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| Phenanthrene |
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| Fluorene |
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| Pyrene |
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| Fluoranthene |
[Update: Please have a look at the four-part response from the biologist who undertook the chemical composition study of the El Sidrón Neanderthal's dental calculus. It's in the comments, at the foot of this post. 2012.08.21]
You may be wondering why I didn't jump on this when it was first announced. Published online on July 18 Hardy et al. claim to have extracted *cough* hard evidence of Neanderthal behaviours akin to those of modern humans from the dental calculus that formed on the teeth of 5 individuals from the Iberian Peninsula.
I've been waiting for the time to look in detail at their claims, and although I haven't finished the fact-checking, I can say that Hardy et al.'s inferences are looking awfully shaky. As before when Very Serious Archaeologists have published chemical analyses, my focus is the authors' interpretation of their findings. I'll give but one example of this from their paper.
They claim the following:
The additional presence [in the calculus] of the main combustion markers, fluorathene [sic], and pyrene, along with smaller amounts of fluorene and phenanthrene, strongly supports the evidence for cooking/smoke inhalation in this sample (621).
For this exciting discovery they cite two studies: one on the chemicals produced during charcoal production, and the other on smoked salmon. Seems reasonable. No? Well, not really.
Pyrene and fluoranthene are the products of combustion, it's true. However, they are characteristic of incomplete combustion, i.e. combustion in a reducing atmosphere--one in which there is insufficient oxygen for complete combustion to occur. That describes perfectly the 'cooking' environment only if you're producing charcoal or, indeed, smoked salmon. Phenanthrene is found in tobacco smoke and that of coal-burning appliances. Fluorene occurs naturally, and according to the EPA, '[f]umes from vehicle exhaust, coal, coal tar, asphalt, wildfires, agricultural burning and hazardous waste sites are all sources of exposure.'
Now, remember that it was the authors' intention to produce evidence for cooking, fire, and plant use. Their inference here--that the Neanderthals were inhaling smoke-- depends entirely on the erroneous claim that the four compounds are present in the Neanderthal's dental calculus because of smoke inhalation. I have to ask, 'How likely is it that this group of Neanderthals inhaled smoke from charcoal production or salmon smoking in a reducing environment, tobacco smoke, vehicle exhaust, or hazardous waste?
I think you can guess the answer to my rhetorical question. But that doesn't address the possibility that the Neanderthals in question weren't dry-distilling birch or coal tar, and sniffing asphalt. After all, as the authors remind us others have made claims that Neanderthals were capable of dry-distilling birch tar and using asphaltum to haft points on wooden shafts. Why couldn't these Neanderthals have been doing just that?
I suppose they could have been dry-distilling birch or coal tar (if either Betula or coal were components of the El Sidrón environment--an open question at the moment). Except, if so, they'd have been inhaling the smoke from the wood fires used to 'cook' the two substances, rather than inhaling any of the compounds given off while the birch or coal were 'cooking,' because it's necessary to keep the substances being 'cooked' in hermetically sealed vessels throughout the process! Likewise, they might have been inhaling asphaltum while they were gluing a rock to a stick. But...well, I think you get the point.
And the point is this. The authors have done nothing, not a thing, to rule out the natural sources of the chemicals they report on. And that is a subversive's take on but one of the claims made by Hardy et al. I'll have more on this cockamamy paper in a subsequent post.
Pyrene and fluoranthene are the products of combustion, it's true. However, they are characteristic of incomplete combustion, i.e. combustion in a reducing atmosphere--one in which there is insufficient oxygen for complete combustion to occur. That describes perfectly the 'cooking' environment only if you're producing charcoal or, indeed, smoked salmon. Phenanthrene is found in tobacco smoke and that of coal-burning appliances. Fluorene occurs naturally, and according to the EPA, '[f]umes from vehicle exhaust, coal, coal tar, asphalt, wildfires, agricultural burning and hazardous waste sites are all sources of exposure.'
Now, remember that it was the authors' intention to produce evidence for cooking, fire, and plant use. Their inference here--that the Neanderthals were inhaling smoke-- depends entirely on the erroneous claim that the four compounds are present in the Neanderthal's dental calculus because of smoke inhalation. I have to ask, 'How likely is it that this group of Neanderthals inhaled smoke from charcoal production or salmon smoking in a reducing environment, tobacco smoke, vehicle exhaust, or hazardous waste?
I think you can guess the answer to my rhetorical question. But that doesn't address the possibility that the Neanderthals in question weren't dry-distilling birch or coal tar, and sniffing asphalt. After all, as the authors remind us others have made claims that Neanderthals were capable of dry-distilling birch tar and using asphaltum to haft points on wooden shafts. Why couldn't these Neanderthals have been doing just that?
