In preparing for a recent Bangkok workshop on the archaeology of rice, I have collected some thoughts of the proposed methodology for tracking rice domestication using rice husk phytoliths (the "double peaked" cells from lemma and palea).
The husks of rice are full of silica and often all the cells of silicified. The rows of cells on the rice husk include trapezoidal phytoliths the upper corners of which often form into peaks, as in the image below. These are diagnostic of the genus Oryza, although a few similar forms may occur more rarely in other grasses. These are often the most frequent form of rice phytolith. Because these derive from husk, disposed of after dehusking, they are an indicator of dehusking waste and useful in crop-processing studies. (See Harvey and Fuller 2005)
Size and shape of these varies and has been suggested to be useful in tracking domestication through measurements on populations (Zhao et al (1998)), although these are not definitive because of large degrees of overlap and because cell size is also impacted by environmental conditions. An explanatory mechanism for how these change during domestication has never been satisfactorily elaborated, although some relationship to grain size change seem plausible.
The husks of rice are full of silica and often all the cells of silicified. The rows of cells on the rice husk include trapezoidal phytoliths the upper corners of which often form into peaks, as in the image below. These are diagnostic of the genus Oryza, although a few similar forms may occur more rarely in other grasses. These are often the most frequent form of rice phytolith. Because these derive from husk, disposed of after dehusking, they are an indicator of dehusking waste and useful in crop-processing studies. (See Harvey and Fuller 2005)
Size and shape of these varies and has been suggested to be useful in tracking domestication through measurements on populations (Zhao et al (1998)), although these are not definitive because of large degrees of overlap and because cell size is also impacted by environmental conditions. An explanatory mechanism for how these change during domestication has never been satisfactorily elaborated, although some relationship to grain size change seem plausible.
The proposed
method for looking at double peak cells and domestication uses 5 measurement on
each phytolith as defined below, left (from Zhao et al 1998)- note that H is
measured twice on each side of the phytolith. Some of these are then used in
squared form. These are combined in discriminant functions that are meant to
assign individual phytoliths to like domesticated or wild (i.e. if the
domesticated score is greater than the wild score: formula at right).
As originally
developed, Zhao et al (1998) reported correct classification in their modern
reference set was correct in >70% of test cases. The formulae were developed
by taking Bayesian approach to discriminant function analysis. In an attempt to
employ and extend this work, we attempted to replicate this in London with
modern rice accessions, but found a correct identification in only 44% of cases
(Harvey 2006). In addition measurements on phytoliths from Chalcolithic sites
in Orissa (Gopalpur and Golbai Sassan) predicted a majority wild rice and on
39% domesticated. However these sites (dating 1500-1000 BC) have spikelets
bases that indicate fully domesticated rice (100% non-shattering at Gopalpur
and ~70% at Golbai out of a small sample size: unpublished UCL data from
Kingwell-Banham 2015). This indicates that this phytolith discrimination method
is unlikely to work in India, raising questions about what biogeographic
contexts it would be useful in, if at all. One problem is that some of the
variation in ancient cultivars may not be well represented in modern landraces.
Indeed some of the measurements on archaeological phytoliths from Orissa fell
outside the range of modern material, both wild and domesticated.
Also,
against this method are two applications in China that have yielded results
that are illogical with regards to what is known about rice domestication. As applied by Zhao (1998) to Diaotonghuan cave
in Jiangxi and change from predicted wild in pre-ceramic layers and predicted
domesticated dominance in early ceramic layers was found. At the time Zhao
wrote this it was assumed the that advent of pottery was Neolithic and sometime
in the early Holocene, but recent dating work on nearby Xianrendong and another
South Chinese cave, Yuchanyuan, indicate the ceramics began to be produced
around the Last Glacial Maximum or just after 18,000-16,000 BP. The ceramics at
Daiotonghuan are comparable and thus this would re-date the alleged rice
domesticated to ~18,000-16,000 BP, nearly 10,000 years earlier than potential
sedentary, agricultural villages. An more plausible alternative explanation is
that rice husk cells (and grains) changes shape in response to the major and
rapid change in climate and atmospheric carbon dioxide levels that took place
after the LGM.
As applied by Itzein-Davey et al (2007) in the
Lower Yangtze region to a stratigraphic sequence of Qingpu rice bulliforms dating
between 2300 BP and 1800 BO (i.e. Warring State through Han Dynasty era), they
found the majority of double peaks were predicted as wild, often as much as 80%
in some samples. Rice was certainly morphologically domesticated in the Lower
Yangtze long before this and we would expect fairly intensive rice agriculture
during Han times. These results also call into question this index.
Nevertheless
plotting double peak measurements over a time series may provide a line of
evidence for rice that is changing and evolving morphologically. This has
recently been applied in South America to argue for a lost rice domesticationin the Amazon (Hilbert et al 2017). In
the context of Chinese rice domestication the study of Wu et al (2014)
demonstrated both that wild and domesticated predictions are very mixed on
sites of early cultivation but also that there is trend for more double peak
cells to fall towards the apparently domesticated end of the spectrum through
time.
The
bottom line: Variation in husk phytoliths exists but its significance in terms
of domestication, varietal changes, cultivation ecology remains unclear and
deserved further study.
3 comments:
I know this will sound dim, but what exactly is a double-peaked phytolith? It looks like it might be a cell wall type analogous to the papillae that we find in the husks of wheat. In other words, what is the anatomical origin of these phytoliths? I have been wandering around the literature for ages an nobody seems to say!
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