New ways with old wheats: the science in detail
The earliest farming communities grew primitive wheats such as emmer, but as agriculture progressed these were replaced by bread wheat and other modern varieties. Bread wheat is a hexaploid species, each cell containing six sets of chromosomes, rather than the four sets found in the tetraploid emmer. The two extra sets of chromosomes (called the D chromosomes) include genes that confer good bread-making quality on flour produced from the grain, which means that bread wheat is of greater culinary value than emmer.
We can deduce the agricultural status of a prehistoric population, with wide cultural and social implications, by identifing the types of wheat preserved at an archaeological site. However, this analysis is not always possible by conventional means. Preserved wheat grains are frequently obtained from archaeological sites but the grains are often burnt and distorted, making it difficult to determine whether they come from a tetraploid or hexaploid wheat.
Our aim was to devise new methods of studying prehistoric wheats, making use of the ancient DNA that is present in some preserved grains. We devised a method to distinguish between primitive tetraploid varieties of wheat (emmer wheats) and modern hexaploid forms (bread wheats).
The DNA is very fragmented: rather than the immensely long molecules (tens of millions of nucleotides long) found in living cells, we are only able to isolate fragments of a hundred or so nucleotides (see graph). But modern molecular biology techniques are very powerful and we have devised ways of working out DNA sequences from these small pieces of ancient DNA.
We made two unexpected discoveries. First, we found that, according to its gene content, one of the Bronze Age wheats we examined would have had excellent bread-making properties. This was surprising because prehistoric wheats are not thought to give rise to particularly good quality bread, though some archaeolinguists hold that Linear B texts from Bronze Age Greece refer to bakers.
The second discovery is that cultivated wheats are more genetically diverse than previously thought. this suggests that wheat might have been domesticated twice, in two different places, rather than just once as currently believed by archaeologists. We are exploring this possibility further in our current experiments.
Our new test involves analysis, by DNA sequencing, of the genes coding for the glutenin proteins. Both tetraploid and hexaploid wheats contain glutenin proteins, but the genes on the D chromosomes have special features which we can recognise by examining their DNA sequences. By looking for these features in ancient DNA from burnt seeds we can tell if a grain comes from emmer or from a bread wheat.
This was a joint project between molecular biologists Terry Brown, Robert Sallares and Robin Allaby (UMIST) and archaeobotanist Glynis Jones (University of Sheffield).
Department of Biomolecular Sciences, UMIST,
Manchester M60 1QD
tel: 0161 200 4173, fax 0161 236 0409
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Department of Archaeology,
University of Sheffield, Sheffield S1 4ET
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Excavation at the Bronze and Iron Age site of Assiros Toumba in Central Macedonia was conducted by Dr. K.A. Wardle between 1975 and 1987. Study and preparation for publication continues.
Dr. K.A. Wardle
Department of Ancient History and Archaeology,
The University of Birmingham
Birmingham B15 2TT
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Ancient Biomolecules Initiative
The Ancient Biomolecules Initiative is a five-year programme to understand the fate of biological molecules in archaeological and fossil materials, and to explore the applications of this new knowledge. The Initiative is funded by the Natural Environment Research Council.