I suppose they could have been dry-distilling birch or coal tar (if either Betula or coal were components of the El Sidrón environment--an open question at the moment). Except, if so, they'd have been inhaling the smoke from the wood fires used to 'cook' the two substances, rather than inhaling any of the compounds given off while the birch or coal were 'cooking,' because it's necessary to keep the substances being 'cooked' in hermetically sealed vessels throughout the process! Likewise, they might have been inhaling asphaltum while they were gluing a rock to a stick. But...well, I think you get the point.
And the point is this. The authors have done nothing, not a thing, to rule out the natural sources of the chemicals they report on. And that is a subversive's take on but one of the claims made by Hardy et al. I'll have more on this cockamamy paper in a subsequent post.
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As the main biomolecular archaeologist/archaeological chemist involved in the Hardy et al Neanderthal study, I felt it might be useful to enlighten both the S.A. author, and members of the group, as well as [later] putting this article into its fuller context with other papers on similar subjects (though I will do this separately). It could also be seen as timely, so perhaps a note of thanks to the Subversive Archaeologist (the world certainly needs them!). I do see PNAS not accepting archaeological science of great significance, because ‘big names’ don’t want it to be so (yes, they have the power to stop research that doesn’t suit, regardless of the scientific merit), yet publishing ‘old news’ of questionable significance (admittedly, unwittingly), as the Subversive Archaeologist rightly suggests. However...
ReplyDeleteFirstly, that we WERE able to extract biomolecules from dental calculus – tartar – from ~50,000 year old material is indeed a major achievement worthy of much scrutiny, but with the samples pre-cleaned for surface contamination we can be sure that the source of the organic compounds was indeed INSIDE the mineralised dental deposit (it is the mineral component which allows survival – cf. pottery where the ceramic matrix protects the organic compounds which have been absorbed into the internal wall of the pot. This work has been used for decades now to determine food consumption in antiquity (e.g. fish, dairying, etc.), and the principle here is the same). So that established, it is reasonable to conclude that the compounds identified reflect ingestion or inhalation by that individual. Given this, it was a question of determining what was ‘the most likely’ explanation.
There will always be some tension between the scientists (e.g. me) and the archaeologists in terms of the degree of interpretation possible. Speaking for myself, findings should also be open to review – that’s how good science works, rather than the ‘My professor’s bigger than your professor’ mentality, prevalent in archaeology, and indeed much of the often so-called ‘hard sciences’ too, for that matter – but in true scientist’s language: “ The findings are consistent with...”, and are a reasonable interpretation given the context.
Actually, that Neanderthals used fire is not a revelation to many, nor is cooking and the exploitation of plants, and nor, for that matter, is the use of medicinal plants, if that is indeed what the chemical evidence represents (note the ‘?’ in the title of the paper, although it’s the most plausible explanation at present). Non-primates self-medicate, so that Neanderthals may well have done so too is far from fanciful. Non-primate animals have worked it out for themselves, WITHOUT the help of modern humans, so in many ways the findings could be described as ‘expected’. Certainly, Richard Wrangham of Harvard University, to name but one, is not surprised by these findings, even if there is [justifiable] debate on the precise significance.
Part 2
ReplyDeleteBut to the supposed flaws: it is true that pyrene and fluoranthene are general markers indicative of combustion. I have studied these and other ‘polynuclear aromatic hydrocarbons’ (PAHs) (also known as ‘polycyclic aromatics’ (PCAs)) for over 25 years, initially with the oil company Shell, where they were connected with bitumen analysis – my main focus for four years there.
It should be noted that many, many processes do indeed produce these (and other) combustion markers (PAHs): the toaster produces them when you toast your bread, they are present in fried and grilled food (McDonald’s workers are exposed to levels which have been suggested to pose a significantly increased risk of cancer), and more - some PAHs have long been known as carcinogens (e.g. benzo-[_]-pyrene), but played down in the many, many cases in which they are produced – no one wants to lose their cars, think that home cooking exposes them to carcinogens (though it does; that doesn’t means we should worry about it: ‘how much’, as well as ‘what’, is key), etc. We consume PAHs when we eat the cooked food we have prepared; we inhale them when we cook.
Yes, we are exposed to (carcinogenic) PAHs when we walk down the street with cars driving by next to us (and by driving the car!), and there are natural sources such as natural petroleum (where PAHs are present in small amounts). Many of us enjoy log fires or wood-based fires (e.g. Bonfire Night, in the UK) – these expose us to PAHs such as pyrene and fluoranthene (the main ones in these processes) and we have certainly inhaled some.
So I’m afraid, that from direct analytical experience (and not only my own), rather than these compounds needing highly reducing conditions to be produced, these combustion markers are common today (less so 45,000-50,000 years ago) and incomplete combustion (e.g. cooking) is the norm, so not just restricted to charcoal and smoking; though smoking (use of fire) is a possibility for what we see in the dental calculus of these Neanderthals.
