Photo credits: German Mining Museum Bochum and Zanjan Cultural Heritage Center, Zanjan Archaeological Museum.
A team of geneticists and archaeologists from Ireland, France, Iran, Germany and Austria has sequenced the DNA of a 1,600 year old sheep mummy from an ancient Iranian salt mine, Chehrābād. This remarkable specimen has revealed sheep farming practices of the ancient Middle East and underscores how natural mummification can affect DNA breakdown.
The incredible results have just been published in the international peer-reviewed journal Biology letters.
The Chehrābād Salt Mine is known for preserving biological material. In fact, human remains of the famous “salt people” were recovered in this mine, which were dried out by the salty environment. The new research confirms that this natural mummification process – the process of removing water from a corpse, leaving soft tissue that would otherwise break down – also preserves animal remains.
The research team, led by geneticists at Trinity College Dublin, took advantage of this by extracting DNA from a small section of mummified skin on a leg recovered from the mine.
While ancient DNA is usually damaged and fragmented, the team found that the sheep mummy’s DNA was extremely well preserved. with longer fragment lengths and less damage normally associated with such an ancient age. The group attributes this to the mummification process, with the salt mine providing ideal conditions for the preservation of animal tissue and DNA.
The influence of the salt mine was also evident in the microorganisms in the sheep bone. Salt-loving archaea and bacteria dominated the microbial profile – also known as the metagenome – and may also have contributed to the maintenance of the tissue.
The mummified animal was genetically similar to modern sheep breeds from the region, suggesting that there has been continuity of sheep ancestry in Iran for at least 1,600 years.
The team also used sheep’s DNA preservation to study genes linked to a wool fleece and a fat tail – two important economic traits in sheep. Some wild sheep – the Asian mouflon – are characterized by a “hairy” fur, which is very different from the woolly fur of many domestic sheep. Fat-tailed sheep are also common in Asia and Africa, where they are valued in the kitchen and can be well adapted to dry climates.
The team made a genetic impression of the sheep and found that the mummy lacked the gene variant associated with a woolly coat, while fiber analysis using scanning electron microscopy found the microscopic details of the hair fibers that matched hairy or mixed breeds of fur. Fascinatingly, the mummy carried genetic variants associated with fat-tailed breeds, suggesting that the sheep resembled the hairy, thick-tailed sheep seen in Iran today.
“Mummified remains are quite rare, so little empirical evidence about the survival of ancient DNA in these tissues was known prior to this study,” says Conor Rossi, PhD student at the Trinity School of Genetics and Microbiology and lead author of the study.
“The astonishing integrity of DNA was unlike anything we had seen before from ancient bones and teeth. This DNA conservation, coupled with the unique metagenomic profile, is an indication of how fundamental the environment is to the dynamics of tissue and DNA decay.
Dr. Kevin G. Daly, also from the Trinity School of Genetics and Microbiology, oversaw the study. He added:
“Using a combination of genetic and microscopic approaches, our team has succeeded in creating a genetic picture of what sheep breeds looked like in Iran 1,600 years ago and how they were used.
“Through interdisciplinary approaches, we can learn what ancient cultures valued in animals, and this study shows us that the people of Sasanid-era Iran may have led flocks of sheep that specialized in meat consumption, suggesting well-developed husbandry practices. ”