Phytoliths don't cut the mustard?

All over my news feed last week were links to news articles reporting on the 'earliest use of spices in Europe', specifically the use of garlic mustard seed (Alliaria petiolata) by Neolithic people. The evidence used to support this is our old favourite microfossil, the phytolith (frequent readers will know they come up in this blog quite frequently), which archaeologists have recovered from burnt food crusts inside Neolithic pots. The original research article is open access in PLoS ONE and can be read here:

I have been debating with myself whether to post these thoughts, as I know the team that did the work, and have great respect for them. But the subject is one I feel strongly about (as anyone who has discussed phytolith analysis with me will know!). So the aim here is to open discussion about the problems with microfossil analysis in general, and this happens to be a case study which illustrates many of these problems very well. It also makes for a great blog title! On a more serious note, I find myself once again feeling uneasy at the way phytoliths are increasingly being used in archaeology to support these big theories (earliest domestication of plant xx, earliest human use of plant yy, Neanderthals were master chefs etc), when phytolith analysis as a discipline still has so many flaws that need to be addressed. I recently reviewed some of these problems here (if you don't have access and would like a copy get in touch!).

There are very few phytoliths that researchers would agree are identifiable beyond genus, yet here we have the exact species being identified, with no explanation except that the sample most closely resembled garlic mustard out of the 120 reference species examined. I can't help but feel that the only reason this species was chosen is because it is the one that happened to be in the reference collection. Other phytolith studies that claim a species level identification (see my review for references) have spent a very long time doing work on modern reference samples, defining the criteria that make the phytoliths distinctive to species.

It is difficult to assess as I cannot find any published reference images of Alliaria petiolata phytoliths apart from the single image in this paper. Personally I am not convinced that the reference image bears much similarity to the archaeological phytoliths. Image A. in particular looks to be a smooth sphere (and about half the size) rather than the distinct pitted apearance in C (the archaeological example). The reference image, B. is similar but by no means (in my opinion) identical. We know from other phytolith studies (rice, maize, wheat) that these details of the phytolith 'decoration' can be subtle, and are essential in distinguishing different types.

This could be a matter of poor image quality. I've looked at enough phytoliths to know that they are often much clearer down the microscope (and in this study the samples were mounted in glycerol, which is not the best refractive index for observing phytoliths). But even if they are similar, this does not preclude other possibilities for their origin. I've seen too many other spherical phytoliths to be convinced (but I do want to be convinced!). How morphologically consistent are these within Alliaria petiolata? How morphologically consistent are these within a single plant? (not very, looking at the large size range in the reference specimen, 4.8 - 11.2 microns). How frequently do they occur within the plant? Although 120 references might seem like a lot, any botanist can tell you this is a fraction of the species present in Europe.

A plus point in the study is that the phytoliths were recovered from within a burnt crust rather than the soil, which narrows down the depositional pathways through which they entered the archaeological record. It is very reasonable to assume that they come from something that was in the pot when the food crust was formed. But whatever the species identification, the very small number recovered does not support the conclusion of the study that "it is demonstrated beyond doubt that the use of spice was practised regularly" (my emphasis).

These are just some of the questions that need to be addressed, not just in this study but phytolith studies in general. This is especially important when research makes the popular news. This is the way that things move from the world of academia into real world 'fact', and it is our responsibility as researchers to be careful in how we present data. Possibilities and suggestions should not become unequivocal and definitive.

On a related but different note, this paper does make a great suggestion that could be tested using experimental archaeology - garlic mustard and marine fats? Yum yum? And ending on a positive note, trying to address cuisine, and the way that plants were prepared and consumed, is an exciting avenue of research, a refreshing change from the endless studies on the nature and timing of domestication.

Please feel free to comment!

Detail from Saul et al. 2013 Figure 1 (original image here)