To quote: ‘Fluorene occurs naturally, and according to the EPA, ‘[f]umes from vehicle exhausts, coal, coal tar asphalt, wildfires, agricultural burning and hazardous waste sites are all sources of exposure.’
COMPARE THIS WITH WHAT Hardy et al PUT (SUPP. MAT. [DUE TO SPACE LIMITS]): ‘Combustion markers (formed from incomplete combustion) possible sources: wood fires, volcanoes, formation of gas, coal or oil (23-28)’
So aside from apparently not reading the full article properly, this indeed serves to reinforce the above, i.e. that PAHs are combustion markers in a wide variety of contexts, and can also derive from natural petroleum sources such as oil and asphalt. The question should then be: ‘which of all of the above would be likely sources of exposure for Neanderthals living in northern Spain some 45,000-50,000 years ago?’. Car exhausts, for one, seems somewhat unlikely.
Part 3
ReplyDeleteIn this context (non-archaeological scientists are often guilty of failing to understand the importance of the archaeological context; archaeologists should, and often do, know better), a fire – whether wood or coal – does not come close to producing complete combustion of any food cooked in that fire, so – speaking as a scientist who has studied these compounds over the 25 previous years – PAHs would be EXPECTED in this scenario (and so the charcoal and smoked salmon are red herrings in the way they are used in the Subversive Archaeologist piece).
The two references were chosen by us because they were related to the sort of activities the Neanderthals were likely to have been carrying out (despite some conservatism in hominid research) – cooking food using fire. It would have been impractical (and unnecessary given the colossal amount of research in this area) to list other references in the main article, despite being aware of them, though we DO reference a selection in the ESM of the article if one reads it.
As said several times already, there are indeed many possible sources, but here it was a question of understanding the context of the material and interpreting what was most likely. That is very often the case with archaeology and anthropology; if we want absolute certainty [though I am guilty of this ‘sin’ myself!] we should stay silent!!
It is that the PAHs pyrene and fluoranthene are the MAIN combustion markers identified, AND their relative abundances to each other, AND that the next two most abundant PAH’s identified were fluorene and phenanthrene, AND that other more minor PAHs were identified (typically lower than these four in combustion processes) which is, collectively, important. If one had looked at the supplementary information online one would also have seen that although minor, others PAH ‘combustion markers’ were also identified*.
*naphthalenes, acenaphthylene, 1-methyl-9Hfluorene, 2-methyl-9Hfluorene, 1-methylanthracene, 2-methylanthracene, 3,6-dimethylphenathrene, 1-methylfluoranthene, 1-methylpyrene, 7H-benzo[c]-fluorene
My experience has included 8 years in the Environmental, Occupational Hygiene and Health field, testing the environment’s exposure to harmful and toxic chemicals, and the exposure of employees working with these substances. Given the long-known carcinogenic properties of PAHs, I am aware of the relative quantities of these compounds (some far more carcinogenic than others) in a variety of environments.
Part 4
ReplyDeleteThe ‘Betula’ (and also note: chemical markers [triterpenoids] characteristic of birch-bark tar would also survive, allowing identification of this specific source if present) and ‘coal’ references in the Sub. Arch. piece are entirely irrelevant. And with the bitumen (or oil shale) source it is likely that they used it as a glue/fixative ‘as is’, and so they would not have needed to ‘dry distil’, or similar. That the bitumen source was only found in one sample, yet combustion markers consistent with roasting or smoking using fire were present in all the individuals subject to chemical analysis (a small sample set, admittedly; sample access means one often has to be pragmatic), is not consistent with bitumen/oil shale being the source. Moreover, it is the combination of chemical markers, not the PAHs exclusively, which allow the inference that cooked food is the most likely explanation for the presence of these chemical markers AS A WHOLE.
Archaeology and anthropology tend, as do humans as a whole, to look for non-existent, infallible, magic bullets, such as radiocarbon dates, age at death, and ancient DNA. In the same way, it is NOT the PAHs in isolation that are key, but rather the biomolecular evidence as a WHOLE.
Would I welcome more samples to analyse in order to provide a fuller picture at El Sidron? A: Undoubtedly. Is this perhaps primarily a show of potential for future studies of ancient diets (consumption)? A: Without question. But the conclusions made are reasonable and justifiable, based on the scientific evidence available to date.
All three: PAHs (general combustion markers), the alkyl phenols (wood smoke [or petroleum oil/coal]*), and methyl esters (heated [in usually acid conditions] animal fat/plant oil**) suggest thermal processes, which are COLLECTIVELY indicative of cooked food, in this case plant-based, from the molecular evidence, and accounting for bacterial input and degradation over time (diagenesis).
