Thursday 7 February 2013

This Argument Has A Few Holes In It. 80-odd-Thousand-Year-Old Shell Beads From La Grotte des Pigeons, Morocco and Blombos Cave, South Africa

Grotte des Pigeons in eastern Morocco, near the village of Taforalt. Photo by Abdellah Elbadaoui (Panoramio/GoogleEarth).
As I continue to wend my way through the literature I've missed during my absence from palaeoanthropology, I'm forever discovering scholarship that makes me think I'm on the other side of Alice's looking glass. Yesterday, while nosing around in search of background knowledge for the Subversive Archaeologist, I found another beaut that I can't ignore. And today I found it a companion. The two articles concern recovery of perforated marine shells from about 80 ka at the site known as Grotte des Pigeons [pictured above], near Taforalt in Morocco and at Blombos Cave, in South Africa. Of greatest interest to me is the way the authors attempt to rule out natural processes in pursuit of an explanation for their presence in the caves.

I hope to show why their efforts are insufficient, and why we should treat with caution these claims for modern human behaviour at such early dates. Remember Rule #1? Rule out natural site formation processes before invoking human or hominid activity. Its corollary is this: to rule out natural processes you need to keep in mind that different processes can result in similar archaeological 'signatures.'

For example. Remember Raymond Dart and the "osteo-donto-kerartic culture" of the australopithecines? Dart had observed the behavior of members of traditional societies in southern Africa. He noted that when attempting to extract marrow from the long bones of their prey, they employed what Dart referred to as "the crack and twist" technique. The marrow seeker would strike the shaft hard enough to crack it. Then, grasping one end in each hand, the hunter would twist the ends in opposite directions until the shaft broke open. On the basis of these observations Dart proposed that the presence of similarly broken fossil bone meant that the Australopithecines had been hunting, then cracking and twisting the long bones to extract the marrow. It was many years before someone pointed out that the same effects can result from carnivore modification, and that it was much more likely that the presence of bones broken in that way was the result of carnivore behavior, rather than that of small, bipedal apes. Dart had premised his explanation on the only analogous process of which he was aware at the time. Such shortcomings permeate the archaeological canon. They remind us that we must always consider the possibility of equifinality---the idea that more than one process can result in a given set of observations. Moreover, to determine whether your observations are humanly or naturally produced, you need to involve as broad a range of present-day analogies as is possible. In the case of Grotte des Pigeons and Blombos Cave, the archaeologists have posited evidence that is sufficient to explain what they recovered, but their evidence is by no means the only possible explanation, nor necessarily the most parsimonious. At best their evidence is equivocal [a accusation that I've used over and over again in my work].

Enough preaching. On to the sea shells.

At La Grotte des Pigeons and at Blombos Cave excavators recovered small marine invertebrate remains that were altered from their natural state. In all, 13 were recovered from Pigeon Cave, and 41 from Blombos. The shells are perforated, and some bear a polish, which, the authors propose, you might expect if these objects had been strung like beads on a necklace. In addition, some very small patches of iron oxide were found on a few, which suggests to the authors that the putative shell beads had been artificially coloured with ochre. In the illustrations below you see the entirety of the collection from La Grotte des Pigeons. [I'm impressed by the presentation---for each artifact all six aspects are illustrated. Compare this to the artful display of the Blombos shells, further down, which is arranged so as to illustrate the path through which some twine might have passed.]

When viewing these images, keep in mind that the shells themselves measure about one centimeter across and slightly more in the longest dimension. Thus, the perforations are on the order of 500 microns in diameter. Note also that a great many of the perforations are irregular, and look more like they've been broken open rather than having been drilled.

From Bouzouggar, A., N. Barton, et al. (2007). "82,000-year-old shell beads from North Africa and implications for the origins of modern human behavior." Proceedings of the National Academy of Sciences 104(24): 9964-9969. Specimen 14 is a modern-day individual.
The photo montage below seeks to illustrate the areas of 'unnatural' polish and other markings on several of the Pigeon Cave shells. The authors claim that these are likely to be the result of having been strung like a strand of ... well, beads. I'd have to say that these aren't patterned alterations.
From Bouzouggar, A., N. Barton, et al. (2007). "82,000-year-old shell beads from North Africa and implications for the origins of modern human behavior." Proceedings of the National Academy of Sciences 104(24): 9964-9969. (Scale bar: A, C, E, and G = 1 cm; B, D, F, and H = 500 mm [sic].) [They must mean microns.] 

