If you put a snail shell to your ear can you hear the sound of your thoughts?

You’ll be seeing a lot of this button around the web today as it’s part of the celebrations for PLoS @ Two. It’s certainly something worth celebrating as PLoSOne is bringing a lot of good science to a wide audience. That’s particularly important with interdisciplinary papers because it’s very easy to publish them in just one field and miss half of the potential audience. That’s a major drawback if you see publication as an iterative process with one paper building on another. An example is this paper, Climate Change, Genetics or Human Choice: Why Were the Shells of Mankind’s Earliest Ornament Larger in the Pleistocene Than in the Holocene? by Teske et. al. in PLoS one. It’s a bit of clever work on the use of snail shells as ornaments in Middle and Late Stone Age Africa. I think there’s one or two problems with the conclusions, but it’s a valuable contribution to the study of human use of snail shells. That’s a much more important subject than it sounds, because these shells might be very important evidence about how your mind came together.

The human mind is a puzzle. One idea is that evolved gradually. Various social and biological factor created feedback which created a mind quite unlike anything seen before on the planet. Not everyone agrees with this. Some archaeologists argue that there is something extraordinary which happens at the end of the Middle Palaeolithic. Art appears. Tools become more complex. There’s evidence of abstract forms of thought. A new species of human arrives in Europe and wipes out another species of human which had been doing quite nicely for over a hundred thousand years. This change has been labelled the Great Leap Forward by Jared Diamond. It’s argued that something changed in human biology that radically altered the human capacity to think. The modern human mind and all its facets in this model came from some special event.

Yet this is all happening in Europe.

Shell beads from Blombos Cave. Photos (cc) Chris Henshilwood.

From the early 1990s there has been ongoing work at Blombos Cave on the south coast of South Africa. If you think that the modern human mind emerged around 30,000 years ago, then the finds are astonishing. There’s bone tools (d’Errico et al 2001). There’s engraved ochre (Henshilwood et al 2002). There’s also quite a few snail shells. The shells in question were the homes of the southern African tick, Nassarius kraussianus, a marine snail that lives in shallow waters and estuaries. Seafood is something that interests archaeologists studying the brain because it’s a rich source of fatty acids which are important in brain development. However these snails would be lousy as a food source. You’d have to spend about an hour gathering them and plucking the meat from the shell to get a gramme of food (d’Errico et al 2005:10). What makes these snail shells special is that they’ve been found punched with holes to make a necklace. That shows someone was spending quite a bit of time planning and working to produce something which wasn’t producing food or safety. To be able to justify wasting time like that you have to be capable of some heavy-duty thinking. D’Errico et al (2005) think it goes further than that. The snail shells are on average significantly larger than shells used in the Late Stone Age. They argue that the Late Stone Age shells are random samples, but the Middle Stone Age shells are evidence of people deliberately choosing to select large shells.

This is where the Teske et al paper, comes in. They argue that there are other possible reasons for the difference in shell size and that d’Errico’s team haven’t fully considered the environmental factors. One of the reasons archaeologists like snails is that they’re very sensitive to their environments. One way that snails change with changes in environment is that they drop dead. They can be very sensitive to change. That leaves an environment that other snails can move into. Often you can hazard a good guess at what sort of environment a site has simply by looking at snail shells. If you have strata with different snail shells in it you can piece together changes over time like the clearing of forests for pasture. So it’s a good idea to ask if we’re dealing with similar snails. If the Middle Stone Age had a different snail, then it would be no surprise to see that they had different shells to Late Stone Age and modern snails. But all we have are the shells. We don’t have the snails, so how can we track what snails were around in the Middle Stone Age?

The way they’ve tackled this is by looking at the mitochondrial DNA of the snails. N. kraussianus are found around the coast of South Africa. Seeing as they move at a snail’s pace, this must be the result of a long period of colonisation, but how long? Teske et al used mutation in the mitochondrial DNA of the snails to work out when the populations diverged from each other. It’s not an exact clock, but they believe it’s likely that the point of divergence was so long ago that N. kraussianus would have been around at the time of the Middle Stone Age. While they were the same snails, could it also be said that the environment was the same?

Teske et al found that N. kraussianus varies according to environmental conditions in a fascinating way. In colder waters N. kraussianus grows a larger shell. They were able to see this by collecting shells across the South African coast. In the case of Olifants on the west coast, the shells were more or less the same as Middle Stone Age Blombos Cave. However at Goukou, near Blombos Cave the modern shells were much smaller on average.

Map of sample sites for the paper Climate Change, Genetics or Human Choice, with Blobos cave highlighted. Image (cc) Teske et al.

So if the genetic data suggests the shells found at Middle Stone Age Blombos cave are the shells of N. kraussianus that still lives in the Goukou estuary, does the change in size show a change in climate? It sounds plausible, but what about the third option, human agency? Unfortunately there’s a hole in the paper there.

We’re comparing assemblages from the Middle and Late Stone Ages, but what exactly is the difference between the two ages? Why is there a divide? One factor is that there’s a much wider toolkit in the Late Stone Age which is more likely to use bone as well as stone. If you have more tools, then more things are possible. Unfortunately the big divide between the Middle and Upper Palaeolithic in Europe where Homo sapiens replaces Homo neanderthalensis isn’t found in Africa. Homo sapiens is around in the Middle Stone Age. As more sites are interpreted in their African context, rather than trying to hook them onto a European chronology, it’s also becoming clear that there’s cultural continuity between the Middle and Late Stone Ages. Sites like Blombos Cave and Klasies River Mouth are pushing back the dates of modern behaviour and a recent study of the Middle Stone Age in Zambia suggests some modern behaviour could date back as far as 300,000 years ago (Barham and Andrews 2000). The cultural evolution of Middle Stone Age peoples is wide open for debate. Teske et al don’t go into any depth about the human aspect and, seeing as the title refers to human choice, that’s surprising.

