable of making boats to cross the broad reaches of Africa's rivers where they meet the sea. After all, the colonizers of Australia crossed at least 90 km of open ocean at around 45 ka. Surely the precocious moderns in south Africa at 70 ka to 90 ka could have done the same. So, the big question is why did they stay put in southern Africa? Huh? Riddle me that, Batman. Apparently the Wall Street Journal has a suggestion.

     So, you see, reason alone tells me that there's much wrong about the early dates from southern Africa. And my instinct tells me that the OSL dates must be wrong. As I've said to you before, will some kindly physicist not come forward to examine the mathematical filmflammery that accompanies OSL age estimates, and thereby save me from this empirical purgatory??? The flimflammery in question.

Please??? I'm beggin' ya.


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Hominid Hunting, Pinnacle Point, and Me. An epilogue.

Hominid Hunting has just covered the latest news from Pinnacle Point, the subject of yesterday's blurt. I couldn't let it go without comment. Hell, I think by now the author, Erin Wayman, probably expects it!
     It's 'awaiting moderation' as I write this. However, in case it's considered too technical or too contrary for publication, I'm gonna quote myself here, verbatim. Here's how it goes.

Hi, Erin. Pinnacle Point is quite an amazing archaeological locality. And the excavators, Brown and Marean, are doing excellent work. I know that it'll sound a bit like sour grapes, however, for years now there have been those who question not the finds, but the age estimates of these and other southern African sites. Like those at Pinnacle Point, the spectacular finds derive from caves, where organic preservation is often enhanced either by protection from the elements or the chemistry of the cave environment, or both. Unfortunately, for those parts of the site that are beyond the radiocarbon limit—around 40,000 years—the excavators have relied on less precise dating techniques. One in particular, optically stimulated luminescence (OSL), begins with the assumption that a given grain of quartz sand was exposed to sunlight for a time before final burial in the cave. Where caves are concerned this assumption is an untenable assumption, because no one is able to say ahead of time, with any certainty, that a single grain was or wasn't exposed to sunlight sufficiently powerful or for long enough. As a result, once the technique has yielded its raw results, there follows a complex mathematical dance based on all sorts of other assumptions, as a means of 'eliminating' the uncertainty of whether or not a given quartz grain had been sufficiently exposed to sunlight. Moreover, if the mathematical assumptions and the inherent complexity of the calculations are going to be in error they'll always overestimate the time since that quartz grain (and the artifacts in proximity that are of interest) was last exposed to sunlight. Logically, there's no under-estimating ages in caves using OSL. This leaves us with a question, one that I think the excavators of Pinnacle Point and elsewhere cannot logically escape. If the dates of unequivocally modern human behavior elsewhere in the world are less than about 45,000 years old, what are the odds that the anomalously early dates for similar behaviour in those southern African sites—all dated by OSL—are systematically overestimating their age? I'd say those odds were far better than the likelihood that all of the finagling involved in arriving at an OSL age estimate is yielding accurate and precise chronometric results.
[Update 0045 UTC November 10, 2012: Doh! I knew I was forgetting something! The bedrock in which the Pinnacle Point caves are developed is described as quartzitic. Wanna know how long it's been since the quartz grains in the bedrock last saw sunlight? Your guess is as good as mine. Suffice it to say that it's unlikely that they had been exposed to very much sunlight once they fell to the floor. All that matters is that the uncertainty introduced by one minimally exposed grain would introduce one hell of a wrench in any age estimate, even one that's been through the math mill.]  

I think that about sums it up. Now, why couldn't I have said that yesterday?

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Throwing in the Towel Thanks to Pinnacle Point, South Africa. As If.

At home. Under the weather. What else to do but watch the news ticker. It is ablaze with the latest news from Pinnacle Point in South Africa. The first word I had of it was a headline about this piece, just published, in Nature. 

In the article, long-time acquaintance and Mousterians 'R' Us advocate Sally McBrearty extolls the collection of more evidence for modern human behaviour at around 70,000 years. Sally was commenting on a Letter to Nature also published electronically yesterday. 