The photomicrographs below illustrate the red oxide on some of the putative beads from Pigeon Cave. The reddish material adhering to some of the shells from the cave was chemically characterized as hematite. Hematite is also known as ochre. As I stated above, there appears to be little rhyme nor reason to these alterations.

From Bouzouggar, A., N. Barton, et al. (2007). "82,000-year-old shell beads from North Africa and implications for the origins of modern human behavior." Proceedings of the National Academy of Sciences 104(24): 9964-9969. (Scale bar: A, C, E, and G = 1 cm; B, D, F, and H = 500 mm [sic].) [They must mean microns.] 
At Blombos Cave the story is much the same. Total number is larger; different species; similar alterations. And I would make the same statement again: beyond the mere presence of fenestrations, their morphology and other modifications to these shells is hardly what I'd call patterned.
Henshilwood,C., d'Errico, F., et al. (2004). "Middle Stone Age Shell Beads from South Africa." Science 304:404. 
Fig. S1. (A) Juvenile (1), sub-adult (2-4) and adult (5) Nassarius kraussianus shells from the modern biocenosis of Duiwenhoks estuary (Cape Province, South Africa) showing gradual size increase and thickening of the lip (solid line). Blombos MSA beads (R) are all made on adult shells (scale bar, 1mm). (B) Perforation types observed on N. kraussianus shells from modern and archaeological collections. The shaded area indicates the variability in hole locations. Types 6 and 8 on the dorsal side, found respectively on 63% and 24% of MSA beads, are observed on 2% and 3% on natural shells (N=2587) from Duiwenhoks and Goukou River estuaries. Type 1 on the ventral side (P, Q), produced by the predator Natica tecta, is present on 60% of shells from modern thanatocoenoses (N=1836) and absent on MSA beads. (C) Perforations on the dorsal side of dead N. kraussianus from the Goukou estuary. (D-I) Macro- and SEM photos of use wear on two MSA N. kraussianus shells (E,H, scale bar, 1 mm; F,I, scale bar, 500μm). (J-O) Macro- and SEM photos of a modern (J-L) and LSA (M-O) N. kraussianus shell showing absence of wear facet on lip (K, scale bar, 1mm; L, scale bar, 200μm; N, scale bar, 500μm; O, scale bar, 200μm). (P-Q) Ventral aspect of a modern N. kraussianus with hole drilled by Natica tecta (Q, scale bar, 500 μm). (R) Perforations on two orange MSA N. kraussianus shell beads from Blombos Cave (scale bar, 1 mm). (S) Typical white N. kraussianus shell beads with large perforations from Blombos Cave LSA levels. Verbatim from Henshilwood,C., d'Errico, F., et al. (2004). "Middle Stone Age Shell Beads from South Africa." Science 304:404. 

So, at La Grotte des Pigeons and at Blombos Cave, archaeologists were compelled to interpret the origin and mode of deposition of these objects. I won't suggest that they were grasping at straws to give these finds importance from an evolutionary standpoint. I'll let you be the judge.

Both sets of authors use evidence from ethnography, natural history, or geology. For example, we know the shells don't occur naturally in these caves. Simple enough. True enough. But, how do we assess their assertion that the shells could have been purposefully transported to these sites only by humans or hominids?

Likewise the perforations. The authors are aware that these tiny mollusks have mollusk predators that are able to drill through the shell and feed on the animal within. In the image immediately above, in P and Q, you see a hole drilled by Natica tecta. The Blombos Cave excavators note that such artifacts of predation show up 60% of the time in the natural death assemblages of this species. They also point out that they see none of these perfectly drilled fenestra in their archaeological sample. What are we to make of that?