One difficulty is while Teske’s team has shown that environment does have an effect, we don’t know how big this effect was. Is the Middle Stone Age assemblage an accurate sample of the shells available to Middle Stone Age peoples? Or is it a selection of larger (easier to work?) shells from a marginally larger population? It’s a question that needs answering because the Blombos Cave sample is a human selected sample, which might have been chosen randomly or for a purpose. In the case of the Late Stone Age deposits at Blombos Cave, we have comparison data from Die Kelders (d’Errico et al 2005:21, approximately site C on the map above) which is matched to modern populations. This is is how d’Errico comes to the conclusion that the Late Stone Age deposits are effectively random samples. The Middle Stone Age deposits at Blombos cave, from the Teske et al paper have no similar comparison data, so the Middle Stone Age sample is one site with twenty-nine shells. Teske’s paper tackles this by saying that the effort in finding large shells would be exceptional. Purely in comparison to modern populations he’s right, but would it be an exceptional effort with a marginally colder climate? If you simply assume that human choice played no part, then it’s no surprise when you reach the conclusion that human choice played no part.

That’s a big assumption. There’s evidence from Klasies River Mouth (between E and F on the map) for sophisticated hunting strategies. A flint point doesn’t wedge in the vertebra of a buffalo by accident. Clearly people of the time were capable of abstract thinking and planning which would make size selection possible. The paper simply doesn’t tackle the archaeological issues. If you’re trying to answer an archaeological problem with an assemblage that only makes sense in an archaeological context and you want to convince archaeologists you’re right, this is a flaw. Teske et al have a particularly hard task if they want to do that as a lot of archaeologists loathe environmentally deterministic explanations. That’s why I think their decision to publish in PLoS One is a smart idea.

A major advantage of publishing in PLoS One or any Open Access journal is visibility. I’m interested in the development and context of human cognition. Work on that can require looking at all sorts of resources; some, from an archaeologist’s point of view, quite obscure. If Teske had published the paper in The Journal of African Estuarine Studies or something like that it would have been just as good, but it’s likely my university wouldn’t have been subscribed to it. That would mean I would have only read the paper if I found the time to put in a inter-library loan request and if I wasn’t already chasing other articles which were in other expensive journals.

Another benefit is the commentary system. In the case of this paper no-one’s commented on it yet, but when they do it’ll be accessible to all. In my case the lack of criticisms of the paper suggests to me that the methods they’ve used are sound and uncontroversial, which can give me some confidence in their conclusions. Additionally as papers like this get cited I’d hope to see trackbacks and comments posted. That way not only can I use the bibliography to look back at what the foundations of the paper are, but I can also see where it led the conversation to. That’s extremely valuable if you see publication as part of a contribution to an ongoing creative process rather than production of a product.

I certainly hope that it’s going to get picked up and contribute further to discussion about Middle Stone Age ornamentation because, human agency aside it’s a good paper. If it had been subtitled ‘Evidence of environmental factors in the selection of shells at Blombos Cave’ I’d have nothing to add. The writing is clear and I can see that their conclusions are sound even if I lack the skills to critique the methodology as I’m no geneticist or ecologist. While I don’t think they’ve shown they can rule out human choice, they have shown that environment must have also played a factor and that the climactic conditions would have had an effect on Middle Stone Age assemblages that should not be ignored. Solving the puzzle of these snail shells would help show if human cognition evolved in the interplay of social, physiological and environmental factors.

Peter R. Teske, Isabelle Papadopoulos, Christopher D. McQuaid, Brent K. Newman, Nigel P. Barker (2007). Climate Change, Genetics or Human Choice: Why Were the Shells of Mankind’s Earliest Ornament Larger in the Pleistocene Than in the Holocene? PLoS ONE, 2 (7) DOI: 10.1371/journal.pone.0000614

See also:
Barham, L., Andrews, P. (2000) The Middle Stone age of Zambia, South Central Africa. Bristol : Published for CHERUB, the Centre for Human Evolutionary Research, at the University of Bristol, by the Western Academic & Specialist Press. (Worldcat)

d’Errico, F., Henshilwood, C.S., Nilssen, P., (2001) ‘An engraved bone fragment from ca. 75 kyr Middle Stone Age levels at Blombos Cave, South Africa: implications for the origin of symbolism and language.’ Antiquity 75 (288), 309-318. (subscription link)

d’Errico F, Henshilwood C, Vanhaeren M, van Niekerk K (2005) Nassarius kraussianus shell beads from Blombos Cave: evidence for symbolic behaviour in the Middle Stone Age. J Human Evol 48: 3–24 (download PDF)

Henshilwood, C., d’Errico, F., Yates, R., Jacobs, Z., Tribolo, C., Duller, G.A.T., Mercier, N., Sealy, J., Valladas, H., Watts, I., Wintle, A., (2002) Emergence of Modern Human Behaviour: Middle Stone Age engravings from South Africa. Science 295, 1278-1280. (subscription link)

The Blombos Cave website has many articles available for download if you want to read more about the site.

If you want to read more about snails, then I highly recommend Snail’s Tales a blog by Aydin Örstan and the best malacology blog on the web.