As you know from reading the SA, there are plenty of claims out there for modern human cognitive abilities or behaviours, or both, going well beyond 100 ka--blade industry at Kathu Pan 1 and Qesem Cave; hafting with bitumen or birch tar, and so on. However, as you also know, I've been critical of a great many of them. 
     Yet, according to the age estimates for unequivocally modern human activities at places in southern Africa, such activities are occurring at an age anywhere from 30 ka to 60 ka earlier than they do in Europe and southwestern Asia. For example, finds of decorated ochre and pigment processing 'kits' at Blombos Cave ca. 75 ka. In today's piece by Brown et al. we're hearing about use of the bow and arrow ca. 70 ka [I haven't had the time to examine the actual evidence for the claim other than to note that they refer to the microlithic portion of the assemblage consisting of small backed blades.]. This of course is one of the inferences frequently made for the kind of stone tools comprising the Howieson's Poort phase of what in Africa is called the MSA (Middle Stone Age). 

PP5-6 excavation. Deeply stratified and sharply inclined, much like a cone of talus [Credit: Erich Fisher; (inset) Simen Oestmo] © Science/AAAS.

     
I've previously said quite a bit about these early dates, here and here. However, this latest publication, and an earlier one from the same [or similar] group of authors, published in Science a few years back  claims [and at this point I have no reason to doubt them] that they have evidence of heat treating lithic raw material to make it more workable, between about 70 ka and 164 ka! This would be a great discovery even if a mere 30,000 years old. Unfortunately for me, these two inferences--of bows and arrows and heat treating--leave me with a new emotion--dismay--at the precocity of the ancient southern Africans. That's because these microliths and heat-treated lithics from Pinnacle Point derive from a part of the Pinnacle Point locality known at PP5-6, which is generally described as a rock shelter.

Pinnacle Point PP5-6. Tarpaulin covers the excavations. © Science/AAAS.

      In my earlier rantings I suggest that using Optically Stimulated Luminescence (OSL) age determinations drawn from buried cave sediments may be the culprit, artificially overestimating the age of these patently modern human assemblages. That's because there's really no way of knowing, a priori, whether or not an individual quartz sand grain had ever been exposed to the sun, much less for how long. [This, you'll remember, is a crucial assumption of the technique.] It's this uncertainty that causes OSL experts to undertake what appears to me to be a complex series of calculations and extrapolations designed to overcome the potential shortcomings of the technique. However, a rock shelter is a bit stickier. Presumably a rock shelter would be exposed to sunlight year in and year out, and thus you might imagine that any quartz grain that came to rest there had been exposed to sunlight long enough to have its 'clock' set to zero [unless, of course, that grain had been stripped from older deposits, redeposited in the rock shelter and immediately covered by sufficient material to preclude its being affected by the sunlight thereafter--like, perhaps, at night]. Now, after about a day and a half of off and on Googling I've been able to sight nothing written that describes the geomorphic history of PP5-6. Nevetheless, I have a hypothesis that will allow me at least some peace of mind. Given its proximity to a series of true caves at Pinnacle Point, and given its location at the edge of the escarpment that contains those caves, it seems highly likely that, while exposed today, the sediment accumulation at the site occurred inside a cave in which the roof and shoreward wall have subsequently collapsed and migrated down slope.
   So, unless I'm mistaken, I have no more reason to accept these early dates from the Pinnacle Point locality than I do the others from caves in South Africa. I know that the Very Serious Palaeoanthropologists like Curtis Marean will simply scoff at my scepticsm. Nevertheless, one can't help but wonder. And one day, perhaps, someone will try to directly date a piece of bone from one of those amazingly old stratigraphic units and Voila!
     I live in hope. No. Scratch that. As John Cleese's supine character in Clockwise moaned, 'It's not the despair...It's the hope.'

   
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Thanks for dropping by! If you like what you see, follow me on Google Friend Connect or Twitter, friend me on Facebook, check out my publications at Academia, or connect on Linkedin. You can also subscribe to receive new posts by email or RSS [scroll to the top and look on the left]. And please don't forget. Oh, and you can always put me on your blogroll! By the way, I get a small commission for anything you purchase from Amazon.com if you go there by clicking through from this site.