Of the facetting noted on several of the shells, both sets of authors claim that these must have arisen due to repeated contact between either bead and bead on a string of beads, or between some form of cordage or other material and the beads that were strung on.

Finally, the ochre. What can we make of the microscopic presence of hematite on some of the shells from La Grotte des Pigeons?

Let's take the transport agent to start with. Although the sea today splashes directly below Blombos Cave, that's not so for Pigeon Cave. The sea was also more or less close to Blombos cave at 80 ka. Not so for Pigeon Cave. If the same transport agent moved the shells from the seashore to the caves, it was capable of moving them several tens of kilometers. So. Was it humans/hominids, or was it something else? I'll admit that my efforts resemble more 'back-of-the-napkin' research than scholarly rigour, I was able to find at least one potential non-human transport agent capable of introducing such shells into the sediments at both caves---terns. Birds. The brown noddy [Anous stolidus] is known to collect such shells and incorporate them in their nests. The noddy is a tern. Terns occur throughout the world, and the brown noddy is native to South Africa and Morocco, as well as almost everywhere else in the world. How could one possibly rule out the possibility that these birds, or other occasional collectors, introduced these items to the archaeological context?

The authors would say that the nature and placement of the holes suggests human agency and the polish and traces of hematite support that conclusion. What about those holes? For starters, another quick trip to the internets tells me that at least two groups of animals are capable of boring through mollusk shell and that it is a commonplace the world over. The one below is riddled with tiny perforations caused by a species of sponge. Yup. Sponge. In addition, as you'll have learned from the Henshilwood et al. paper, many species of predatory mollusks bore through other mollusk's shells to get at the living creature within. You'll be thinking "Yes, but the archaeological shells from Pigeon Cave and Blombos are not as beautifully regular as those left by sponges, or, for that matter by Natica tecta." True enough. They appear to be ragged, not drilled. More on that after the pretty picture.

Far be it for me to accuse palaeo people of having a courser aesthetic sense than I, but the majority of the perforations claimed to be the result of human/hominid activity look as if they've been punched out by pressure applied either from the outside or from the inside. Who's to say that those rough perforations weren't the result of inadvertent pressure [underfoot, for example, or in pounding surf]? It's logical to assume that pressure placed on a part of the shell where a sea creature's boring had previously weakened that portion of the shell would be capable of punching out a larger hole. Damned difficult to choose. Don't you think? Natural? Or not?

I realize that I'm merely casting doubt on the conclusions of the Blombos and Pigeon Cave archaeologists. Unlike the law, I can't employ the 'beyond a reasonable doubt' criterion when I'm interpreting an archaeological site. But let's face it. These claims are at best tentative, and would require, I believe, extensive experimental, ethnoarchaeological, and ethological examination before we accept them as fact.

Ooops. almost forgot. The polish. I think the presence of polish on just a few specimens should rule out any notion that they'd been strung like beads on a necklace. At a minimum the authors would want to examine ethnographic strings of similar or identical shells before claiming that the rare facets they find are due to rubbing of one shell against another on a string. Moreover, if such shells are susceptible to wear when suspended on a string, they can just as easily get worn on a beach that comprises numerous other such shells. After all, they all originated at the water's edge. And, although the authors acknowledge that certain kinds of damage might be possible at the beach, their investigations of present-day death assemblages of these animals reveals to them that the modifications to the archaeological specimens are unlike those occurring on present-day strands. To that I would only say that time passes and circumstances change. What effect would over-harvesting of sponges have on the frequency of perforations on modern day mollusk death assemblages in Morocco? Or pollution in the Mediterranean? Or different substrates at different times at Blombos? I think you get my point.

And finally the ochre. Again, if ochre can occur in a cave at a distance from the ocean, it can occur at the sea-side.

For all of the reasons I've given here, it's clear to me that the claims from La Grotte des Pigeons and Blombos Cave should be viewed with extreme caution, if only because they're extreme claims for modern human behaviour at extreme time depth.

I hope you enjoyed this little road trip to the seaside. See you *cough* next time.

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