A team of geneticists and archeologists from Ireland, France, Iran, Germany,
and Austria has sequenced the DNA from a 1,600-year-old sheep mummy from an
ancient Iranian salt mine, Chehrābād. This remarkable specimen has revealed
sheep husbandry practices of the ancient Near East, as well as underlining
how natural mummification can affect DNA degradation.
The incredible findings have just been published in the international,
peer-reviewed journal Biology Letters.
The salt mine of Chehrābād is known to preserve biological material. Indeed,
it is in this mine that human remains of the famed "Salt Men" were
recovered, dessicated by the salt-rich environment. The new research
confirms that this natural mummification process—where water is removed from
a corpse, preserving soft tissues that would otherwise be degraded—also
conserved animal remains.
The research team, led by geneticists from Trinity College Dublin, exploited
this by extracting DNA from a small cutting of mummified skin from a 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 that 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—also known as the metagenome—and may have also contributed to the
preservation of the tissue.
The mummified animal was genetically similar to modern sheep breeds from the
region, which suggests that there has been a continuity of ancestry of sheep
in Iran since at least 1,600 years ago.
The team also exploited the sheep's DNA preservation to investigate genes
associated with a woolly fleece and a fat-tail—two important economic traits
in sheep. Some wild sheep—the asiatic mouflon—are characterized by a "hairy"
coat, much different to the woolly coats seen in many domestic sheep today.
Fat-tailed sheep are also common in Asia and Africa, where they are valued
in cooking, and where they may be well-adapted to arid climates.
The team built a genetic impression of the sheep and discovered 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 consistent with hairy or mixed coat breeds. Intriguingly,
the mummy carried genetic variants associated with fat-tailed breeds,
suggesting the sheep was similar to the hairy-coated, fat-tailed sheep seen
in Iran today.
"Mummified remains are quite rare so little empirical evidence was known
about the survival of ancient DNA in these tissues prior to this study,"
says Conor Rossi, Ph.D. 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.
Dr. Kevin G Daly, also from Trinity's School of Genetics and Microbiology,
supervised the study. He added, "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 specialized for meat
consumption, suggesting well developed husbandry practices."
Reference:
Exceptional ancient DNA preservation and fibre remains of a Sasanian
saltmine sheep mummy in Chehrabad, Iran, Biology Letters (2021). https://doi.org/10.1098/rsbl.2021.0222