Wheat at the museum

Revealing the agricultural past using DNA from a 3,000-year-old museum specimen of Egyptian emmer wheat.

Sequenced specimen of emmer wheat
Sequenced specimen of emmer wheat

Human societies throughout history need food, and that often means wheat, which was first cultivated over 12,000 years ago in the 'fertile crescent' region of the Middle East. Since this time, agricultural practices and tastes have changed and we have even shifted between growing different species of wheat. Now, genomic resources allow us to look at the DNA of ancient wheats to get another perspective on these changes.

In a recent study, I analysed DNA sequence from an Egyptian wheat harvested over 3,000 years ago. The sequenced chaff was excavated over 90 years ago and is now on display in the UCL Petrie Museum of Egyptian Archaeology.

In Pharaonic Egypt, the most common wheat was emmer wheat, a progenitor of the free-threshing wheats that are now widely grown. Egyptian preference was so notable that it is claimed that hulled wheats are called 'farro' because Romans referred to emmer as "Pharaoh’s wheat". Bread made with emmer wheat has a nutty flavour, but the gluten is weaker than in common wheat and it tends to rise less.

Accession of wheat chaff on display at the Petrie Museum of Egyptian Archaeology, UCL, alongside Egyptian artefacts
Accession of wheat chaff on display at the Petrie Museum of Egyptian Archaeology, UCL, alongside Egyptian artefacts

The first thing I looked at was the overall genetic similarity between the ancient Egyptian sample and modern emmer wheats from around the world. Emmer wheat is no longer grown in Egypt, but this analysis can be used to infer how it spread to Egypt.

One surprise was that the most closely-related modern emmer wheats were not those that are grown geographically closest to Egypt. The Egyptian emmer wheat was most similar to modern emmer grown in Turkey, Oman and India. This connects the spread of agricultural practices into Egypt (from ~4500 BCE) with early expansions into the Indus valley (by ~6000 BCE).

Furthermore, I examined genomic regions associated with pre-historic artificial selection. Coincidentally or deliberately, humans have changed crop plants to be better suited for cultivation. Domesticated emmer wheats retain their seeds on the plant, where they can be easily harvested. This would be disadvantageous in the wild because it prevents dispersal. Domesticated emmer wheat seeds are also consistently large and easy to germinate. The variants carried by the ancient Egyptian emmer at key loci indicate that it likely had these domestication traits.

Sourdough loaves made with 100% emmer wheat (left) and with a 50:50 mix of emmer wheat and bread wheat (right)
Sourdough loaves made with 100% emmer wheat (left) and with a 50:50 mix of emmer wheat and bread wheat (right)

Finally, I examined whether there was evidence of genetic exchange between wild wheats and the ancient specimen. Genetic material from wild populations can get into domesticated wheats if farmers cultivate a mixture of wild and domesticated crops or if they are grown near to wild populations. The ancient Egyptian wheat genome had signs of historical hybridisation with wild wheats in the Southern Levant region. This could have occurred before the wheat's introduction to Egypt from the Levant. Alternatively, it's possible that Egyptian trade and military campaigns in the Southern Levant allowed their wheat to breed with the wild populations there.

Our results show that museum archaeobotanical collections, even those stored for decades without climate control, have great potential for further genomic analysis. Some have speculated that alleles found in ancient crops but absent from modern domesticates might be targeted for re-introduction from the wild, as some of these lost variants may have been useful. Certainly, we can expect further genomic analyses of ancient crops to uncover historical details about the spread of cereals around the globe and the changes in genetic diversity that have occurred.

Michael F Scott - MECEA 2020 Post Doc winner - @mscott0106, Google Scholar

Scott, M.F., Botigué, L.R., Brace, S. et al. A 3,000-year-old Egyptian emmer wheat genome reveals dispersal and domestication history. Nat. Plants 5, 1120–1128 (2019). https://doi.org/10.1038/s41477-019-0534-5

comments powered by Disqus