Ancient mummified sheep shows same genetic traits as modern breeds

14 Jul 2021

The mummified sheep leg. Image: Deutsches Bergbau-Museum Bochum and Zanjan Cultural Heritage Centre, Archaeological Museum of Zanjan

Genetic analysis of the 1,600-year-old mummified sheep showed it had a hairy fleece and fat tail, just like sheep in the region today.

Sitting 75km north-west of Zanjan City in Iran and at an elevation of 1,350 meters above sea level, the Chehrābād salt mine is famous for its timeless quality.

This historic site is less a tourist attraction however, and more a mother lode for archaeologists and geneticists. This is because of its remarkable ability to preserve dead organic matter.

In this mine, researchers discovered part of a mummified sheep and with it, have deduced insights into the historic farming preferences of the region.

A small cutting from the sheep leg was sent to the Smurfit Institute of Genetics at Trinity College Dublin. From this, researchers showed it is likely an ancestor of modern flocks in the area.

This sheep is estimated to be at least 1,600 years old and joins the ranks of the already famous ‘salt men of Chehrābād’. The salt mine has been in use through at least four distinct periods in history and to date, eight mummified bodies have been found in these caverns.

These ‘salt men’ have captured the public imagination since they were first discovered in 1993. Their remarkable preservation makes for a visually striking, direct view into the area’s past.

Articles in biology and archaeology journals describe the mine as a highly saline, anhydrous environment that provides an excellent opportunity “to investigate potential differences in nucleotide degradation resulting from this unusual taphonomic context.”

In other words, the salt in the air sucks the water from dead tissue before it has a chance to decay, creating amazingly preserved natural mummies that can show how DNA degrades.

A research team, led by geneticists from Trinity, exploited this by extracting DNA from a small cutting of mummified skin from the sheep leg recovered in the mine.

While ancient DNA is usually damaged and fragmented, the team found that the sheep mummy DNA was extremely well preserved with longer fragment lengths and less damage than would usually be associated with such an ancient age.

The group attributes this to the mummification process, with the salt mine providing conditions ideal for preservation of animal tissues and DNA.

The salt mine’s influence was also seen in the microorganisms present in the sheep leg skin. Salt-loving archaea and bacteria dominated the microbial profile and may have also contributed to the preservation of the tissue.

“Mummified remains are quite rare so little empirical evidence was known about the survival of ancient DNA in these tissues prior to this study,” said Conor Rossi, PhD candidate in Trinity’s School of Genetics and Microbiology, and the lead author of the paper.

“The astounding integrity of the DNA was not like anything we had encountered from ancient bones and teeth before. This DNA preservation, coupled with the unique metagenomic profile, is an indication of how fundamental the environment is to tissue and DNA decay dynamics.”

The team were particularly interested in similarities and differences between the genetic makeup of the sheep mummy and modern flocks in the region.

By investigating the DNA of the mummified sheep, the team looked at two traits that were associated with certain genes – the presence of a woolly fleece and a fat tail. Some sheep have a hairy fleece rather than a woolly one, while other sheep have slimmer hind quarters than their fat-tailed counterparts.

Through building a genetic impression and using fibre analysis, the researchers found the hairs of the leg to be more consistent with a hairy fleece. Equally, the genes were similar to those of fat-tailed breeds.

This lines up with the hairy-coated, fat-tailed sheep seen in the area today, suggesting a continuity of ancestry of sheep in Iran that is at least 1,600 years old.

Dr Kevin G Daly, also from Trinity’s School of Genetics and Microbiology, supervised the study and said: “Using a combination of genetic and microscopic approaches, our team managed to create a genetic picture of what sheep breeds in Iran 1,600 years ago may have looked like and how they may have been used.

“Using cross-disciplinary approaches we can learn about what ancient cultures valued in animals, and this study shows us that the people of Sasanian-era Iran may have managed flocks of sheep specialised for meat consumption, suggesting well developed husbandry practices.”

Sam Cox was a journalist at Silicon Republic covering sci-tech news