Friday, 8 December 2017

Neolithic wine drinkers in Georgia or wishful thinking

Strong inference I was always taught comes from thinking through multiple working hypotheses and assessing which hypothesis is best supported by available evidence and trying to falsify alternatives. It is unfortunate if the quest for headlines and high profile publication gets in the way of clear thinking and an scientific approach. Of course sometimes evidence and conclusions are only partly certain, but when that is the case it should not be carefully hidden in online supplementary text and onreference claims of background fact as is the case in this study. I have to conclude weak inference reigns in the recent headline grabbing claim that the first grape wine makers anddrinkers were to be found in the Neolithic Georgia. While McGovern et al present a compelling read in their PNAS paper, and some apparently very technical scientific support, their presentation seems to me aimed to grab headlines and appeal to journalist or generalist and not really to convince scientifically the specialist. The fine print in the supplement raises many unanswered questions that undermine their conclusion. The failure to reject alternative plausible hypotheses for their result, the lack of reference to scientific names, regional flora inventories or vegetation surveys, as these would so clearly support alternative hypotheses…

Here is the claim: some interesting liquid storage vessels that could well be for wine storage have produced tartaric acid residues. This would indeed constitute part of an evidential base to argue for early wine, but on its own this is necessary but not sufficient evidence for the claim. Tartaric acid occurs as background in the soil—thus one learns in the supplement that some 11 sherds were rejected as having levels not sufficiently above background soil levels. What is more tartaric acid occurs in many fruits, not only grapes. Oddly they reject other sources with a flippant line in their supplement, “Other plants with high tartaric acid–e.g., hawthorn fruit and star fruit from east Asia, tamarind from the Indian sub-continent, and yellow plum from the New World—can be ruled out”- notable for being without any Latin names, without any citation of botanical sources on  these taxa, their chemistry, or on the regional flora from which to based claims about their distribution. What is worrying here is the inclusion of hawthorns (Crataegus spp.), and plums in this list. They claim that “yellow plum” is exclusively American—and while this is ture if what they mean is the species Prunus americana, but the broader Prunus genus, including numerous Cerasus cherries and Padus bird cherries, has high endemic diversity in the Caucusus regions, as well as numerous Crataegus species. There are ~70 species of Prunus native to Eurasia: are we really to believe that none contains tartaric acid in contrast their common American cousin? If so, how have these been excluded. Contrary to the dismissive statement in the supplement there are half a dozen Crataegus reported form the Caucasus region in the old Flora of the USSR, and several more to the South or to the north in Ukraine! (Flora of the USSR Vol. IX. Rosales and Sarraceniales, by Botisova et al 1939, English Language 1971 from Jerusalem, online here)

Grape pip with measurements (from
Bouby & al 2013 PLOSone
One might support an argument for grape wine production on archaeobotanical grounds if flotation samples were rife with grape pips and no other fruits, but in fact we learn that all AMS dates run grape pips turn out to be intrusive, Bronze Age and later. This does not support major use of grapes for wine in the Neolithic but quite the opposite! For Neolithic grape finds one must go to the Fertile Crescent, or indeed to parts of Mediterranean Europe, like Greece or Italy! While some of the co-authors have have done some cutting age work on the geometric morphometrics on grapes, e.g. Laurent Bouby, whose work on archaeological grape diversity in France is indeed cutting edge (e.g. the Vegetation History and Archaeobotany paper by Bacilieri, Bouby et al earlier this year), the deployment of these techniques on Georgian grapes from the Bronze Age and later (based on direct dates) does little to support a sequence of grape domestication in the Neolithic Caucausus.

Some sort of fermented fruit wine—the conclusion is plausible. But grape wine? That still seems wishful thinking. At best the sloppy and journalistic presentation of the evidence might be attributed to weak editing and inadequate peer reviewing combined with authors’ excitement, but at worst it represents obfuscation of the science to claim headlines and citations indices.

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