See Electronic Supplementary material: *alkyl phenols: Wood smoke and wood smoked foods (23,24); formation of gas, coal or oil; **fatty acid methyl esters: Strongly heated fat/oil derived acyl lipids, not easily formed (in nature); minor in fungal spores, bacteria (4-6)
The Subversive Archaeologist fundamentally fails to understand that the ‘combustion markers’ can be, and often are, formed by wood fires, WITHOUT the need to make an effort to exclude air. Most methods of combustion are inefficient (whether the car’s combustion engine, or fires we make in the woods, or on bonfire night), and so incomplete. Consequently, PAHs are often indicative of combustion in general. In the context of the FAR, FAR more abundant and no less persistent chemical markers from natural petroleum sources, coals, or similar, PAHs as a group are relatively minor. Consequently, one [again] then has to ask the question ‘what is the MOST LIKELY source of these compounds TRAPPED WITHIN ~50,000 year old Neanderthal dental calculus, long before the Industrial revolution and man’s impact on the planet threw out these compounds far more widely, making them as ubiquitous as they are today?’
In this full context explained above, I would suggest that the inferences made are reasonable, although, like all science (NOTE: recently, scientists announced that it was “probably a Higgs Boson” they had identified at CERN, so let’s not get too clever), it should always be open to review.
Apologies to all for the length, but I wanted all to have the full context. IF you’ve managed to read it all(!), thanks for listening.
If by my ineffective prose this response raises hackles, please accept my apologies.
ReplyDeletePhew! Thanks for that, Stephen. I'd very much like to transfer your four-part comment to the comment section of the Subversive Archaeologist, so the rest of its readers can attend to the points you make. And thanks for reminding me that it's worthwhile reading everything attached to a paper in this day of "online supplements." Mind you, I hope I can be forgiven for interpreting the two statements quoted below from the main article as being a little unequivocal, because that's what they sound like to me. 1. From the paper's final paragraph: "We believe that our findings offer the first direct molecular evidence for the ingestion of carbonized food and the inhalation of smoke by a Neanderthal individual." This, together with your comments throughout the paper, implies very strongly that the Neanderthals cooked that food and that the smoke they inhaled was from a campfire. Notwithstanding your confidence in the inferences of other archaeologists regarding the capacity of the Neanderthals to make and use fire, cook food and who knows what all else, the fact remains that those inferences are, themselves, interpretations based on observations for which the origin is by no means unambiguous. Unfortunately for your conclusions in "Neanderthal Medics?," they are not facts to be assumed a priori. 2. From the results on Adult 2—SDR-007c: "Also identified were a series of hopanes (carbon numbers C29 to C33), indicative of an oil shale or bitumen and corroborated by the presence of the isoprenoid hydrocarbon biomarkers phytane and pristane (Williams and Douglas 1986; Connan 1999)." This pretty much sets in stone the idea that these compounds were there because of the presence near the Neanderthal of either bitumen or oil shale. In the first place, I wonder whether either substance is to be found in the environment of El Sidrón. But that's not the important datum. At the risk of being a pain in the ass, I'm hoping that by now you've seen my own online supplement--"Pyrene, Good Night, Pyrene." All of these substances could have been incorporated in the dental calculus by the natural oral flora and the ingestion of completely different substances than your paper strongly implies--the rumen of an artiodactyl, for example. Finally, with reference to your gentle nudge about my selective reading of your article, neither statement quoted above includes a signal to your reader to consult another document (i.e. the online supplement) for the full results. If you want to avoid someone like me coming down on you for unwarranted inferences, you might wish to include any disambiguating information in the main article. But, besides all this, and despite your decades of experience, there remains the essential point--that you and your colleagues have not ruled out the other natural possibilities for your observations. Instead, you prefer to argue from the, as yet, unwarranted assumptions about the cognitive capabilities of the Neanderthals. Your conclusions are undermined by the possibility that the materials you've seen in the calculus of your Neanderthals could have come to rest there as a result of other processes that you simply haven't considered. That's not intellectual dishonesty, but it's intellectual short-sightedness, and its presence in the majority of what I comment on is what compels me to do what I do. Anyone wishing to make sense of observations made on the traces of past events must do all in their power to rule out natural circumstances before imputing the results to hominid behaviour. If you don't like that, I can't help it. I appreciate your careful work in isolating the substances you report. However, I wish your archaeologist co-authors were as careful with their reasoning, and more circumspect about their conclusions. Rob
Hi Rob,
ReplyDeleteYes, I'm happy for you to transfer my comments to the comments section, though apologies if I'm slow in responding myself; it will only be due to workload.
I agree that it might have been helpful if the electornic supplemetary info was more 'visible' - always a problem with science journals. I also suspect that media attention has forced - as it necessarily does - a more positive tone than would otherwise be the case. As I said, there is always some tension between the scientist and those interpreting the findings. Future work will hopefully address some of these problems.
I've answered your "Pyrene, Good Night, Pyrene." - plus a recap of all, following it.
Stephen
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